Keywords von Willebrand factor - ADAMTS13 - COVID-19 - schistocytes - coagulopathy
Introduction
Coronavirus disease 2019 (COVID-19) is a respiratory disease with heterogeneous manifestations
ranging from asymptomatic illness in some, to systemic inflammation, multiorgan failure,
and a rapid death in others.[1 ]
[2 ] The first stage of disease manifests as an upper respiratory infection followed
by pneumonia when the virus invades the respiratory epithelium via binding to angiotensin
converting enzyme 2 (ACE2) receptors.[3 ] A second, more severe, phase may be manifested as multiorgan damage, including respiratory,
cardiac, hepatic, and renal injury. At this stage, the ACE2 receptors on the endothelium
can also be involved, causing direct damage to blood vessels and inducing a coagulopathy.[3 ]
Systemic inflammation and coagulopathy are characteristic hallmarks of this phase.
“COVID coagulopathy” manifests mainly as a prothrombotic state affecting both large
and small blood vessels, and presenting as arterial, venous, and microangiopathic
thrombotic events.[4 ]
[5 ] Coagulopathy with D-dimer elevations is reported in most hospitalized COVID-19 patients.[6 ]
[7 ]
[8 ] A recent study showed that markers of endothelial damage such as von Willebrand
factor (VWF) and soluble thrombomodulin were also increased in COVID-19 hospitalized
patients. All these markers were even higher in intensive care unit patients and correlated
with mortality.[9 ] VWF has three main functions: binding to collagen in the wounded subendothelial
matrix, binding to glycoprotein-1b on platelets, and carrying then subsequently delivering
coagulation factor VIII (FVIII) to the surface of activated platelets bound to wounded
endothelium.[10 ] Whether the increased VWF reported in COVID-19 is a result of increased production,
abnormal and/or increased release, or decreased destruction is unclear. Since ADAMTS13
(a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13),
a VWF-cleaving protease, plays a key role in regulating both VWF quantity and multimer
size, analysis of this enzyme would be important in elucidating the pathophysiology
of COVID coagulopathy. Although there have been some COVID-19 data involving ADAMTS13
activity levels, the small sample size in these reports precluded any major conclusions.[11 ]
[12 ]
[13 ]
The primary objective of our study was to establish the relationship of VWF-related
biomarkers with coagulation, thrombosis, intravascular hemolysis, and end-organ damage
in a large cohort of COVID hospitalized patients. The secondary objective was to study
the correlation of VWF-related biomarkers with disease severity and mortality.
Methods
Study Population
We included confirmed COVID-19 cases in Montefiore Medical Center who were hospitalized
and had routine and/or advanced coagulation tests done between March 26, 2020 and
May 5, 2020. All included patients tested positive for SARS-CoV2 with reverse-transcriptase–polymerase-chain-reaction
real-time assay of the nasal and the pharyngeal swabs. We excluded the patients younger
than 18 years of age. The medical records of the patients were reviewed to obtain
epidemiological, demographic, clinical, and laboratory data. The management and clinical
outcomes were followed-up until June 20, 2020. All cases had final disposition (deceased
or discharged alive) and none were censored. The study was approved by the Albert
Einstein College of Medicine Institutional Review Board.
Laboratory Investigations
A total of 3,672 plasma samples were aliquoted from sodium citrate tubes shortly after
blood draw and stored at −80°C until needed. Samples were deidentified, coded with
a unique arbitrary number, sorted by this arbitrary number (lowest to highest), and
grouped into two categories: deceased versus discharged alive cases. To study the
association of disease severity with VWF antigen, VWF activity, and ADAMTS13 activity,
samples were then selected down the list (top to bottom) with balanced number of discharged
alive versus deceased patients across the range of D-dimer levels from normal to very
high (0.26 to >20 μg/mL) (n = 128). These cases were stratified by D-dimer categories: low (<2 μg/mL), moderate
(2–10 μg/mL), and high (>10 μg/mL) (study design chart in [Supplementary Fig. S1 ]). To study if initial ADAMTS13 activity at presentation has predictive value, we
selected an additional 40 cases with a sample collected within 72 hours since admission.
This selection, like the original selection, was random going down the list with similar
numbers of discharged alive and deceased patients. To study the association between
platelet count and ADAMTS13 activity, we selected 13 patients who had a platelet count
< 70 × 106 /μL. This platelet count was predefined, as it is a common cut off to differentiate
mild from moderate thrombocytopenia in our institute. Altogether the total number
of cases was 181 (study design chart in [Supplementary Fig. S1 ]).
Statistical Analysis
Data analysis was performed using R studio, V.3.6.2 and graphs generated in Prism
V.8.3.1. Differences in demographic, clinical variables, and laboratory assessments
between patients who were deceased, and patients discharged alive were compared using
Chi-square or Fisher's exact tests for categorical variables, two-sample Student t -tests, and one-way ANOVA for three group comparisons. Youden's J statistics was used to find the optimal cut point for ADAMTS13 activity for mortality.
Logistic regression of initial ADAMTS13 activity adjusted by age was represented in
a density plot against mortality. Logistic regression was also used to adjust BMI
by age. A Kaplan-Meier cumulative curve was generated for patients discharged alive.
Laboratory Testing
Coagulation tests (VWF Antigen, VWF Ristocetin activity [VWF:RCo], FVIII activity
levels, D-dimer, and fibrin monomer [FM]) were performed by STA-R Max instruments
using STAGO reagents as per manufacturer recommendations. We validated the Siemens
BC VWF Ristocetin cofactor reagents in the STA-R Max as previously described.[14 ] STA Liatest LIA D-dimer assay was performed as per manufacturer recommendations
and reported as fibrinogen-equivalent-units (FEU) μg/mL. Complete blood counts (CBCs)
were performed by Sysmex XN9000. Chemistry assays were performed by Roche instrumentation
and reagents as per manufacturer recommendations.
After thawing the patient samples in a 37°C water bath, a 1:1 dilution was created
using patient plasma and STAGO Owren-Koller buffer. Using this dilution, VWF antigen,
VWF:RCo, FVIII activity levels, D-dimer, and FM levels were obtained. Our reportable
range for VWF antigen is 420%, VWF activity 650%, and FVIII activity is 600%. If the
STA-R Max instrument was not able to accurately report the value due to it being above
the reportable range, a 1:20 dilution was made with Owren-Koller buffer and subsequently
a 1:100 dilution until the value was within reportable range. All results were multiplied
by the dilution factor to obtain the final value.
Data Gathering
Chart reviews were performed to document demographic attributes (age, sex, and self-reported
race and/or ethnicity) and baseline comorbidities (body mass index, previous history
of hypertension, diabetes, kidney, pulmonary, liver, autoimmune, cancer, or sickle
cell disease) on presentation collected for calculation of comorbidity indexes. We
gathered data on initial vital signs and laboratory values within the first 48 hours
of hospital admission. The laboratory assessments consisted of a CBC, blood chemical
analysis, coagulation testing, assessment of liver and renal function, measures of
electrolytes, and markers of inflammation. Additionally, we noted the D-dimer, fibrinogen,
hemoglobin, creatine, lactate dehydrogenase (LDH), indirect bilirubin, and platelet
count of the patients within 48 hours of the time of collection of the samples we
tested. We also noted the trajectory of these parameters in the week following the
collection of the sample, noting whether these parameters increased by 10% or more,
decreased by 10% or more, or remained stable. We accessed each patient for thrombosis
and clot formation. A patient was considered to have thrombosis if a thrombus was
identified on radiological imaging. We also noted if a patient experienced ex vivo
clotting while on hemodialysis (HD) or continuous renal replacement therapy based
on the need for kit/filter change and/or visual clots as documented in the clinical
progress notes. We documented anticoagulation medications given to each patient within
48 hours preceding the thrombus or clotting event.
STROBE Criteria
This study followed the STROBE criteria for retrospective studies including: (1) providing
a summary in the abstract of the objectives, the study type, outcome, and conclusion;
(2) providing scientific background, rationale, and hypothesis in the introduction;
(3) providing details of the study design in the methods including setting, participants,
sample size, variables, data sources, and measurements; (4) providing details of statistical
methods; (5) describing demographics of the population in the results; (6) providing
95% confidence intervals in the results when appropriate; (7) providing a discussion
of the limitations, potential bias, and generalizability.
VWF Multimers
VWF multimers were generated using a western blot technique previously described.[15 ] Briefly, samples were first prepared by normalizing the amount of thawed plasma
added to the loading dye according to the measured VWF antigen level. The samples
were then heat inactivated for at least 15 minutes in a 56°C water bath. The samples
subsequently were loaded onto a 1% agarose gel. A normal control consisting of normal
pooled plasma and an abnormal control consisting of plasma from a patient with Type
2A Von Willebrand disease were used. The gels were run using the PhastSystem Separation
and Control unit. After this, the gels were transferred to a polyvinylidene difluoride
membrane also using the PhastSystem Separation and Control unit. After the transfer,
the membranes were blocked for at least 40 minutes using 1.2% bovine serum albumin
(BSA) (Sigma-Aldrich) in tris-buffered saline plus 0.04% Tween 20. After blocking,
rabbit anti-human F. VIII-related antigen antibody (Accurate Chemical AXL 205) was
applied as the primary antibody for 45 minutes at a concentration of 5.58 µg/mL diluted
in 1.2% BSA. Anti-rabbit IgG-alkaline phosphatase-conjugated antibody (Sigma A8025)
was applied as the secondary antibody for 45 minutes at a concentration of 1.5 µg/mL
diluted in 1.2% BSA. After this, SigmaFast 5-Bromo-4-chloro-3-indolyl phosphate/nitro
blue tetrazolium (BCIP/NBT) solution was used to develop the blot.
Western blots were then analyzed using the ImageJ analysis software. Briefly, each
sample well was segmented into low, intermediate, and high molecular weight VWF multimers.
Low molecular weight multimers were defined as the bottom three bands of the well.
The division between intermediate and high molecular weight multimers (HMWMs) was
established using the abnormal control that was run in each gel, as the abnormal control
has no HMWMs. A straight line was drawn across the gel where the HMWM signal in the
abnormal control started to taper. Intermediate size multimers were those between
the cut-off established with the abnormal control and the highest band of the low
molecular weight multimers. The measure function in ImageJ was used to measure the
raw integrated density of each size of the multimers. All values were normalized to
total VWF protein loaded per well. Values are reported as fold change from normal
control.
VWF Collagen Binding Activity
Previously thawed plasma was centrifuged for 10 minutes at 24,328xg and the supernatant
was used for the VWF collagen binding activity (VWF:CB) enzyme-linked immunosorbent
assay (ELISA) and ADAMTS13 activity assay. Human VWF:CB was measured using Zymutest
VWF:CB ELISA Kit (#RK038 from Hyphen BioMed). The ELISA was completed and analyzed
using the manufacturer's recommendations.
ADAMTS13 Activity
The ADAMTS13 protease activity on previously thawed and centrifuged plasma supernatant
was measured using ATS-13 activity assay based on fluorescence resonance energy transfer
(Immucor ATS-13). The assay was performed and analyzed using the manufacturer's recommendations.
For patients whose plasma samples had an ADAMTS13 activity level of less than 30%,
we ran an inhibitor assay according to the manufacturer's recommendations. Briefly,
this assay involved mixing equal volumes of normal pooled plasma with the patient's
plasma and measuring the ADAMTS13 activity of the mixed sample relative to that of
the normal pooled plasma to find the percent inhibition. We considered a value between
25 to 40% mild inhibition and greater than 40% inhibition to indicate a true inhibitor.
ADAMTS13 Antigen
The ADAMTS13 antigen level was measured on previously thawed and centrifuged plasma
supernatant using Technozym ADAMTS13 Fluorogenic Activity/Antigen (cat# 5450551).
The ELISA was completed and analyzed using the manufacturer's recommendations.
ADAMTS13 Antibody Detection
The presence of human IgG autoantibodies against ADAMTS13 was determined using Technozym
ADAMTS13 Inhibitor ELISA (cat# 5450451). The ELISA was completed and analyzed using
the manufacturer's recommendations.
Peripheral Blood Smear
CBCs were performed on admission and when clinically indicated during hospitalization.
Manual differentials were performed when reflexed due to a count threshold or scattergram
abnormality. We analyzed all the smears available in which a smear review was reflexed
due to an abnormality in the white blood cells (WBCs), red blood cells (RBCs), platelet
count, or scattergram. Smear photos were obtained from CellaVision. Schistocytes,
RBC fragments, and ghost cell count were based on at least 1,000 RBCs. RBC count was
performed by using the digital manual counter on Image J. We noticed that cases in
which the schistocytes were less than 1%, the smear review was not prompted by RBC
or platelet flags but were prompted by unrelated flags, e.g., WBC flags. As previously
reported, the sensitivity of RBC or platelet flags to detect schistocytes/RBC fragments
is less than 1% (0.6–0.9%).[16 ] Thus, we classified all cases with no RBC/platelet flags as <1% schistocytes/RBC
fragments.
Results
Study Population
Samples from 90 patients who died and 91 who were discharged alive with a wide and
balanced distribution of D-dimer levels (0.26 to >20 μg/mL) were selected ([Supplementary Fig. S1 ]). The characteristics of the study population are summarized in [Table 1 ]. Consistent with many other studies, nonsurvivors were older (median [interquartile
range or IQR]; 72.5 [63.3, 79.8] vs. 62.0 [50.5, 70.0] years) and the majority were
males (67% [60/90], p = 0.03) ([Table 1 ]). No difference between survivors and nonsurvivors by ethnicity or comorbidity was
observed. Although the median BMI was higher in survivors, when BMI was adjusted by
age, there was no significant difference between survivors and nonsurvivors (p = 0.54, data not shown). As expected, the number of patients that required a ventilator
was higher in nonsurvivors 51% (46/90) versus 26% (24/91) in survivors. No significant
difference in the average length of stay or treatment was observed.
Table 1
Characteristics and initial clinical laboratory data of 181 patients with COVID-19
stratified by mortality outcome
Characteristics
Discharged alive (n = 91) (median [IQR] or n (%))
Deceased (n = 90) (median [IQR] or n (%))
p -Value
Age
62.0 [50.5, 70.0]
72.5 [63.3, 79.8]
<0.001
Sex (male)
46 (51)
60 (67)
0.03
Race
Black or African-American
36 (40)
34 (38)
0.81
White
9 (10)
11 (12)
0.81
Other/Patient declined/not reported
46 (51)
44 (49)
0.82
Ethnicity
Spanish/Hispanic/Latino
36 (40)
33 (37)
0.69
BMI (kg/m2 )
30.1 [27.2, 34.0]
28.5 [25.5, 31.8]
0.04
Elixhauser comorbidity index
4.0 [1.0, 6.3]
5.0 [2.0, 8.0]
0.14
Length of stay (days)
9.0 [5.0, 27.0]
10.5 [5.3, 16.0]
0.73
Date of sample after admission (days)
3.0 [1.0, 6.5]
3.0 [1.0, 8.0]
0.75
In vivo thrombosis or ex vivo clot
23 (25)
20 (22)
0.63
Invasive ventilator use
24 (26)
46 (51)
<0.001
Vasopressor use
21 (23)
31 (34)
0.09
Hemodialysis or CRRT use
17 (19)
23 (26)
0.27
Anticoagulation use[a ]
34 (37)
33 (37)
0.92
Steroid use[b ]
29 (32)
31 (34)
0.71
Initial clinical laboratory values measured upon admission (units) [reference range]
eGFR (mL/min/1.73 m2 ) [90–120]
74.5 [34.3, 98.8]
40.5 [25.6, 70.8]
<0.001
Creatinine (mg/dL) [0.84–1.21]
1.0 [0.7, 1.9]
1.7 [1.1, 2.5]
<0.001
Anion gap (mEq/L) [8–16]
17.0 [15.0, 20.0]
18.0 [16.0, 22.0]
0.03
Aspartate transaminase (IU/L) [8–48]
44.0 [28.3, 68.0]
51.0 [37.0, 78.8]
0.08
Hemoglobin (g/dL) [12–17.5]
12.9 [11.5, 13.8]
13.0 [10.8, 14.2]
0.63
Mean corpuscular volume (fL) [80–95]
89.9 [85.2, 92.9]
89.8 [84.2, 96.0]
0.64
White blood cell (103 /µL) [4.8–10.8]
6.8 [5.1, 10.0]
7.3 [5.5, 11.4]
0.21
Neutrophil to lymphocyte ratio [0.78–3.53]
5.9 [3.0, 9.0]
7.1 [4.6, 11.3]
0.02
Platelet (Count) (k/µL) [150–450]
213.0 [157.3, 288.0]
183.0 [136.0, 270.0]
0.14
Mean platelet volume (fL) [7–12]
10.8 [10.1, 11.8]
11.1 [10.4, 11.9]
0.18
D-dimer (µg/mL) [<0.27]
1.8 [0.7, 3.9]
2.6 [1.3, 5.7]
0.03
International normalized ratio [<1.1]
1.1 [1.0, 1.2]
1.1 [1.0, 1.3]
0.65
C-reactive protein (mg/L) [<10]
12.4 [4.9, 20.3]
15.9 [6.9, 26.1]
0.06
Lactate dehydrogenase (U/L) [140–280]
451.0 [277.0, 658.0]
542.0 [391.0, 652.0]
0.03
Troponin (ng/mL) [<0.04]
0.01 [0.01, 0.02]
0.03 [0.01, 0.06]
<0.001
Pulse oximeter (%) [95–100]
96.0 [91.0, 99.0]
94.0 [87.3, 97.8]
0.04
Diastolic blood pressure (mm Hg) [60–80]
75.0 [66.0, 85.0]
62.0 [42.8, 75.0]
<0.001
Abbreviations: BMI, body mass index; CRRT, continuous renal replacement therapies;
eGFR, estimated glomerular filtration rate; IQR, interquartile range.
a Within 48 h prior to clot or ADAMTS13 activity measurement.
b Within 24 h prior to ADAMTS13 activity measurement.
Initial Clinical Laboratory Data
Initial markers of renal function were significantly worse in nonsurvivors compared
with survivors (creatinine, 1.7 [1.1, 2.5] vs. 1.0 [0.7, 1.9] mg/dL, p < 0.001) whereas markers of liver function were not significantly different. Oxygen
saturation was lower in nonsurvivors compared with survivors (94.0 [87.3, 97.8] vs.
96.0 [91.0, 99.0]%; p = 0.04). Also, initial diastolic blood pressure was significantly lower in nonsurvivors
versus survivors (62.0 [42.8, 75.0] vs. 75.0 [66.0, 85.0] mm Hg; p < 0.001). CBC parameters were not significantly different with the exception of neutrophil
to lymphocyte ratio, (7.1 [4.6, 11.3] in nonsurvivors vs. 5.9 [3.0, 9.0] in survivors;
p = 0.02).
The only hemolysis marker that was significantly higher in nonsurvivors was LDH (542.0
[391.0, 652.0] vs. 451.0 [277.0, 658.0] U/L; p < 0.001). Initial D-dimer was significantly higher in nonsurvivors (2.6 [1.3, 5.7]
vs. 1.8 [0.7, 3.9] μg/mL; p = 0.03) ([Table 1 ]).
Correlation of VWF Antigen and Activity, ADAMTS13 Activity, and Coagulation Markers
We analyzed ADAMTS13 activity , VWF antigen and activity, FVIII activity, D-dimer,
and FM concentration, a precursor of D-dimer concentration, on the same samples irrespective
of the time since admission. Nonsurvivors had significantly lower ADAMTS13 activity
levels (48.8 [36.2, 65.1] vs. 63.6 [47.2, 78.9]%; p ≤ 0.001) and higher FM (13.2 [5.0, 129.1] vs. 5.0 [5.0, 29.40]μg/mL; p = 0.02) and D-dimer levels (4.93 [1.83, 20.00] vs. 2.90 [0.92, 14.47]ug/mL; p = 0.04) than survivors ([Table 2 ]). As expected, VWF antigen directly correlates with VWF:RCo (r = 0.58; p ≤ 0.0001) and FVIII activity (r = 0.34; p ≤ 0.001) ([Supplementary Fig. S2A, C ]). Correspondingly, VWF:RCo correlates with VWF activity collagen binding (VWF:CB)
(r = 0.77; p ≤0.0001, [Supplementary Fig. S2B ]), although VWF:CB levels were proportionally higher than VWF:RCo levels, perhaps
due to increased sensitivity of VWF:CB to HMWM.[17 ] Also as expected, ADAMTS13 activity inversely correlates with VWF:RCo (r = −0.28; p = 0.0001) and VWF:CB (r = −0.3; p = 0.009) ([Supplementary Fig. S2D ] and [E ]).
Table 2
ADAMTS13 activity levels and concurrent markers of endothelial activation, intravascular
hemolysis, coagulation, and organ damage of 181 patients with COVID-19 stratified
by mortality outcome
Clinical laboratory values (units) [reference range]
Discharged alive (n = 91[a ]) (median [IQR] or n (%))
Deceased (n = 90[a ]) (median [IQR] or n (%))
p -Value
ADAMTS13 activity (%) [70–110]
63.6 [47.2, 78.9]
48.8 [36.2, 65.1]
<0.001
Schistocyte/RBC fragment count (%)[b ] [<0.5]
0.56 [0.16, 1.12] n = 31
1.06 [0.49, 2.63] n = 42
0.008
Schistocyte/RBC fragment[b ] >1%
10 (11)
22 (24)
0.02
Lactate dehydrogenase (U/L) [140–280]
424.0 [275.0, 594.0]
562.5 [437.3, 664.8]
<0.001
Indirect bilirubin (mg/dL) [0.2–0.8]
0.2 [0.1, 0.4]
0.2 [0.1, 0.4]
0.89
Hemoglobin (g/dL) [12–17.5]
11.8 [10.1, 13.9]
11.7 [9.7, 13.7]
0.85
Platelet count (k/µL) [150–450]
241.0 [163.5, 344.5]
196.0 [124.8, 312.8]
0.06
Decreased platelet trajectory[c ]
18 (20)
27 (30)
0.11
Creatinine (mg/dL) [0.84–1.21]
1.0 [0.7, 2.3]
1.9 [1.2, 3.5]
<0.001
Increased creatinine trajectory[c ]
14 (15)
37 (41)
0.001
VWF activity (ristocetin) (%) [50–150]
282.0 [214.0, 400.0]
321.0 [238.0, 451.0]
0.05
VWF activity (collagen binding) (%) [50–150]
383.2 [235.2, 458.2] n = 35
368.7 [261.1, 585.2] n = 37
0.40
VWF antigen (%) [50–150]
362.0 [261.0, 540.0]
441.0 [307.6, 598.0]
0.05
D-dimer (µg/mL) [<0.27]
2.9 [0.9, 14.4]
4.93 [1.8, 20.0]
0.04
Fibrin monomer (µg/mL) [<10]
5.0 [5.0, 29.4]
13.2 [5.0, 129.1]
0.02
Factor VIII activity (%) [50–150]
175.0 [118.0, 247.5] n = 27
160.0 [106.5, 246.5] n = 58
0.46
HMW multimer fold change from normal[d ]
0.95 [0.71, 1.25] n = 57
1.04 [0.83, 1.46] n = 58
0.14
Fibrinogen (mg/dL) [200–400]
572.0 [462.0, 727.0] n = 59
496.0 [376.3, 744.3] n = 50
0.27
Abbreviations: IQR, interquartile range; ADAMTS13, a disintegrin and metalloproteinase
with a thrombospondin type 1 motif, member 13; VWF, Von Willebrand factor; HMW, high
molecular weight.
a Unless otherwise stated.
b Within 3 days of ADAMTS13 activity testing.
c Based on a change >10% over 1 wk period.
d Refer to [Fig. 1 ].
Thus, we analyzed the trends of ADAMTS13 activity and VWF antigen and activity levels
stratified by D-dimer. When stratified by D-dimer <2, 2–10, >10 μg/mL, ADAMTS13 activity
levels incrementally decrease with higher D-dimer ([Supplementary Fig. S2G ]). These D-dimer cut offs were based on our previous studies of D-dimer correlation
with mortality.[18 ] Likewise, VWF:RCo and VWF antigen incrementally increase based on D-dimer levels
([Supplementary Fig. S2H ] and [I ]). In addition, similar trends are seen when VWF:CB and VWF:RCo were stratified by
FM levels ([Supplementary Fig. S2K ] and [L ]).
Increased High Molecular Weight Multimers in COVID-19 Inpatients
VWF multimer analysis was performed in the first 115 samples analyzed. We observed
that many COVID-19 patients had an increased density of HMWMs compared with normal
pooled plasma ([Table 2 ], [Fig. 1A ]). Increased HMWM correlated with higher VWF:RCo (r = 0.5; p < 0.0001, [Supplementary Fig. S2F ]) and increasing D-dimer (p < 0.01, [Supplementary Fig. S2J ]). However, the relative increased HMWM was not significantly different between survivors
and nonsurvivors ([Table 2 ]). Therefore, no further VWF multimer analysis was performed in the remaining cases.
Nonetheless, serial time points in a discharged alive patient showed how HMWM changed
during hospitalization ([Fig. 1B ]).
Fig. 1 Cross sectional and longitudinal analysis of VWF multimers. For each multimer western
blot, patient plasma was run on each lane, and the loading of all samples was normalized
to measured VWF antigen levels. For each western blot, bands 1–3 were considered low
molecular weight multimers, the bands between band 4 and the last band of the abnormal
control were considered intermediate molecular weight multimers, and the bands above
the last band of the abnormal control were considered HMW multimers. (A ) The Western Blot to the left shows a pattern of VWF multimer cleavage in five patients.
Lane 1 is the negative control, which was derived from normal pooled plasma, and lane
4 is the abnormal control, which was derived from the plasma of a patient with Type
II von Willebrand disease. The abnormal control is missing HMW multimers. Lanes 2,
3, and 5 are from COVID-19 positive patients. The patient in lane 6 is from a COVID-19
negative patient with a normal multimer pattern. The COVID-19 positive patients have
increased high molecular weight multimers, except for the patient in lane 3 who was
on extracorporeal membrane oxygenation at the time of sample collection. The image
to the right of the blot is the densitometry of the lanes represented in the western
blot. The black filled in area represents the density of the normal control, and the
red line indicates the abnormal control. The other lines indicate the densitometry
of the multimers of the patients. (B ) Longitudinal trends of coagulation parameters of a discharged alive COVID-19 patient.
Western blot on the left shows the change in HMW multimer patterns throughout this
patient's hospital stay. Lane 1 is the negative control and Lane 3 is the abnormal
control. Multimer pattern between lane 1 and 2 was blocked as it was derived from
an unrelated COVID-19 patient. Lane 2 corresponds to the day of admission, Lane 4
corresponds to day 25 after admission, and Lane 5 corresponds to day 30 after admission.
The image to the right of the blot is the densitometry of the lanes represented in
the western blot. The black filled in area represents the density of the normal control,
and the red line indicates the abnormal control. The other lines indicate the densitometry
of the multimers at various timepoints of the patient. Note that by day 25, HMW multimers
decreased to normal size. COVID-19, coronavirus disease 2019; HMW, high molecular
weight.
Increased Schistocytes and LDH Are Associated with Low ADAMTS13 Activity and Higher
Mortality
Upon smear review, many RBC and platelet abnormalities were observed including fibrin
strands, platelet clumps, giant platelets, echinocytes, elliptocytes, ghost cells,
tear drops, schistocytes, RBC fragments, and RBC agglutination ([Fig. 2 ]). Importantly, schistocytes and RBC fragments along with microspherocytes were among
the most remarkable and predominant findings ([Fig. 2 ]). Increased percentage of schistocytes/RBC fragments correlated with high VWF antigen
and activity levels (r = 0.24; p = 0.04) ([Supplementary Fig. S3A–C ]). Increased percentage of schistocytes/RBC fragments also correlated with decreased
platelet count (r = −0.26; p = 0.02) and low ADAMTS13 activity (r = −0.45; p < 0.0001) ([Supplementary Fig. S3D–F ]). Increased percentage of schistocytes/RBC fragments correlated with markers of
hemolysis, such as LDH (r = 0.51; p < 0.0001) and indirect bilirubin ([Supplementary Fig. S3G–I ]). Similarly, increased LDH strongly correlated with high VWF:RCo (r = 0.25; p = 0.002) and VWF antigen (r = 0.34; p < 0.0001) levels ([Supplementary Fig. S4A,B ]). Importantly, LDH strongly correlated with indirect bilirubin (r = 0.46; p < 0.0001), supporting their use as hemolysis markers ([Supplementary Fig. S4C ]). High LDH correlated with increasing creatinine (r = 0.16; p < 0.05), and creatinine also correlated inversely with hemoglobin (r = −0.18; p = 0.02) and trended with decreasing platelet count (r = −0.14; p = 0.06) ([Supplementary Fig. S4D–F ]). The percentage of schistocytes/RBC fragments was higher in those who died than
those who survived (1.06 [0.49, 2.63] vs. 0.56 [0.16, 1.12], p = 0.008) ([Table 2 ]).
Fig. 2 Blood smear abnormalities in COVID-19. Morphology evaluation of peripheral blood
smear was performed by reviewing digital images from CellaVision. Schistocytes/RBC
fragments (red arrow ) was a predominant finding in many COVID-19 patients. Spherocytes and microspherocytes
(red S ) were very abundant, especially in smears with high number of schistocytes, thus
only one is pointed in each smear for illustration (A–F ). Ghost red blood cells (blue G ) was also a common finding. Several morphologies of ghost cells were seen: smudge
and diffuse hemoglobin staining without intact membrane (A and C ), vacuolated red blood cells with intact membrane, and vacuolated ghost without intact
membrane (B ). Polychromatophils (purple P ) were also seen (B, C, D, F ). Fibrin strands (brown F ) were seen in several patients (C ). Echinocytes, aka Burr cells (orange B ), were seen in association with renal injury (D ). Giant platelets (pink G ) were seen in many COVID-19 patients (E ). Elliptocytes (green E ) and tear drops (black T ) were seen in several smears (E ). Platelet clumps (black C ) and agglutination (blue A ) were also seen in some COVID-19 patients (F ). Scale bar = 8.5 µm. COVID-19, coronavirus disease 2019.
Thrombosis, Coagulation Activation, and VWF
We documented thrombosis, type of thrombosis, anticoagulation, and temporal relationship
of thrombosis to ADAMTS13 activity testing ([Supplementary Table S1 ]). 19% (34 of 181) of the patients developed in vivo thrombosis during hospitalization.
Patients with thrombosis exhibited significantly higher D-dimer (mean difference,
52.9; 95% confidence interval [CI] 27.3–l78.6 μg/mL), FM (517.4;168.8–865.9 μg/mL),
VWF activity (62.3; 2.9–127.4%), higher number of cases with schistocytes/RBC fragments
>1%, LDH (314.5;56.4–573.0 U/L), and creatinine (1.36;0.24–2.49 mg/dL) ([Supplementary Fig. S5 ]).
A significant number of patients did not receive anticoagulation within 48 hours prior
to clot detection (18 of 43 [42%]) ([Supplementary Table S1 ]). Although the number of patients with documented thrombosis (both in vivo and ex
vivo) was not significantly different based on ADAMTS13 activity levels, ex vivo clots,
such as clots in the HD lines, were mainly observed in patients with ADAMTS13 activity
levels lower than the normal range, which is less than 70% activity (10 patients [7.8%]
vs. 1 [1.9%]; p = 0.181). Anticoagulation did not seem to change the risk of these thromboses ([Supplementary Table S1 ]). However, VWF antigen and LDH levels were higher among the patients that received
anticoagulation (mean difference, 105.9; 95% CI 13.4–198.4%, and 239.9; 54.0–425.8
U/L, respectively) ([Supplementary Fig. S6 ]).
Initial ADAMTS13 Activity Predicts Hospital Course and Discharge Outcome
Given that ADAMTS13 activity levels seem to fluctuate during the course of hospitalization,
we studied whether the initial ADAMTS13 activity within 72 hours of admission is predictive
of mortality. A total of 102 patients had ADAMTS13 activity levels performed within
72 hours of admission ([Table 3 ]). Using Youden's J statistic, we determined that the best cut-off of initial ADAMTS13 activity to predict
mortality was 43%, p -value <0.01 ([Fig. 3A ]). As expected, the demographic and clinical characteristics were similar to the
larger original cohort ([Table 1 ]). There was no difference by age or gender, although Hispanic patients represented
49% of patients with ADAMTS13 activity levels <43% ([Table 3 ]). The logistic regression model of ADAMTS13 activity adjusted by age as a continuous
variable showed that patients presenting with lower ADAMTS13 activity levels had higher
risk of mortality ([Fig. 3B ]). Only 30% (10/33) of patients with an ADAMTS13 activity <43% within 72 hours of
admission survived compared with 60% of patients (41/69) with ADAMTS13 activity ≥43%
who survived ([Fig. 3C ]). Patients presenting with low ADAMTS13 activity (<43%) had significantly higher
VWF:RCo activity (352.00 [225.00, 490.00] vs. 258.00 [200.00, 322.00]%; p = 0.04). The number of patients that required ventilation with an initial ADAMTS13
activity <43% was more than twice that of patients with initial ADAMTS13 activity
≥43% (12/33 [36%] vs. 11/69 [16%]; p = 0.04) ([Table 3 ]).
Table 3
Clinical laboratory data within the first 72 hours of admission from patients with
COVID-19 stratified by ADAMTS13 activity level
Characteristics
Low ADAMTS13 activity (<43%)
(n = 33[a ]) (median [IQR] or n (%))
High ADAMTS13 activity (>43%)
(n = 69[a ]) (median [IQR] or n (%))
p -Value
Age
71.0 [62.0, 80.0]
68.0 [59.0, 79.0]
0.31
Sex (Male)
22 (67)
41 (59)
0.50
Race
Black or African-American
7 (21)
28 (40)
0.09
White
1 (3)
10 (15)
0.10
Other/Patient declined
25 (76)
31 (45)
0.003
Ethnicity
Spanish/Hispanic/Latino
16 (49)
23 (33)
0.21
BMI (kg/m2 )
30.9 [26.2, 32.8]
28.9 [26.9, 31.3]
0.25
Elixhauser comorbidity index
4.0 [2.0, 6.0]
4.0 [1.0, 6.0]
0.71
Mortality
23 (70)
28 (41)
0.01
Invasive ventilator use
12 (36)
11 (16)
0.04
In vivo thrombosis or ex vivo clot
5 (15)
11 (16)
>0.99
Vasopressor use
6 (18)
10 (15)
0.85
Hemodialysis or CRRT use
5 (15)
7 (10)
0.52
Anticoagulation use[b ]
8 (24)
9 (13)
0.26
Steroid use[c ]
10 (30)
19 (28)
0.96
Clinical laboratory values measured at time of ADAMTS13 activity (units) [reference
range]
ADAMTS13 activity (%) [70–110]
34.5 [26.8, 38.6]
66.2 [50.9, 79.9]
<0.001
Schistocyte count (%) [<0.5]
2.04 [1.05, 2.85] n = 13
0.48 [0.18, 0.68] n = 20
0.01
Schistocyte >1%
10 (30)
1 (1)
<0.001
Ghost cell count (%)
0.39 [0.2, 0.55] n = 13
0.27 [0.08, 2.77] n = 20
0.07
Lactate dehydrogenase (U/L) [140–280]
555.0 [417.0, 693.0]
452.0 [283.0, 625.5]
0.03
Indirect bilirubin (mg/dL) [0.2–0.8]
0.25 [0.10, 0.40]
0.25 [0.10, 0.40]
0.61
Hemoglobin (g/dL) [12–17.5]
12.9 [11.2, 14.6]
12.5 [10.4, 14.3]
0.35
Platelet count (k/µL) [150–450]
197.0 [149.0, 270.0]
211.0 [145.0, 304.0]
0.54
Platelet trajectory[d ]
Decrease
16 (49)
9 (13)
<0.001
Increase
11 (33)
37 (54)
0.06
Creatinine (mg/dL) [0.84–1.21]
1.6 [0.8, 2.4]
1.1 [0.9, 2.0]
0.40
Increased creatinine trajectory[d ]
14 (42)
16 (23)
0.08
VWF activity (Ristocetin) (%) [50–150]
352.0 [225.0, 490.0]
258.0 [200.0, 322.0]
0.04
VWF antigen (%) [50–150]
442.0 [282.0, 656.0]
346.0 [256.0, 440.0]
0.06
D-dimer (µg/mL) [<0.27]
2.6 [1.8, 13.9]
1.9 [0.8, 6.7]
0.09
Fibrin monomer (µg/mL) [<10]
5.0 [5.0, 61.8]
5.0 [5.0, 24.5]
0.29
Factor VIII activity (%) [50–150]
144.0 [112.0, 153.0] n = 11
154.0 [98.0, 188.0] n = 37
0.81
HMW multimer fold change from normal[e ]
0.90 [0.68, 1.70] n = 13
1.01 [0.87, 1.20] n = 36
0.63
Fibrinogen (mg/dL) [200–400]
510.0 [377.5, 681.0] n = 19
588.0 [458.8, 747.5] n = 48
0.43
Initial clinical laboratory values measured upon admission (units) [reference range]
eGFR (mL/min/1.73 m2 ) [90–120]
39.0 [26.0, 83.0]
56.0 [30.8, 83.4]
0.73
Aspartate Transaminase (IU/L) [8–48]
53.5 [42.5, 106.8]
44.0 [28., 69.0]
0.06
Mean platelet volume (fL) [7–12]
11.2 [10.7, 11.6]
10.7 [10.0, 11.8]
0.15
Lymphocyte (count) [1.5–4.5]
0.8 [0.6, 0.9]
1.1 [0.7, 1.5]
0.02
International normalized ratio [<1.1]
1.1 [1.0, 1.3]
1.2 [1.1, 1.3]
0.38
C-reactive protein (mg/L) [<10]
17.0 [5.3, 27.1]
13.7 [4.5, 20.4]
0.19
Troponin (ng/mL) [<0.04]
0.04 [0.01, 0.08]
0.01 [0.01, 0.04]
0.05
Pulse oximeter (%) [95–100]
92.0 [84.0, 96.0]
96.0 [91.0, 99.0]
0.03
Diastolic blood pressure (mm Hg) [60–80]
70.0 [46.0, 80.0]
72.0 [61.0, 82.0]
0.45
Abbreviations: ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin
type 1 motif, member 13; BMI, body mass index; CRRT, continuous renal replacement
therapies; eGFR, estimated glomerular filtration rate; HMW, high molecular weight;
IQR, interquartile range; VWF, Von Willebrand factor.
a Unless otherwise stated.
b Within 48 h prior to clot or ADAMTS13 activity measurement.
c Within 24 h prior to ADAMTS13 activity measurement.
d Based on a change >10% over 1 wk period.
e Refer to [Fig. 1 ].
Fig. 3 Initial ADAMTS13 activity as a predictor of mortality. (A ) Youden index measuring the optimal cut point for ADAMTS13 activity as a differentiating
marker when equal weight is given to sensitivity and specificity for the values in
the cohort. The optimal cut-point for the initial ADAMTS13 activity within 72 hours
since admission to predict mortality was found to be at 43% with an accuracy of 0.63,
sensitivity of 0.82, and AUC of 0.63. (B ) Logistic regression model of initial ADAMTS13 activity adjusted by age. Patients
that deceased (each dot at the top classified as event 1) presented with lower ADAMTS13
activity levels compared to patients that were discharged alive (each dot at the bottom
classified as event 0). The gray zone represents 95% of the confidence interval. (C ) Kaplan-Meier curve shows cumulative number of discharged COVID-19 positive patients
over time (n = 102) based on initial ADAMTS13 activity. Patients with ADAMTS13 activity ≥ 43 show
higher rate of discharge alive compared to patients with ADAMTS13 activity < 43 (log
rank, p = 0.016). C1 table shows the number of COVID-19 positive patients admitted and at
risk of mortality over time. C2 table shows the cumulative number of discharged alive
patients in each group in increments of every 10 days. Each dot represents a discharged
alive patient. ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin
type 1 motif, member 13; COVID-19, coronavirus disease 2019.
Severe thrombocytopenia at presentation was rare, with only one patient having a platelet
count of 1 k/µL and an ADAMTS13 activity level of 118%. Although admission platelet
count was not significantly different between patients with ADAMTS13 activity <43%
versus ≥43%, the trajectory (defined as a change >10% within 7 days) was significantly
different. Sixteen (49%) patients admitted with ADAMTS13 activity <43% had a decrease
in their platelets compared with nine (13%) patients with ADAMTS13 activity ≥43%;
p < 0.001 ([Table 3 ]). The majority (16/25, 64%) of patients with a negative platelet trajectory died.
D-dimer, FM, fibrinogen, and FVIII activity were not significantly different in patients
with an initial ADAMTS13 activity <43% compared with patients with an initial ADAMTS13
activity ≥43%. Although a strong correlation was observed between initial D-dimer
and FM, no correlation was observed between initial D-dimer and prothrombin time (PT),
FVIII activity, platelet count, and hemoglobin ([Supplementary Fig. S7 ]).
ADAMTS13 Activity Levels <30% Are Not Caused by Immune-Mediated Antibodies
To investigate the etiology of the decreased ADAMTS13 activity, we assessed inhibitor
status for the protease in all cases with an ADAMTS13 activity <30%, which is a routine
cut off for further work-up for antibody detection. 12% (22 of 181) of patients in
our cohort had ADAMTS13 activity levels <30% and most had mild <40% inhibition ([Table 4 ]), but when these samples were tested for specific antibodies against ADAMTS13 by
ELISA, none were found in any of these patients. Since IL-6 can inhibit ADAMTS13 activity,
we correlated ADAMTS13 activity with IL-6 levels[19 ]; however, we did not observe a linear correlation between IL-6 and degree of ADAMTS13
activity inhibition (r = 0.06). Likewise, ADAMTS13 activity level did not directly correlate with eGFR or
AST (not shown). However, more than 80% of these patients had albumin levels below
the normal reference range. Assessment for dysfunctional ADAMTS13 was unrevealing:
ADAMTS13 antigen levels were correspondingly low in the nine patients in whom it was
measured (0.1–0.4, normal range 0.6–1.6 UI/mL, data not shown).
Table 4
ADAMTS13 activity inhibitor, kidney function, liver function, and immunological analysis
of patients with very low (≤30) ADAMTS13 activity
Patient
Dispo.
ADAMTS-13 activity (%) [70–110]
% Inhibition ADAMTS-13 activity [<30%][a ]
ADAMTS-13 antibody
Clotting evidence
Schistocyte count (%) [<0.5]
Platelet (count) [150–450]
eGFR (mL/min/1.73 m2 ) [90–120]
Albumin (g/dL) [3.4–5.4]
VWF antigen (%) [50–150]
D-dimer (µg/mL) [<0.27]
IL-6 (pg/mg) [<17][b ]
Patient A
Died
2.6
36.2
Neg
No
<1
161.0
42.0
3.9
513.0
2.0
77.3 (3)
Patient B
Died
7.7
37.2
Neg
No
<1
218.0
>120
4.3
282.0
20.0
71.4 (−2)
Patient C
Died
11.9
37.1
Neg
No
<1
102.0
74.0
2.9
1020.0
0.6
Patient D
Alive
16.0
29.1
Neg
No
4.3
49.0
5.0
2.7
386.0
0.7
43.6 (0)
Patient E
Died
16.2
33.6
Neg
No
<1
267.0
43.0
3.5
442.0
105.0
12.1 (−2)
Patient F
Alive
18.0
QNS
Neg
No
1.3
238.0
76.0
2.8
648.0
16.2
47.9 (−6)
Patient G
Died
18.6
21.9
Neg
No
6.9
60.0
41.0
2.1
1325.0
64.6
7982.5 (−1)
Patient H
Died
19.7
7.0
Neg
Ex vivo clot
1.5
485.0
16.0
2.6
498.0
7.9
226.0 (2)
Patient I
Alive
22.6
4.7
Neg
Pulmonary embolism
<1
264.0
114.0
2.9
309.0
2.1
27.1 (0)
Patient J
Died
23.0
27.9
Neg
No
<1
190.0
64.0
2.5
244.0
3.7
132.3 (−17)
Patient K
Died
23.2
21.6
Neg
No
8.6
37.0
26.0
2.6
273.0
0.8
120.6 (−1)
Patient L
Alive
23.4
40.5
Neg
No
<1
452.0
101.0
2.7
264.0
5.4
Patient M
Died
24.3
25.7
Neg
No
<1
124.0
84.0
3.1
656.0
1.0
187.9 (0)
Patient N
Died
25.4
32.1
Neg
Stroke
2.9
187.0
62.0
2.5
880.0
189.0
Patient O
Died
25.6
23.1
Neg
No
<1
197.0
>120
2.8
598.0
197.6
1338.5 (0)
Patient P
Died
26.1
37.0
Neg
Arterial thrombus
<1
161.0
10.0
3.1
120.0
3.5
16.90 (0)
Patient Q
Alive
26.8
18.2
Neg
No
<1
82.0
44.0
3.4
78.0
0.8
25.50 (0)
Patient R
Died
27.6
29.0
Neg
No
2.9
498.0
14.0
2.5
514.0
20.0
283.4 (−1)
Patient S
Alive
27.7
33.0
Neg
No
<1
190.0
>120
3.3
262.0
0.5
Patient T
Died
28.8
27.3
Neg
No
<1
187.0
18.0
2.0
456.0
3.5
1826 (−12)
Patient U
Alive
29.2
22.3
Neg
No
1.6
147.0
10.0
3.1
255.0
0.7
45.7 (0)
Patient V
Died
30.8
11.8
Neg
Ex vivo clot
6.8
96.0
75.0
2.1
738.0
20.0
173.7 (−32)
Abbreviations: ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin
type 1 motif, member 13; LDH, lactate dehydrogenase; Hgb, hemoglobin; eGFR, estimated
glomerular filtration rate; AST, aspartate aminotransferase; VWF, Von Willebrand factor;
IL-6, Interleukin 6; QNS, quantity not sufficient.
a Missing values are due to insufficient quantity of sample.
b Missing values are due to the test not being ordered for the patient. Number in parenthesis
indicates the days relative to ADAMTS13 activity measurement.
Discussion
The main hypothesis of this retrospective study is that VWF biomarkers are associated
with coagulation in COVID-19. We performed a balanced retrospective study of COVID-19
hospitalized patients with similar demographics and comorbidities and a wide range
of D-dimer levels to study how VWF biomarkers correlate with coagulation, intravascular
hemolysis, and outcome. Indeed, we show a clear association of elevated VWF antigen
and activity levels with high D-dimer and FM levels. We also show that mild ADAMTS13
activity deficiency is common in COVID-19 inpatients.
Elevated VWF antigen and activity levels have been documented in COVID-19.[9 ]
[11 ]
[20 ] Likewise, inflammatory markers such as CRP and IL-6 are known to be elevated in
COVID-19. Thus, a potential explanation for elevated VWF antigen and activity levels
in COVID-19 could be that this represents an acute phase response.[21 ]
[22 ] However, the magnitude of increases in D-dimer, FM levels, and VWF antigen and activity
cannot be explained solely by acute phase response and/or inflammation. In addition,
ADAMTS13 activity is not expected to significantly decrease in acute inflammation,
yet the majority of COVID-19 patients had decreased ADAMTS13 activity, indicating
a profound endothelial dysregulation or an intrinsic ADAMTS13 activity deficiency.
Possible mechanisms of ADAMTS13 activity deficiency include decreased production,
inhibition, or consumption of ADAMTS13. 12% of patients in our cohort had ADAMTS13
activity levels less than 30% but none had detectable anti-ADAMTS13 antibodies. Many
of these patients had increased IL-6 levels, but the IL-6 level did not correlate
linearly with reduced ADAMTS13 activity, thus favoring consumption or decreased production
rather than inhibition. ADAMTS13 antigen levels were also reduced and >80% of patients
with ADAMTS13 activity levels <30% had albumin levels below normal reference range,
thus liver dysfunction may explain a low ADAMTS13 activity in these patients. In addition,
consumption of ADAMTS13 due to excess of its substrate, VWF, or excess of plasmin,
has been observed in sepsis, disseminated intravascular coagulopathies (DICs), and
thrombotic microangiopathy (TMA).[23 ]
[24 ]
[25 ]
[26 ]
[27 ] Indeed, elevated VWF antigen and activity levels, D-dimer levels, FM levels along
with moderately reduced ADAMTS13 activity levels is a repertoire of hallmarks shared
by critical illnesses that result in severe microvascular endothelial cell injuries.[28 ]
[29 ]
[30 ]
Thrombocytopenia is not common in COVID-19 and was not directly associated with low
ADAMTS13 activity levels in our cohort.[31 ] Also, lack of severe ADAMTS13 activity deficiency (only two patients had ADAMTS13
activity <10%) and lack of anti-ADAMTS13 antibodies in our patients excludes thrombotic
thrombocytopenic purpura (TTP)[32 ] and may be more suggestive of secondary TMA, sepsis, or DIC.
TMA is defined by the triad of microangiopathic anemia, thrombocytopenia, and end-organ
damage.[33 ] In our cohort we found evidence of microangiopathic anemia (schistocytes/RBC fragments)
and intravascular hemolysis (high LDH, indirect bilirubin) in the majority of patients
with low ADAMTS13 activity. The correlation of schistocytes/RBC fragments with markers
of hemolysis (LDH, indirect bilirubin), and elevated VWF antigen and activity levels
with a concomitant presence of decreased ADAMTS13 activity may indicate a TMA pattern.
Also, the correlation of high D-dimer and FM levels with LDH and the occasional finding
of fibrin strands in peripheral blood smears suggests that high D-dimer levels may
be a direct product of small vessel thrombosis (arterial and venous), which have been
documented in COVID-19 autopsies.[34 ]
[35 ]
[36 ]
[37 ]
[38 ]
[39 ] Microvascular thrombosis leads to ischemic end-organ damage, most commonly affecting
kidneys, but other organs can also be affected. COVID-19 primarily manifests as respiratory
failure, however, renal and cardiovascular complications are common in COVID-19. In
our cohort approximately 40% of patients required ventilation, 20% developed thrombosis,
and 15% required hemodialysis, of which 28% developed ex vivo clots. We observed a
trend of both in vivo and ex vivo thrombosis in cases with lower ADAMTS13 activity
but did not reach a significance of p < 0.05, probably due to the small sample size. The lack of thrombocytopenia in the
majority of the patients with low ADAMTS13 activity argues against TMA. Although thrombocytopenia
was not common in our cohort, evidence of platelet consumption in peripheral blood
smears (large immature platelets and platelet clumps) and decreasing platelet trajectories
was prevalent among patients with low ADAMTS13 activity.
High D-dimer levels, coagulation factor consumption, platelet consumption, and anemia
along with multiple organ damage are hallmarks of DIC.[40 ] However, in our cohort high D-dimer levels did not correlate with prolonged PT,
and unlike overt DIC, COVID-19 patients presented with elevated fibrinogen, FVIII
activity, and FM along with D-dimer ([Supplementary Fig. S7 ]). Furthermore, high D-dimer did not correlate with decreasing hemoglobin or platelets,
excluding the classic overt DIC. Nonetheless, nonovert DIC which has been described
in COVID-19, may explain these findings.[41 ]
Approximately 60% of patients that developed thrombosis were on anticoagulation at
least 48 hours prior to clot detection. However, patients receiving anticoagulation
had higher LDH and VWF antigen and activity levels, suggesting these patients were
sicker ([Supplementary Fig. S6 ]). Many other studies have shown that despite anticoagulation, certain COVID-19 patients
still thrombose.[42 ] Anticoagulation alone is not an effective treatment for DIC.[44 ] Nafamostat, a synthetic serine protease inhibitor, used to treat DIC and pancreatitis,
has been shown as beneficial in COVID-19 treatment in several case reports.[45 ] Additional treatment may be required to decrease high levels of VWF antigen and
activity and increase ADAMTS13 activity. A nanobody, caplacizumab, that inhibits the
binding of VWF to gp-1b on platelets has been effective in treating TMAs.[46 ] ADAMTS13 replacement via plasma exchange is a standard TTP treatment.[47 ] Although the main rationale of convalescent plasma (CP) treatment is to provide
passive immunity to acutely ill COVID-19 patients, replacement of ADAMTS13, and other
plasma proteins can possibly contribute to benefits attributed to CP.[48 ]
[49 ]
[50 ] Although we did not measure complement levels in our cohort, as serum samples were
not preserved, we noticed ghost cells in several cases, which suggest complement activation
of RBCs.[51 ] Eculizumab, a monoclonal antibody, binds C5, inhibiting the terminal complement
complex, and has been shown to be effective in treating COVID-19 in several case reports.[52 ]
[53 ]
[54 ]
An advantage of our study is that cases were selected based on a repository of frozen
plasma, and thus multiple tests with serial dilutions were performed, allowing us
to accurately correlate D-dimer concentration, FM concentration, VWF activity, VWF
antigen, VWF multimers, FVIII activity, and ADAMTS13 activity levels, all derived
from the same samples. In addition, serial dilutions of samples that reach the upper
limit of detection allow us to accurately measure the actual elevated levels of D-dimer,
FM, and VWF antigen and activity. Herein, we showed cases with unprecedented levels
of VWF antigen and activity >1,000%, FM >2,000 μg/mL, and D-dimer >300 μg/mL FEU.
In summary, we present the most comprehensive and largest study to date analyzing
correlations of D-dimer levels with VWF activity and antigen, size of VWF multimers,
ADAMTS13 activity levels, markers of intravascular hemolysis, and smear pathology
in hospitalized COVID-19 patients. A subset of COVID-19 inpatients presents a unique
microangiopathy characterized by elevated VWF antigen and activity, D-dimer, schistocytes/RBC
fragments, and evidence of macrothrombosis but also microthrombosis ([Fig. 4 ]). In particular, the markedly elevated D-dimer levels, along with mildly reduced
ADAMTS13 activity and lack of thrombocytopenia argues against TTP or TMA. In contrast
to TTP, D-dimer levels are significantly higher in DIC.[28 ] Furthermore, presence of elevated D-dimer (>4 mg/mL) is a negative predictor of
TTP in the Bentley score, the first clinical diagnostic score system for TTP.[28 ] The elevated VWF antigen and activity levels are derived from endothelial cells
that are activated and/or damaged by the SARS-CoV2 virus infection ([Fig. 4 ]). The resulting moderate decrease in ADAMTS13 activity is likely a combination of
decreased production due to liver impairment and/or consumption by excess of VWF ([Fig. 4 ]). Presence of schistocytes/RBC fragments, elevated D-dimer, and hallmarks of platelet
activation and consumption correlate with a growing collective evidence of platelet-fibrin
macro- and microthrombosis in the lungs and other organs of COVID-19 patients.[28 ]
[39 ] In our cohort we demonstrate that low ADAMTS13 activity and increased schistocytes/RBC
fragments on admission correlated with mortality. Thus, in addition to elevated D-dimer,
presence of schistocytes/RBC fragments on admission may warrant further work-up including
ADAMTS13 activity and VWF antigen and activity levels since these patients may be
at increased risk of mortality and may benefit from more aggressive therapy.
Fig. 4 Unique COVID-19 microangiopathy. (A ) Normal endothelium. (B ) SARS-CoV-2 enters the endothelial cells of capillaries via the ACE2R. Injured endothelial
cells release HMW multimers of VWF which unfold in shear forces of the microvasculature.
HMW multimers recruit platelets to the wounded endothelium. Unfolded HMW multimers
consume circulating ADAMTS13 activity, allowing for increased platelet binding to
uncleaved HMW multimers downstream. In turn, activated platelets aggregate activating
coagulation and forming microvascular thrombi. In high shear stress, schistocytes
are formed as a result of RBC shearing while forced through small vessels with thrombi.
High D-dimers result from plasmin degradation of microthrombi. ACE2R, ACE2 receptor;
ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin type 1 motif,
member 13; COVID-19, coronavirus disease 2019; SARS-CoV-2, severe acute respiratory
syndrome coronavirus 2. HMW, high molecular weight; VWF, von Willebrand factor.
Like any other retrospective studies, limitations include intrinsic confounders and
bias. Choosing samples from a limited repository bank could create bias. We tried
to compensate by randomly selecting a balanced cohort with equal distribution of survivors
and nonsurvivors and similar demographics. We could only demonstrate correlations
but no causality. Major confounders include: a wide spectrum of disease severity at
presentation, and possibly over imposed sepsis. Thus, we cannot exclude the possibility
that low ADAMTS13 activity is a simple passive biomarker and an indirect consequence
of disease severity. Therefore, prospective randomized clinical studies are needed
to determine the relationship and causality between ADAMTS13 activity, complement,
endothelial, and coagulation activation and to study the efficacy of treatments aiming
at preventing and/or ameliorating COVID-19 microangiopathy.
