Obstetrical hemorrhage is a potentially preventable cause of maternal morbidity and
mortality, and its incidence is steadily increasing.[1]
[2] Standardized approaches are being adopted to improve the care of these patients.[3]
[4] Since poor outcomes can result from both delayed recognition and denial of the occurrence
of significant bleeding[3] and changes in maternal vital signs or laboratory parameters often provide late
or misleading information,[5]
[6]
[7] effective measurement of ongoing blood loss is critical to early recognition.
Existing techniques for determining cumulative blood loss during cesarean procedures
include visual estimation and a gravimetric method that involves weighing of soiled
sponges and measurement of fluid in suction canisters. Since visual estimation frequently
either over or underestimates the amount of bleeding[8]
[9] and requires continual retraining and constant vigilance during surgery,[10] national organizations such as California Maternal Quality Care Collaborative (CMQCC),
the Association of Women's Health, Obstetric and Neonatal Nurses (AWHONN), and the
Council on Patient Safety in Women's Healthcare recommend weighing sponges to quantify
blood loss. While this gravimetric method focuses providers on the importance of quantitatively
assessing blood loss, it is cumbersome and has mixed data to validate its accuracy.[11]
[12]
The Triton system (Gauss Surgical, Inc., Los Altos, CA) is a novel U.S. Food and Drug
Administration-cleared mobile application on a tablet computer (iPad) that uses the
enabled tablet camera to capture images of surgical sponges. It performs colorimetric
image correction and analysis and uses cloud-based machine-learning models to quantify
hemoglobin (Hgb) mass on surgical sponges in real time. The technology can also be
used to measure the Hgb content of fluid collected in suction canisters during surgery
and is accurate despite dilution with amniotic or other fluids. The performance of
the device has been validated in bench-top and clinical settings.[13]
[14]
[15]
The objective of this study was to evaluate and compare the accuracy of visual estimation,
quantitative gravimetric and colorimetric methods in determining cumulative blood
loss during cesarean delivery procedures using a validated Hgb extraction assay method
as the reference standard.
Materials and Methods
The protocol was approved by the Santa Clara Valley Medical Center Institutional Review
Board (San Jose, CA) reference #12–003; August 12, 2013. Canister and sponge samples
from 50 consecutive patients having cesarean deliveries on weekdays between October
and December 2015 were studied, and relevant patient and procedural information were
collected and deidentified. Patients with known human immunodeficiency virus, hepatitis
B virus or hepatitis C virus, were excluded. Standard methods of care were used throughout
the procedures including fluid administration and the use and management of surgical
sponges (RFDetect L1818–04P01C-1 18”×18”, RF Surgical Systems, Inc.) and suction canisters
(Medi-Vac Guardian™ 65651–230 3000 mL, Cardinal Health, Inc.). Soiled laparotomy sponges
were individually stored in sponge counting bags, and suction canisters were affixed
with a label for recording amniotic and irrigation fluid volumes. Preprocedure and
postoperative day 1 Hgb values (g/dL) and all blood product transfusions given in
the operating room were documented. Clinicians used only visually estimated blood
loss (EBL) in making patient management decisions; they were blinded to results of
the other assays.
For each patient, the cumulative blood loss was calculated from direct extraction
assays of Hgb content on surgical sponges and in suction canisters. This result was
compared with the attending obstetrician's visual estimate of blood loss, the measured
blood loss using a quantitative gravimetric method and the blood loss determined by
the colorimetric system.
Extraction assay: The Hgb recovery process draws from previously published methodology.[16]
[17]
[18]
[19]
[20] Upon completion of each procedure, all soiled laparotomy sponges, and suction canisters
were transferred to an on-site benchtop facility for Hgb extraction. Sponges were
individually soaked in 400 mL of normal saline, compressed by hand for 60 seconds
to a mean weight of 50 g. This process was repeated four times. Hemoglobin concentration
of the final extraction fluid was measured using the plasma/low spectrophotometer
(HemoCue AB, Ängelholm, Sweden) and incorporated into the following formula to determine
the total Hgb content of the sponge:
where mfluid represents the mass of the extraction fluid, mresidual the mass of the fully extracted sponge, mdry the average dry weight of the sponge, ρ the density of the extraction fluid approximated
as 1.0 g/mL, [Hgb]fluid the Hgb concentration of extraction fluid, and %yield the yield of the manual rinse extraction method. The yield was independently characterized
by depositing banked blood on sponges in known quantities and performing the same
mechanical extraction. A linear regression analysis revealed mean mHgb recovery rates
of 89.5% (95% confidence interval [CI] = 86.8–92.1%) for individual sponges (n = 116).
Canister Hgb was determined by gently remixing the effluent, and transferring a 10
mL aliquot into a centrifuge tube. Two samples were drawn from the tube and measured
using the plasma/low spectrophotometer for canisters ranging from 0 to 2.00 g/dL,
or Hb201+ (HemoCue AB) for samples ranging from 2.0 to 25.6 g/dL, per instrument guidelines.
The canister fluid mass was measured using the digital scale, with an approximated
density conversion of 1.0 g/mL:
where meffluent represents the mass of the canister fluid, ρ its density, and [Hgb]effluent the Hgb concentration averaged over two samples.
The Hgb concentration in the canisters was separately assayed by using either a whole
blood or low-concentration Hgb analyzer and converted to a canister blood volume first
by converting the blood concentration (g/dL) of the canister to Hgb mass (g) by multiplying
by the known total fluid volume in the canister, and then by dividing this canister
Hgb mass (g) by the patient's baseline Hgb concentration (g/dL). All blood loss measurements
(mL) were calculated by dividing Hgb mass readings by the patient's baseline (preoperative)
Hgb value (g/dL). The blood loss in the canisters was then combined with the blood
loss from the sponges to give a total assayed blood loss.
Visually EBL: At the conclusion of the procedure, the attending obstetrician visually estimated
total blood loss based on examination of the surgical sponges and suction canisters,
knowledge of the procedural specifics, and his or her estimate of the amount of amniotic
fluid. The results were recorded independently, and the providers were blinded to
the results of gravimetric, colorimetric or reference assay measurements to prevent
confounding.
Quantitative blood loss (QBLGrav): Quantitative gravimetric measurement methods were adopted from published guidelines.[3] At the time of the uterine incision, the surgical technician or nurse recorded the
canister volume using the graduated markings. After aspirating all of the amniotic
fluid, a second measurement was made, and the difference was recorded as the estimated
amniotic fluid volume. At the conclusion of the procedure, the surgical technician
recorded the total amount of irrigation fluid used. Immediately following the case,
all sponges and suction canisters were individually weighed using a calibrated digital
scale (A&D Co. Ltd., Tokyo, Japan). Dry sponge weights were determined by weighing
three packs of five sponges each before the study (mean = 21 g, standard deviation
[SD] = 0.96 g) and premeasured canister weights were subtracted. To determine the
total QBLGrav estimate, all individual sponge and canister QBLGrav measurements were tallied, and the amount of amniotic and irrigation fluid used was
subtracted. The sponge fluid weight was expressed as a blood volume using a 1.0 g/mL
mean density conversion.
where mwet represents the mass of a soiled sponge, VspongeQBL the gravimetric blood volume estimate on a sponge, VcanisterQBL the gravimetric blood volume estimate in a canister, Vamniotic the amniotic fluid volume estimate, and Virrigation the measured amount of irrigation fluid.
Colorimetric: Following the case, all surgical sponges were collected and scanned using the Triton
sponge application (Version 2.0.9). This resulted in a measured amount of Hgb loss
per sponge (g) that was converted to a volumetric measure based on the patient's preprocedure
Hgb value (g/dL). Also, the surgical canisters used to collect blood and fluid from
the operative field were scanned using the Triton canister application (Version 1.0.37–61),
and the concentration of Hgb in the canisters was determined. This concentration was
multiplied by the volume of fluid in the canister, and the resultant total Hgb in
the canister was converted to a volumetric measurement of blood loss based on the
patient's preprocedure Hgb value.
Statistical Analysis
Variables are expressed in mean ± SD, median/interquartile range or count (%) as appropriate.
Kolmogorov–Smirnov test was used to evaluate whether the continuous variables followed
a normal distribution. For parameter estimates, 95% CIs are provided. Additional analyses
were performed using t-test, Mann–Whitney U test, Wilcoxon signed ranks test, and Pearson or Spearman correlations
as appropriate. Volumetric blood loss measurement using the extraction assay and the
other measurements (EBL, QBLGrav, and colorimetric) were compared using a two-sided paired t-test.
Agreement between the extraction assay and other measurements (EBL, QBLGrav, and colorimetric) was evaluated using the Bland–Altman method, an analysis framework
that has been widely established as the standard for the comparison of the clinical
differences between two different measurement methods.[21] The Bland–Altman bias (mean the difference between the two measures) and upper and
lower limits of agreement (mean ± 1.96 × SD) with their respective 95% CIs were computed.
As in previous studies, an acceptance criterion of ± 30 g of Hgb per case was set
a priori as the clinically acceptable maximum bias.[22] This difference represents approximately 5% of the total blood volume of an average
adult (Hgb content of ~250 mL [approximately 1/2 unit] of whole blood). Prior studies
with Triton[13]
[14]
[15] indicated that the SD of the Hgb mass bias was relatively low (∼10 g or less) compared
with the acceptance criterion (± 30 g), and therefore a sample size of 50 cases was
deemed adequate as it provided a 95% CI of ± 0.5 × SD (approximately ± 5 g) around
the limits of agreement.[23] This sample size would allow 90% certainty that the limits of a two-sided 95% CI
will exclude a bias of 7.25 × SD if there were truly no difference between the two
measurement methods. Statistical analyses were performed using SPSS (version 13.0,
SPSS, Inc.).
Results
Data were successfully collected from all 50 cases. Mean pre- and postoperative Hgb
levels were 12.2 ± 1.0 and 10.8 ± 1.2 g/dL, respectively (p < 0.001 for the paired comparison of pre- and postoperative Hgb levels). One patient
received a single unit packed red cell transfusion intraoperatively.
The mean patient age was 31.9 years (range = 19–44 years). Overall, 44 mothers were
multiparas. All babies were singleton. A total of 41 procedures were elective. The
indications for delivery included 32 elective repeat deliveries, eight breech presentations,
and the remainder for a variety of other reasons. Six mothers labored before delivery
although all had intact membranes. Gestational ages ranged from 31 to 41 weeks and
one day with 38 being 39 weeks or greater.
The mean amniotic fluid volume as recorded intraoperatively by marking the suction
canister for volumetric assessment was 632 ± 507 mL (median = 500 mL, [Fig. 1]) and the mean measured irrigation fluid volume was 759 ± 437 mL. As measured by
the assay the mean amount of blood contained in a laparotomy sponge was 24.5 ± 20.3
mL, and the average canister contained 236 ± 137 mL of blood ([Fig. 2]). An average of 15 sponges and one canister were used for each procedure. The mean
blood loss per procedure as measured by the assay was 470 ± 296 mL (range = 113–1,614
mL). In four cases, the total blood loss exceeded 1,000 mL ([Fig. 3]).
Fig. 1 Distribution of amniotic fluid volume recorded by marking the suction canister for
volumetric assessment.
Fig. 2 (A) Distribution of sponge blood content as measured by the assay method. (B) Distribution of canister blood content as measured by the assay method.
Fig. 3 Distribution of blood loss as determined by the assay method.
The visual, gravimetric, and colorimetric methods of estimating blood loss all demonstrated
positive bias (mean difference between two methods of measurement) about the extraction
assay, at 458, 352, and 102 mL, respectively. Both the visual and gravimetric methods
systematically overestimated blood loss more than the clinical tolerance of 1/2 a
unit of whole blood, or 250 mL ([Table 1]).
Table 1
Blood loss determinations
|
Method
|
Extraction Assay
|
Visual EBL
|
QBLGrav (adjusted)[a]
|
Colorimetric
|
|
Sponge (mL)
|
Mean ± SD
|
280 ± 222
|
|
759 ± 317
|
332 ± 255
|
|
Median (IQR)
|
203 (138)
|
|
695 (426)
|
251 (181)
|
|
Bias (95% CI)
|
|
|
480 (428–531)
|
52 (32–71)
|
|
p Value
|
|
|
< 0.001
|
< 0.001
|
|
Canister (mL)
|
Mean ± SD
|
190 ± 133
|
|
63 ± 335
|
240 ± 137
|
|
Median (IQR)
|
142 (173)
|
|
−13 (285)
|
199 (202)
|
|
Bias (95% CI)
|
|
|
−127 (−228 to −26)
|
50 (26–74)
|
|
p Value
|
|
|
0.014
|
< 0.001
|
|
Total blood loss per procedure (mL)
|
Mean ± SD
|
470 ± 296
|
928 ± 261
|
822 ± 489
|
572 ± 334
|
|
Median (IQR)
|
384 (296)
|
800 (200)
|
651 (475)
|
481 (332)
|
|
Bias (95% CI)
|
|
458 (396–520)
|
352 (237–467)
|
102 (72–132)
|
|
p Value
|
|
< 0.001
|
< 0.001
|
< 0.001
|
Abbreviations: CI, confidence interval; EBL, estimated blood loss; IQR, interquartile
range; QBLGrav, quantitative blood loss; SD, standard deviation.
a Adjusted by subtracting the measured amniotic fluid volume and irrigation volume.
Note: p Values reflect the statistical significance level of paired t-tests comparing each method with the extraction assay (reference standard).
The gravimetric method was evaluated to understand the source of its inaccuracy better.
Of the 757 sponges measured, QBLGrav exceeded the blood content determined by the assay method in all but 8. The QBLGrav measurements exhibited poor correlation (r
2 = 0.2682) with the assay and systematically overestimated sponge blood content ([Fig. 4]) most likely due to the addition of absorbed amniotic and irrigation fluid to the
blood collected on the sponges. This inaccuracy persisted despite the corrections
for amniotic and irrigation fluid that were made to the total blood loss measures.
Fig. 4 Scatter plot of the blood content of each sponge comparing the assay and gravimetric
methods.
The relationship between blood losses measured by the various methods versus the extraction
assay method is described in [Table 2] and illustrated by scatter plots in [Fig. 5]. Assessment of agreement between the various measurements and the extraction assay
method according to Bland–Altman method is provided in [Table 3] and [Figs. 6]
[7]
[8].
Table 2
Linear correlation of blood loss measurements versus extraction assay (reference standard)
|
Method
|
Correlation coefficient (95% CI)
|
Standardized coefficient
|
p Value
|
Adjusted R2
|
|
Visual EBL
|
0.700 (0.523–0.819)
|
0.700
|
< 0.001
|
0.479
|
|
QBLGrav (adjusted)
|
0.564 (0.339–0.728)
|
0.564
|
< 0.001
|
0.304
|
|
Colorimetric
|
0.951 (0.915–0.972)
|
0.951
|
< 0.001
|
0.902
|
Abbreviations: CI, confidence interval; EBL, estimated blood loss; QBLGrav, quantitative blood loss.
Fig. 5 Scatter plots of blood loss measured by (A) visual estimation, (B) gravimetric method, and (C) colorimetric method compared with the assay method. A line of unity representing
perfect correlation is shown for comparison.
Table 3
Assessment of agreement between methods of measuring blood loss and the extraction
assay (reference standard)
|
Visual EBL (mL)
|
QBLGrav (adjusted) (mL)
|
Colorimetric (mL)
|
|
Bias (95% CI) (mL)
|
458
|
352
|
102
|
|
SD (error) (mL)
|
218
|
405
|
106
|
|
Upper limit of agreement (95% CI) (mL)
|
886
|
1,145
|
309
|
|
Lower limit of agreement (95% CI) (mL)
|
31
|
−441
|
−105
|
|
RMSE (mL)
|
507
|
533
|
146
|
|
CI (bias)
|
62
|
115
|
30
|
|
CI (LOAs)—calculated as 
|
105
|
194
|
51
|
Abbreviations: CI, confidence interval; EBL, estimated blood loss; LOA, limits of
agreement; QBLGrav, quantitative blood loss; RMSE, root mean square error; SD, standard deviation.
Fig. 6 Bland–Altman plot: Visual method.
Fig. 7 Bland–Altman plot: Gravimetric method.
Fig. 8 Bland–Altman plot: Colorimetric method.
Comment
This study demonstrates that when using an extraction assay method as a reference
standard, cumulative blood loss is more accurately assessed by the colorimetric method
than by either visual estimation or the widely recommended quantitative gravimetric
method. Visual estimation demonstrated a slightly better correlation to the assay
than gravimetric estimation because, despite adjustments, the quantitative gravimetric
method tended to overestimate blood loss consistently. This was presumably due to
the effects of amniotic fluid and irrigants that inevitably mix with the blood collected
on surgical sponges and within suction canisters confounding the results of weighing
the sponges and measuring the volume in the canisters.
The regression curve for visual estimation demonstrates a slope well below 1.0 suggesting
that the providers' estimates of the amount of blood loss were virtually unrelated
to the reference values ([Fig. 5A]). In fact, if a few of the high estimates were removed the slope would have approached
zero. Clinicians never estimated a blood loss of less than 500 mL, and yet the reference
data demonstrates that 66% of patients had such values. These data confirm that surgeons
and other medical personnel are inaccurate in visually estimating blood loss.[9]
Historical data supporting the quantitative gravimetric method of measuring blood
loss is mixed. For example, Lilley et al,[12] concluded that in a mixed group of vaginal and cesarean deliveries gravimetric assessment
of blood loss during postpartum hemorrhage (PPH) was effective, while Johar et al,[11] determined that in surgical procedures the technique was frequently inaccurate due
to issues such as recording bias, amniotic fluid/saline corruption, and human error.
Data from this study illustrate the persistent challenges posed by this methodology
in cesarean deliveries where blood and nonsanguineous fluids frequently mix. Specifically,
the quantitative gravimetric technique showed a lower correlation with actual blood
loss than the colorimetric method as evidenced by the lower R-value. This variation
includes several cases where there was a significant deviation, both above and below
the actual blood loss. In 34% of the cases the quantitative gravimetric method overestimated
the blood loss by greater than 500 mL when compared with the reference standard (mean = 761
± 370 mL for those cases) and in two cases the quantitative gravimetric method underestimated
the blood loss by more than 500 mL. However, unlike visual estimation, gravimetric
methods did effectively quantify blood losses of less than 500 mL in many patients.
The cases where the gravimetric estimate of blood loss was greater than the blood
loss calculated from the Hgb loss determined by the extraction assay method can be
plausibly explained by visual underestimation of the amount of irrigation and amniotic
fluid in the canister, lower preoperative Hgb concentration (since the weight of additional
plasma is included) or significant amounts of irrigation and/or amniotic fluid on
the sponges. Underestimation by the quantitative gravimetric method likely resulted
from the reverse of these conditions.
Accurate blood loss estimation is clinically valuable and may substantially alter
the timing of interventions to control hemorrhage. Overestimation during cesarean
delivery may lead patients, particularly those who have minimal postpartum blood loss
following the procedure, to have unnecessary laboratory evaluation and exposure to
unneeded medications and/or transfusions. Conversely, underestimation may lead to
a delay in evaluation and treatment, particularly if further blood loss occurs postpartum.
This risk may be exacerbated by the fact that patients with presumed low blood loss
may be placed in care environments with the lower nurse to patient ratios and less
intensive monitoring. Furthermore, patients with underestimated blood loss may not
receive appropriate blood, or blood component therapy is potentially leading to excessive
hemorrhage from dilutional anemia and/or coagulopathy.
A limitation of this study is that it investigated a patient population having surgical
blood losses mostly within the normal range. The population studied did not have a
substantial number of patients with excessive blood loss, and therefore comparisons
between the various methods could not be made for that situation. Nonetheless, the
colorimetric method is likely to be accurate in patients experiencing massive hemorrhage
since the study validates the comparative accuracy of colorimetry in measuring blood
loss on individual sponges and in each canister. In cases with increased hemorrhage,
one would expect that there would simply be more sponges and larger volumes in the
canisters. In contrast, both visual estimation and the quantitative gravimetric method
would be prone to greater variation with increased blood loss. A strength of this
study is that a rigorous and detailed evaluation of all three methods was conducted
and compared the results to a validated reference standard. Although all sponge/canister
image capture and analysis in this study was done at the conclusion of surgery, the
use of this tool has previously been effectively implemented “real-time” during surgical
procedures, thus providing continuous and ongoing monitoring of blood loss.[15]
The blood losses measured in this study were typically less than that commonly estimated
for cesarean delivery. This may be due to the failure of the extraction assay, colorimetric
and gravimetric methods to account for blood loss on surgical drapes. Alternatively,
the data could be interpreted as demonstrating that those traditional estimates are
often incorrect. Further studies are needed to determine whether data using the colorimetric
method is sufficiently accurate to predict postoperative Hgb levels and guide therapy.
This study demonstrates that both visual and quantitative gravimetric methods of measuring
blood loss during cesarean deliveries are unreliable and colorimetric image analysis
using a computer-based algorithmic system provides more accurate results. Accurate,
real-time measurement of blood loss has the potential to facilitate proper implementation
of obstetric hemorrhage protocols to improve patient care. Further study of these
methods and workflows, particularly in patients with larger amounts of perioperative
bleeding, is warranted.
