Abstract
Mixing tests are a relatively simple procedure used in the hemostasis laboratory as
a first-line investigation into the cause of an abnormal screening test, typically
a prolonged activated partial thromboplastin time and/or a prolonged prothrombin time.
The mixing test involves combining the test plasma with normal plasma, then repeating
the screening test on the mixture to assess whether the clotting time becomes normal
or remains prolonged. The primary purpose of a mixing test is to guide further investigations.
When mixing test results “normalize,” this suggests the test plasma is deficient in
clotting factor(s) and thus specific factor assays can be performed to determine which
are reduced. When the mixing test result does not “normalize,” this suggests the presence
of an inhibitor or other type of interference (e.g., the presence of an anticoagulant
such as high-dose heparinoids), and so the laboratory needs to determine if this is
a lupus anticoagulant or a specific coagulation factor inhibitor, or another type
of inhibitor. Because these follow-up investigations are more costly and time-consuming
than the basic screening tests, the appropriate performance and interpretation of
mixing tests is advantageous for the laboratory. Moreover, the correct laboratory
approach is also clinically relevant, as patient management is ultimately affected,
and an incorrect interpretation may lead to inappropriate therapies being established.
Components of a mixing test that can influence result interpretation include the sensitivity
of the used screening reagents to various factor deficiencies and inhibitors, the
source or composition of the normal plasma, and the setting of cutoffs for the formula
used in expressing mixing test results. Numerous and differing criteria for mixing
test interpretation have been suggested historically, which can lead to confusion
as to which approach is the most appropriate. The use of differing criteria will also
lead to differing interpretations regarding “normalization.” For this pivotal reason,
standardized mixing test procedures and a consistent set of validated interpretive
criteria represent the most favorable approach to maximizing the utility of a mixing
test, and ensure the most accurate diagnosis for investigated patients.
Keywords
PT - APTT - mixing test - factor deficiency - inhibitors - correction - non-correction
