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An International Study on the Interpretation of Test Results of the APTT Mixing Test

1ECAT Foundation, Voorschoten, the Netherlands, 2University Hospital Son Espases, Dept. of Laboratory Medicine, Palma de Mallorca, Spain, 3Sz-Sz-B County's Teaching Hospital, Central Laboratory, Nyíregyháza, Hungary, 4Halland Hospital, Dept. of Clinical Chemistry and Transfusion Medicine, Halmstad, Sweden, 5Haukeland University Hospital, Lab. of Clinical Biochemistry, Bergen, Norway, 6Norwegian Quaility Improvement of Primary Care Lab. (Noklus), Bergen, Norway

Abstract Number: PB0582

Meeting: ISTH 2020 Congress

Theme: Diagnostics and OMICs » Laboratory Diagnostics

Background: The APTT mixing test is a useful tool to gain insight in the potential cause of a prolonged APTT. However, it has been demonstrated that, using case scenarios, it can be problematic to correctly interpret APTT mixing test results (Ajzner et al, Clin Chem Lab Med, 2015: 1593-603). Therefore a follow-up study was conducted to investigate the interpretation of APTT mixing test results on the basis of real test results.

Aims: A follow-up study was conducted to investigate the interpretation of APTT mixing test results on the basis of real test results.

Methods: Two lyophilized citrated plasma samples together with case histories were distributed to 340 European laboratories. Laboratories were asked to perform an APTT and mixing test according to their own in-house procedures. The participants were also asked to interpret the obtained test results in the perspective of the provided case history and classify the sample according to the following categories: normal, coagulation factor deficiency, presence of an inhibitor, presence of anticoagulant or unknown. Multiple selections were allowed.

Results: Results of 270 laboratories were evaluated. For case 1 (sample contaminated with heparin), 100% found a prolonged APTT and about 90% a prolonged initial and incubated mixing test result. For case 2 (factor VIII deficiency) almost all participants reported a prolonged APTT result and 78% found a correction to normal in the mixing test. The results of the classification of both samples is given in the table below.

Conclusions: This study demonstrates again that for particular scenarios the interpretation of APTT mixing test results can be problematic.

Classification Case 1 (heperin contamination) Case 2 (factor VIII deficiency)
Normal 0.0% 1.%
Coagulation Factor Deficiency 4.5% 85.5%
Inhibitor (specific or non-specific) 35.1% 3.4%
Presence of anticoagulant 14.9% 0.0%
Normal / Coagulation Factor Deficiency 0.0% 0.4%
Factor Deficiency / Inhibitor 4.1% 1.9%
Factor Deficiency / Anticoagulant 3.0% 3.8%
Inhibitor / Anticoagulant 33.2% 0.0%
Factor Deficiency / Inhibitor / Anticoagulant 3.4% 2.3%

[The results of the classification of case 1 and 2]

To cite this abstract in AMA style:

Meijer P, Bauça JM, Ajzner É, Hillarp A, Kristoffersen AH, Working Group on the Postanalytical Phase (EFLM / EQALM) . An International Study on the Interpretation of Test Results of the APTT Mixing Test [abstract]. Res Pract Thromb Haemost. 2020; 4 (Suppl 1). https://abstracts.isth.org/abstract/an-international-study-on-the-interpretation-of-test-results-of-the-aptt-mixing-test/. Accessed May 15, 2021.

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