Background: The College of American Pathologists accreditation standards require that laboratories have a procedure to detect antithrombotic drugs that inhibit coagulation in samples with positive mixing study results. With increasing use of direct Xa inhibitors (DOAC) in our patient population, our laboratory is increasingly, perhaps inadvertently, receiving mixing study samples from patients with recent DOAC use. While review of medication history can reveal recent DOAC use, sometimes, the only clue that a DOAC is present is an inhibitor pattern in the mixing study results.

Aims: We sought to determine whether an inhibitor pattern is a reliable indicator of DOAC presence.

Methods: We performed a retrospective review of mixing studies performed on patients with recent Rivaroxaban use and for which a concurrent anti-Xa Rivaroxaban level was available.

Results: Nineteen test results met the study criteria. We found that the difference between the predicted mixing study result based on factor X prothrombin time reagent sensitivity and the actual prothrombin time mixing study result significantly correlated with the Rivaroxaban level in a linear fashion (P=0.66, p=0.0076).

Conclusions: A prothrombin time mixing study inhibitor pattern indicated by the difference between predicted and actual mixing study results based on factor X reagent sensitivity can provide a clue that a DOAC may be present in a mixing study sample.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/identifying-doacs-in-mixing-study-samples/