Reagent-Dependent Perioperative Activated Partial Thromboplastin Time Prolongation Leading to Diagnosis of Antiphospholipid Syndrome: A Case Report

Activated partial thromboplastin time (APTT) prolongation in antiphospholipid syndrome (APS) varies according to reagent sensitivity to lupus anticoagulant (LA). Additionally, direct oral anticoagulant (DOAC) therapy can interfere with LA testing, requiring careful consideration of timing. We report a surgical case in which preoperative APTT, despite DOAC cessation, became markedly prolonged after the APTT reagent was replaced with one with higher LA sensitivity, prompting suspicion of APS. Postoperatively, LA positivity was confirmed during interruption of DOAC therapy. This case highlights reagent-dependent variability in APTT and suggests that perioperative DOAC interruption may provide an opportunity for accurate laboratory assessment of APS.