Abstract
High-risk patients with antiphospholipid syndrome (APS) experience increased risk of thrombosis when treated with direct oral anticoagulant (DOAC) therapy compared with warfarin. It is essential to establish the APS diagnosis to choose therapy and determine treatment duration. It requires testing for antiphospholipid antibodies, including lupus anticoagulant (LAC). In this viewpoint, we discuss the options for timing of LAC testing, which includes testing before starting anticoagulant treatment (DOAC or warfarin), after switching to heparin or after withdrawal of anticoagulant treatment. DOACs interfere with LAC testing and recommendations emerge stating not to conduct on-therapy LAC testing. All approaches are to some extent currently practised, but have limitations and the area is therefore seemingly a catch 22. We put forward that the anticoagulant effect of DOAC can be eliminated in the laboratory and therefore patients can be tested on-therapy. While it may not eliminate all cases of interference, it could aid the interpretation in these situations and this approach is attractive from the patient and clinician's perspective. Nevertheless, to prevent misdiagnosis the diagnostic workup for APS requires collaboration between the clinician and the laboratory. We advocate for standardisation in laboratory and clinical practice when diagnosing APS.