Unexpected high international normalised ratio

45-year-old man who was suspected to have the antiphospholipid syndrome, presented with persistently high international normalised ratio despite not being on any oral anticoagulant. He was previously put on Rivaroxaban at another institution after multiple episodes of thromboembolic events, but was stopped prior to his admission at our clinic. No bleeding episodes were recorded. Subsequent measurements of prothrombin time (PT) were performed using Sysmex CS-2500, Sysmex CA-104 and Instrumentation Laboratory (IL) ACL Top analysers. PT results by both Sysmex analysers were prolonged at 34s and 30s, respectively. Conversely, the PT result using IL ACL Top analyser was within normal range. Both Sysmex analysers used a PT reagent, which incorporated a recombinant thromboplastin. When PT was repeated using a tissue derived PT reagent, PT result was within the reference range. The patient was later found to have antiphospholipid antibodies with triple positivity.