Unexpected High International Normalised Ratio

Abstract

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.