Summary

Antiphospholipid antibodies can influence the results of clottingtests in a subset of patients, which can be a major obstacle inmonitoring warfarin.The aim was to determine if point-of-caretesting of the International Normalized Ratio (INR) is influencedby antiphospholipid antibodies. We compared 59 patientsreceiving warfarin for a diagnosis of antiphospholipid antibodysyndrome (APS) to 49 patients receiving warfarin for atrial fibrillationto evaluate the consistency between INR results obtainedby different methods. INR results obtained by finger stick (capillarywhole-blood) and venipuncture (non-citrated and citratedwhole-blood) were compared with our laboratory plasmabasedprothrombin time assay. Five patients (8%) with APS andboth elevated anti-ß 2 glycoprotein I levels and positive lupus anticoagulantshad non-measurable ProTime® INR results and generallyhigher Hemochron ® Signature INR results than the plasmabased method, but the corresponding chromogenic factor X resultswere not supratherapeutic. For the remaining patients, differencesbetween the plasma-based INR and the point-of-careINR results ranged from 0.2±0.2 to 0.4±0.3. The differenceswere similar for patients with APS and atrial fibrillation for allINR comparisons with the exception of the plasma-basedmethod compared with the ProTime, which showed a meanabsolute difference of 0.4±0.3 for APS patients and of 0.2±0.2for atrial fibrillation patients (p=0.02).In a subset ofAPS patients,the ProTime® system will not yield an INR result and the HEMochronSignature (citrate and non-citrate whole-blood) INRresults will exhibit elevated INR results. For this subset of APSpatients, we suggest using an alternative method to monitorwarfarin.