Warfarin monitoring and interference by lupus anticoagulant in patients with antiphospholipid syndrome

Introduction: Patients with antiphospholipid syndrome (APS) receive vitamin K antagonists, which warrants international normalized ratio (INR) monitoring. Research has indicated that presence of lupus anticoagulant (LA) can interfere with INR results obtained by point-of-care testing (POCT) devices. We aimed to investigate whether a systematic difference exists between POCT-INR and plasma-INR in patients with APS. Materials and methods: We compared 291 paired POCT-and plasma-INR results from 52 LA-positive APS patients receiving warfarin with each patient having a minimum of three paired measurements of paired POCT-INR (CoaguChek, Roche Diagnostics) and plasma-INR. Agreement limits were considered satisfactory if differences were within +/- 0.4 INR for plasma-INR < 2.0, within +/- 20% for plasma-INR 2.0 to 3.5, within +/- 20% for plasmaINR > 3.5 to 4.5, within +/- 25% for plasma-INR > 4.5 to 6.0 and within +/- 30% for plasma-INR > 6.0. Results: A strong positive correlation was found between POCT-and plasma-INR, Spearman's rho (95% CI) = 0.72 (0.65-0.78), p < 0.001. The average bias was 0.1 INR (3.7%), p < 0.001. 79% of paired INR results met the agreement limits with 67% of the diverging POCT-INRs being from a subset of five patients, who had consistently higher POCT-than plasma-INR. Conclusions: The majority of LA-positive APS patients had no clinically significant difference between POCT-INR and plasma-INR. However, in a subset of patients, clinically significant systematic differences were found. Consequently, systematic comparison of a minimum three paired POCT-and plasma-INR results is recommended before implementing POCT-INR monitoring in LA-positive APS patients.