Control of warfarin treatment in patients with antiphospholipid syndrome and falsely high INR at point-of-care-testing

Background: The standard of care for thrombotic antiphospholipid syndrome (APS) is anticoagulation with vitamin K antagonists (VKA). Several studies have indicated that lupus anticoagulant (LA) can interfere with the international normalized ratio (INR) results obtained by point-of-care testing (POCT) devices. A subset of patients with APS has clinically significant systematic differences between POCT INR and plasma INR (P-INR) measured in the hospital laboratory. Objectives: We aimed to investigate a potential correlation between POCT INR (CoaguChek, Roche Diagnostics) and P-INR in these patients. Materials and methods: In our anticoagulation clinic, we compared 363 paired CoaguChek-INR (CC-INR) with PINR results using Owren's method from 37 patients with APS receiving self-managed VKA. Each patient had a minimum of three paired measurements of CC-INR and P-INR and a median CC-INR/P-INR ratio > 1.20. Results: In all patients, we found a strong linear correlation between CC-INR and P-INR (median R-2: 0.89; IQR: 0.78-0.94). Each patient had their own characteristic regression line from which CC-INR could be converted to PINR. Thirty-four patients continued with self-managed treatment and the use of a conversion table. However, 3 patients had to switch to INR control in the anticoagulation clinic, as the upper limit of their therapeutic CC-INR interval exceeded the maximum (INR: 8.0) on their CoaguChek device. Conclusions: In a subset of patients with APS who, during self-managed VKA therapy, had persistently higher POCT INR than P-INR, the majority could continue with self-managed therapy using a conversion table, as the falsely elevated POCT INR values were linearly correlated to P-INR.