Explaining the Differences Between Capillary INR and Plasma INR in Patients With Antiphospholipid Syndrome

Antiphospholipid syndrome (APS) is an autoimmune and prothrombotic disorder that can affect up to 10% of young people experiencing a thrombotic event. Its treatment relies on long-term anticoagulation with vitamin K antagonists (VKAs). Direct oral anticoagulants, which are simpler to use because they do not require regular blood monitoring, are contraindicated because they are associated with an increased risk of thrombotic recurrence in some patients with APS.

Patients with APS receive VKAs and must regularly have their Index Normalized Ratio (INR) measured via a cumbersome venous blood draw. Capillary INR measurement systems are already used in certain situations, such as in patients with mechanical heart valves. The use of these systems improves the quality of life of these patients and, above all, the stability of VKA therapy, thus preventing potentially serious hemorrhagic complications or thrombotic recurrences.

In antiphospholipid syndrome (APS), these systems are discouraged due to perceived differences between capillary and venous INR (the reference method). However, among the few studies on the subject, none demonstrated significant discrepancies between patients with APS and controls, and when such discrepancies were observed, the origin of this variability could not be determined. We hypothesize that the biological profile of antiphospholipid antibodies is responsible for the INR differences.