Background: Recommended therapeutic international normalized ratios (INRs) for oral anticoagulation in patients with lupus anticoagulants who sustain a thromboembolic event are controversial. Patients with lupus anticoagulants often have a prolonged prothrombin time, which may complicate management of anticoagulant therapy.
Objectives: To determine the validity of the INR as a monitor for warfarin therapy in patients with lupus anticoagulants and to investigate alternate approaches to monitoring warfarin therapy in these patients.
Design: Prospective case series.
Setting: Tertiary care hospital.
Patients: 34 patients with lupus anticoagulants.
Measurements: Prothrombin times were determined by using several thromboplastins, and INRs were calculated for the patients receiving warfarin. Factor II levels, chromogenic factor X levels, and prothrombin-proconvertin times were determined for patients receiving warfarin.
Results: For patients with lupus anticoagulants who were not receiving warfarin, prothrombin times were often elevated and varied significantly with different thromboplastins. Individual thromboplastins differed in sensitivity to the presence of a lupus anticoagulant. For patients receiving warfarin, INRs obtained by using different thromboplastins greatly varied and often overestimated the extent of anticoagulation. Chromogenic factor X levels and prothrombin-proconvertin times correlated well with each other and with established therapeutic ranges.
Conclusions: Lupus anticoagulants can influence prothrombin times and lead to INRs that do not accurately reflect the true level of anticoagulation. Use of the INR to standardize prothrombin times is invalid for some patients with lupus anticoagulants. To prevent supratherapeutic or subtherapeutic anticoagulation, these patients must be individually monitored with a test that is insensitive to lupus anticoagulants.