Anti-Xa and Anti-IIa Drugs Alter International Normalized Ratio Measurements: Potential Problems in the Monitoring of Oral Anticoagulants

Several of the newly developed anti-Xa and anti-IIa agents have been shown to influence the International Normalized Ratio (INR) values. During phase I trials with normal healthy volunters and phase II study patients who were given warfarin and concomitant anti-IIa or anti-Xa agents, it has been reported that INR values were falsely elevated. It is of critical importance to know of the effects of these agents on INR to avoid dosage errors. To study the influence of these agents on INR, we used several anti-IIa agents (argatroban, recombinant hirudin, efegatran, and PEG-hirudin) and anti-Xa drugs (pentasaccharides such as fondaparinux and idraparinux, DX-9065a and JTV-803). The anti-IIa drugs were supplemented in citrated plasma at a concentration of 0 to 1 μg/mL level and anti-Xa drugs in the range of 0 to 25 μg/mL. The IC50 values for each of these agents were calculated. Four different commercially available prothrombin time (PT) reagents were used to perform the PT assays and to calculate the relative INR values. Direct synthetic factor IIa and Xa inhibitors exhibited a concentration-dependent increase in the INR values. Hirudin, efegatran, and PEG-hirudin showed a weaker effect, whereas argatroban showed a much higher elevation of the INR values. Synthetic indirect anti-Xa agents such as the pentasaccharide did not show any effect on the INR values. Furthermore, prothrombin time reagents with high ISI values exhibited disproportionally higher INR values for both the direct anti-Xa and anti-IIa agents. Elevation of INR values has therapeutic implications when non-oral anticoagulant drugs are used in combination with drugs such as warfarin. Because of the false elevation of INR values with some of the non-oral anticoagulant drugs, patients who are on concomitant warfarin therapy should be carefully evaluated for their corresponding INR values for proper dosing. To avoid dosing errors it is best not to use the INR values in the therapeutic monitoring of anti-Xa and anti-IIa agents either in the monotherapeutic or polytherapeutic modalities. These data also warrant the development clinically relevant methods for the monitoring of the concomitant use of newly developed anti-Xa and anti-IIa drugs with oral anticoagulants.