To establish a method to determine warfarin concentration in plasma by ultra performance liquid chromatography (UPLC), and to explore the relationship between warfarin plasma concentration and international normalized ratio (INR) in patients after cardiac valve replacement, so as to seek more safe and reliable monitoring indicators and guide rational use of anticoagulant therapy.

An ultra performance liquid chromatography method has been developed for measuring warfarin concentration in plasma by using 6-methoxy naphthaleneacetic acid (6-MNA) as internal standard (IS). At the some time, the correlation of INR, warfarin dosage and plasma warfarin concentration in 79 samples was analyzed.

The retention time of warfarin and the IS were 2.2 and 1.1 min, respectively. The average extraction recovery of plasma warfarin was 96.0%, The concentration of warfarin was 15.6~4000 ng/ml with good linearity (R²=0.9996). The overall accuracy of the method was 98.5%~99.9% and the lowest detection limit was 5.0 ng/ml. The intra-and inter-day variations were less than 1.34% and 2.69%, respectively. The correlation between INR and warfarin dose was r²=0.006 (P=0.481), INR and plasma warfarin concentration was r²=0.006 (P=0.497), and the correlation between warfarin dose and plasma concentration was r²=0.298 (P=0.001) in 79 cases of samples.

A simple, rapid, accurate and sensitive UPLC method for quantifying warfarin levels in human plasma was validated. The determination of warfarin blood concentration has guiding significance for anticoagulant therapy of patients after heart valve replacement.