Study on warfarin plasma concentration and its correlation with international normalized ratio.

A sensitive high-performance liquid chromatographic (HPLC) method was developed for warfarin determination in plasma of patients who undertook cardiac valve replacement and were on anticoagulation with warfarin. The method described proved to be accurate, sensitive, easy to perform, reproducible and specific for plasma warfarin measurement with relative standard deviation (R.S.D.) of <5.27% for inter-day and <6.89% for intra-day. The assay was linear in warfarin concentration ranges of 0.12-3 microg/ml (r=0.9995) with mean recovery of 94.6%. The mean warfarin plasma concentration of 58 patients with heart valve replacement within 1 month of post operation was 567.6+/-122.3 ng/ml. The anticoagulant effect of the drug was monitored by international normalized ratio (INR). The correlation of warfarin dosage and concentration with INR was analysed, and the coefficients were 0.21, 0.1<p<0.2 and 0.30, 0.02<p<0.1, respectively. The correlation of warfarin dosage or concentration with INR is very poor, and hence in order to adjust the dosage more objectively and accurately, concentration monitoring is necessary and helpful for the patient management. It is needed especially when the ideal INR is difficult to target.