EFFECT OF WARFARIN ON ACTIVATED PARTIAL THROMBOPLASTIN TIME IN PATIENTS RECEIVING HEPARIN

Background: The activated partial thromboplastin time (APTT) is used t o adjust heparin sodium dosage. However, warfarin sodium is often admi nistered concomitantly with heparin and may also affect the APTT and, therefore, heparin dose. We performed a prospective cohort study to qu antify the effect of warfarin on the APTT in patients who are being tr eated with heparin. Methods: Serial assays of APTT, international norm alized ratio, heparin levels, and functional levels of prothrombin (fa ctor II) and factors VII and X were performed in 24 patients with acut e venous thromboembolism who were treated with concomitant continuous intravenous heparin and warfarin. The effects of warfarin, as expresse d-by international normalized ratio and coagulation factor levels, on APTT were determined. Results: Warfarin markedly affected APTT; for ea ch increase of 1.0 in the international normalized ratio, the APTT inc reased 16 seconds (95% confidence interval, 10-22 seconds). The effect s of warfarin and heparin on APTT were additive. Consequently, warfari n markedly altered the relationship between APTT and heparin levels; o f the 29 blood samples with supratherapeutic APTT, 13 had a therapeuti c heparin level and 10 had a subtherapeutic heparin level. Conclusions : In patients receiving concomitant heparin and warfarin therapy, APTT reflects the combined effects of both drugs. Because of the marked ef fect of warfarin on the APTT, decreasing heparin dose in response to a high APTT frequently results in subtherapeutic heparin levels.