title: Effect of warfarin on activated partial thromboplastin time in patients receiving heparin.
abstract: BACKGROUND
  The activated partial thromboplastin time (APTT) is used to adjust heparin sodium dosage. However, warfarin sodium is often administered concomitantly with heparin and may also affect the APTT and, therefore, heparin dose. We performed a prospective cohort study to quantify the effect of warfarin on the APTT in patients who are being treated with heparin.
  
  
  METHODS
  Serial assays of APTT, international normalized ratio, heparin levels, and functional levels of prothrombin (factor II) and factors VII and X were performed in 24 patients with acute venous thromboembolism who were treated with concomitant continuous intravenous heparin and warfarin. The effects of warfarin, as expressed by international normalized ratio and coagulation factor levels, on APTT were determined.
  
  
  RESULTS
  Warfarin markedly affected APTT; for each increase of 1.0 in the international normalized ratio, the APTT increased 16 seconds (95% confidence interval, 10-22 seconds). The effects of warfarin and heparin on APTT were additive. Consequently, warfarin markedly altered the relationship between APTT and heparin levels; of the 29 blood samples with supratherapeutic APTT, 13 had a therapeutic 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 effect of warfarin on the APTT, decreasing heparin dose in response to a high APTT frequently results in subtherapeutic heparin levels.
authors.affiliation: Hamilton Civic Hospitals Research Centre and McMaster University, Ontario, Canada.
authors.forename: C
authors.initials: C
authors.lastname: Kearon
authors.forename: M
authors.initials: M
authors.lastname: Johnston
authors.forename: K
authors.initials: K
authors.lastname: Moffat
authors.forename: J
authors.initials: J
authors.lastname: McGinnis
authors.forename: J S
authors.initials: JS
authors.lastname: Ginsberg
mesh_terms: D000925:Anticoagulants / Q000494:pharmacology*; D001777:Blood Coagulation / Q000187:drug effects*; D006493:Heparin / Q000494:pharmacology*; D006801:Humans; D019934:International Normalized Ratio; D010314:Partial Thromboplastin Time; D011446:Prospective Studies; D012044:Regression Analysis; D014859:Warfarin / Q000494:pharmacology*
chemical_list: D000925:Anticoagulants; D014859:Warfarin; D006493:Heparin