title: Variations in prothrombin time and international normalized ratio over 24 hours in warfarin-treated patients.
abstract: OBJECTIVE
  To determine the variation of prothrombin times and international normalized ratio (INR) over 24 hours in humans.
  
  
  METHODS
  Prospective, parallel study.
  
  
  METHODS
  University-affiliated general clinical research center.
  
  
  METHODS
  Six patients receiving long-term warfarin therapy and six sex-matched controls.
  
  
  METHODS
  Warfarin was administered to the patients at 6:00 P.M.
  
  
  RESULTS
  Prothrombin times and INR were determined every 2 hours over 24 hours. Time of study entry, meals, and sleep cycles were controlled. A significant cosinor rhythm for prothrombin times and INR (p < or = 0.03) occurred in warfarin-treated patients, suggesting that diurnal variation occurs. The mean difference between the peak and trough prothrombin times was 1.8 +/- 0.9 seconds (range 0.8-3 sec) with a mean change of 9.3% +/- 3.7%. The peak prothrombin time and INR values occurred between 4:00 A.M. and 8:00 A.M. in five patients, and trough values between 6:00 P.M. and midnight in five. No significant cosinor rhythm was noted for controls (p > 0.5).
  
  
  CONCLUSIONS
  Significant variations in prothrombin time and INR occurred in patients receiving warfarin therapy, with the highest values occurring in the morning and the lowest in the evening. These results may have clinical implications for patients receiving either high- or low-intensity warfarin therapy.
authors.affiliation: University of Michigan, College of Pharmacy, Ann Arbor 48109-1065, USA.
authors.forename: B E
authors.initials: BE
authors.lastname: Bleske
authors.forename: L S
authors.initials: LS
authors.lastname: Welage
authors.forename: E W
authors.initials: EW
authors.lastname: Warren
authors.forename: M B
authors.initials: MB
authors.lastname: Brown
authors.forename: M J
authors.initials: MJ
authors.lastname: Shea
mesh_terms: D000328:Adult; D000368:Aged; D000925:Anticoagulants / Q000494:pharmacology* / Q000627:therapeutic use; D002940:Circadian Rhythm / Q000187:drug effects*; D005260:Female; D006785:Hospitals, University; D006801:Humans; D008297:Male; D008875:Middle Aged; D010349:Patient Compliance; D011446:Prospective Studies; D011517:Prothrombin Time; D013997:Time Factors; D014859:Warfarin / Q000494:pharmacology* / Q000627:therapeutic use
chemical_list: D000925:Anticoagulants; D014859:Warfarin