Effect of warfarin nonadherence on control of the International Normalized Ratio.
OBJECTIVE
The frequency and causes of ab- errant International Normalized Ratios (INRs) in warfarin recipients and the percentage explainable by warfarin nonadherence were studied.


METHODS
The medical records of patients whose warfarin therapy was monitored by a telephone-based anticoagulation service in a Midwestern urban hospital between March 2000 and March 2001 were reviewed for causes of out-of-range INRs, the percentage of out-of-range INRs attributable to warfarin nonadherence, and demographic and clinical variables predictive of nonadherence.


RESULTS
Data from 347 patients were studied. The cohort yielded 4305 INRs, of which 1002 (23%) were out of range (lower than 1.8 or higher than 3.4). Thirty-six percent of the out-of-range INRs were due to warfarin or dietary nonadherence, 9% were due to medical systems problems (including drug interactions), 18% were due to a change in clinical status, and 38% were idiopathic. Age younger than 65 years, age greater than 80 years, and living closer to the laboratory were predictive of warfarin nonadherence.


CONCLUSIONS
Warfarin nonadherence was the most common cause of explainable aberrant INRs in patients taking warfarin. Age younger than 65 years, age greater than 80 years, and living close to the laboratory were predictive of warfarin nonadherence.
Waterman
Amy D
AD
Division of General Medical Sciences, School of Medicine, Washington University, Campus Box 8005, 660 S. Euclid, St. Louis, MO 63110, USA. awaterma@im.wustl.edu
Milligan
Paul E
PE
Bayer
Laura
L
Banet
Gerald A
GA
Gatchel
Susan K
SK
Gage
Brian F
BF
D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000925:Anticoagulants / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use*; D000933:Antifibrinolytic Agents / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use; D016903:Drug Monitoring / Q000379:methods; D005260:Female; D006786:Hospitals, Urban; D006801:Humans; D019934:International Normalized Ratio; D007407:Interviews as Topic; D008297:Male; D008875:Middle Aged; D015146:Midwestern United States; D010349:Patient Compliance / Q000706:statistics & numerical data*; D010353:Patient Education as Topic; D015995:Prevalence; D016016:Proportional Hazards Models; D012646:Self Administration / Q000706:statistics & numerical data*; D014812:Vitamin K / Q000627:therapeutic use; D014859:Warfarin / Q000008:administration & dosage / Q000009:adverse effects / Q000627:therapeutic use*
D000925:Anticoagulants; D000933:Antifibrinolytic Agents; D014812:Vitamin K; D014859:Warfarin