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1 at time of writing, Primary Care Pharmacy Practice Resident, Pharmacy
Department, Clinical Center, National Institutes of Health, Bethesda, MD; now,
Medical Information Scientist, GlaxoSmithKline, Philadelphia, PA
2 Senior Scientist and Assistant Chief of Clinical Chemistry, Department of
Laboratory Medicine, Clinical Center, National Institutes of Health
3 at time of writing, Primary Care Pharmacy Practice Resident, Pharmacy
Department, Clinical Center, National Institutes of Health; now, Assistant
Professor of Pharmacy Practice, School of Pharmacy, Purdue University, West
Lafayette, IN
4 at time of writing, Primary Care Pharmacy Practice Resident, Pharmacy
Department, Clinical Center, National Institutes of Health; now, Assistant
Clinical Professor, School of Pharmacy, University of Connecticut, Storrs,
CT
5 at time of writing, Hematology Service, Department of Laboratory Medicine,
Clinical Center, National Institutes of Health
6 Chair, Medical Executive Committee, and Clinical Director, National Heart,
Lung and Blood Institute, National Institutes of Health
7 at time of writing, Primary Care Pharmacy Practice Resident, Pharmacy
Department, Clinical Center, National Institutes of Health; now, Clinical
Associate Professor, Pharmacy Practice, School of Pharmacy and Allied Health
Sciences, University of Montana, and Home Health/Hospice Pharmaceutical Care
Coordinator, Partners in Homecare, Missoula, MT
8 Chief, Clinical Epidemiology and Biostatistics Service, Office of the
Director, Clinical Center, National Institutes of Health
Reprints: Dr. Chen, Department of Pharmacy Practice, Purdue University, R. Heine Pharmacy Bldg., Rm. 502D, 575 Stadium Mall Dr., West Lafayette, IN 47907-2091, fax 765/496-1886, jtchen{at}pharmacy.purdue.edu
BACKGROUND: The effectiveness and safety of warfarin require maintaining an international normalized ratio (INR) within the therapeutic range.
OBJECTIVE: To identify predictors of nontherapeutic INR results in patients receiving warfarin.
METHODS: A retrospective study was conducted using 350 ambulatory care patients from a broad geographic region, all receiving long-term warfarin therapy and followed in a tertiary-care cardiology clinic. Possible predictors of nontherapeutic INR results (gender, age, body weight, body mass index, height, race, tobacco use, alcohol use, warfarin dose, therapeutic indication, regimen intensity, INR monitoring frequency/category, interacting medications, adverse events) were assessed with logistic regression models. Subset analysis involved 146 patients concurrently monitored with capillary whole blood INR (CoaguChek).
RESULTS: As measured on venous specimens, 52% (182/350) of the
patients had subtherapeutic INR results and 13% (44/350) had supratherapeutic
INR results despite frequent (
4 wk) monitoring in 75% of the patients. Due
to the small sample size, supratherapeutic INR results could not be further
analyzed. Of 19 predictors tested, only daily warfarin dose (p < 0.02) and
regimen intensity (p < 0.03) were significant independent and additive
predictors of subtherapeutic results. Patients on the high-intensity regimen
(INR 2.5-3.5) and receiving warfarin 6 mg/day had >50% risk of having
subtherapeutic INR results. Subtherapeutic CoaguChek results were independent
predictors of subtherapeutic venipuncture INR results in the subset (p =
0.001).
CONCLUSIONS: In the absence of readily identifiable predictors, only higher warfarin dosing and/or more frequent monitoring (possibly with point-of-care/home monitoring devices) may minimize the time that INRs are subtherapeutic, especially in patients receiving low-dose and/or high-intensity anticoagulation therapy.
Key Words: warfarin monitoring, predictors, atrial fibrillation, mechanical heart valve replacement
Published Online, November 15, 2005. www.theannals.com, DOI 10.1345/aph.1E381
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