Search for predictors of nontherapeutic INR results with warfarin therapy.
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 (< or =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 < or =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.
McGriff-Lee
Nayahmka J
NJ
Primary Care Pharmacy Practice Resident, Pharmacy Department, Clinical Center, National Institutes of Health, Bethesda, MD, USA.
Csako
Gyorgy
G
Chen
Judy T
JT
Dang
Devra K
DK
Rosenfeld
Kathryn G
KG
Cannon
Richard O
RO
Macklin
L Rose
LR
Wesley
Robert A
RA
D000328:Adult; D000368:Aged; D000925:Anticoagulants / Q000008:administration & dosage / Q000627:therapeutic use*; D001780:Blood Coagulation Tests; D003625:Data Collection; D003627:Data Interpretation, Statistical; D005260:Female; D006801:Humans; D019934:International Normalized Ratio; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011379:Prognosis; D012189:Retrospective Studies; D018570:Risk Assessment; D014859:Warfarin / Q000008:administration & dosage / Q000627:therapeutic use*
D000925:Anticoagulants; D014859:Warfarin