ABSTRACT
In a prospective case control study, 289 outpatients (mean age 70 years) on warfarin therapy for at least one month (target INR of 2.0 to 3.0) were followed within 24 hours of the reported INR. Case patients were identified as those with INRs greater than 6.0 (n = 93) and controls were randomly selected from patients with INRs between 1.7 and 3.3 (n = 196). Although mean INRs were higher in the case group (8.3, range: 6.1 to 29.8) when compared to the control group (2.4, range: 1.7 to 3.3), the average weekly warfarin dose was similar between the two groups (28.9 mg vs 26.7 mg). Case patients were more likely to ingest acetaminophen (56 vs 36 percent) and in greater amounts on a weekly basis (mean doses 6756 mg vs 2938 mg). The odds ratios for elevated INRs (6.0) were increased with advanced malignancy (16.4), newly started medications (8.5), warfarin dose more than prescribed (8.1), decreased oral intake (3.6), acute diarrheal illness (3.5) and acetaminophen intake between 2275 mg to 4549 mg weekly (3.5). The risk for elevated INRs with concurrent acetaminophen therapy was dose dependent. The odds of having an INR greater than 6.0 were increased seven-fold when weekly acetaminophen intake was between 4550 to 9099 mg, but was increased ten-fold when weekly intake was 9100 mg.
