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Determination of the lack of a drug interaction between azithromycin and warfarin
Kenneth L McCall1, H Glenn Anderson, Angela D Jones
1School of Pharmacy, Texas Tech University Health Sciences Center, Amarillo, Texas 79106, USA. kenneth@ama.ttuhsc.edu
Study Objective:
To determine the effect on the international normalized ratio (INR) of adding azithromycin to patients receiving stable dosages of warfarin.
Design:
Retrospective chart review.
Setting:
Outpatient clinic.
Patients:
Ambulatory patients receiving warfarin and azithromycin concurrently who had a documented therapeutic INR value before the start of azithromycin therapy (pre-INR) and a documented INR value within 30 days after the start of azithromycin therapy (post-INR).
Measurements And Main Results:
Patients given felodipine during long-term warfarin therapy formed a comparative control group. Patient demographics were similar in both treatment groups. Mean age of the azithromycin group (17 patients) was 59 +/- 13 years and of the control group (20 patients) 65 +/- 12 years. All 17 patients in the azithromycin group and 16 of the controls were women. Mean change from pre-INR to post-INR in the azithromycin and control groups, respectively, was 0.14 +/- 0.64 (pre-INR 2.46, post-INR 2.61) and 0.19 +/- 0.54 (pre-INR 2.46, post-INR 2.66) (p = 0.74). A post hoc power analysis based on a pooled standard deviation of 0.60 revealed that the study had 68% power to detect a 0.5 change in the INR value.
Conclusion:
No interaction between azithromycin and warfarin was observed in ambulatory patients with therapeutic baseline INR values.
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