Warfarin-associated hypoprothrombinemia: an unusual presentation

Azithromycin and warfarin interaction

A 57-year-old Caucasian woman came to the clinic with symptoms of an upper respiratory tract infection. She was treated with a 5-day course of oral azithromycin 500 mg on day 1, then 250 mg/day for 4 days. During this period, the patient decreased her cigarette smoking from 1 pack/day to 1 pack every 3 days. No additional confounding variables were present. Two days after the completion of therapy, her international normalized ratio (INR) was 8.32. Six case reports documented in the literature have suggested an azithromycin-warfarin interaction with a resultant increase in INR. Many confounding variables existed in each of these cases, such as hepatic dysfunction, poor appetite, and concomitant drugs that resulted in an increased anticoagulant response. We report a case that involved only one potential confounding variable. Continued documentation of azithromycin-warfarin interactions is valuable considering no mention of this drug interaction exists in most tertiary references and in the package insert for azithromycin, the demonstration that no drug interaction occurred in a retrospective review of 52 cases, and the widespread use of azithromycin in the community. Clinicians should be mindful when prescribing azithromycin in combination with warfarin, and INR values should be monitored.

Determination of the lack of a drug interaction between azithromycin and warfarin

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.

Enhanced hypoprothrombinemia with warfarin due to azithromycin

OBJECTIVE

To report a case of probable azithromycin-warfarin drug interaction with enhanced hypoprothrombinemic effect of warfarin.

CASE SUMMARY

An 83-year-old African American man stabilized on warfarin therapy (10 mg on Wednesdays, 7.5 mg on other days) developed a prolonged prothrombin time one day after starting azithromycin 500 mg. The elevated prothrombin time normalized 3 days after azithromycin was discontinued. After the initial increase in the international normalized ratio, the absence of any significant confounding factors affecting the anticoagulant effect of warfarin in our patient and the numerous reports of such interactions indicate that an interaction between azithromycin and warfarin may have been responsible for the elevated prothrombin time seen in this patient. An objective causality assessment revealed that the adverse event was probably related to the combination of these drugs.

DISCUSSION

Azithromycin, unlike erythromycin and clarithromycin, is not known to inhibit the cytochrome P450 enzyme system and is presumed to be the macrolide of choice in patients already on warfarin. However, previously reported cases of azithromycin-warfarin interactions support the possibility that azithromycin does interact with warfarin, although the exact mechanism is not understood.

CONCLUSIONS

Azithromycin may interact with warfarin and enhance its hypoprothrombinemic effects. This effect may be delayed for 4-8 days after a course of azithromycin has been completed. Periodic monitoring of the prothrombin time is recommended when using azithromycin in patients taking warfarin.