Zithromax drug interaction with Coumadin

Zithromax (Azithromycin) is a very popular antibiotic in primary care. It is widely used for a variety of indications such as community aquired respiratory tract infections and tonsillitis. I find it especially useful because of it's easy once daily dosing and short five day course. Patient compliance is improved. When Zithromax was first introduced and I educated myself on the drug, I noted that there was a possibility of Zithromax increasing the effect of Coumadin and elevating a patient's INR. For a few years, every patient I had on Coumadin had their INR's checked a couple of extra times during therapy with Zithromax. Not once did I find any significant changes in their INR. Because of cost and time, I stopped doing the extra lab work. I recently treated a male patient in his mid sixties with Zithromax for community aquired bronchitis. He is on Coumadin for anticoagulation for a mechanical aortic valve. For months, his INR had been steady with no changes in dosing of his Coumadin. Two weeks after starting his Zithromax, his respiratory symptoms had improved. He came in to the office however, for severe left flank and anterior thigh pain. He required urgent evaluation and eventual hospitalization for a severe anemia. His hemoglobin had dropped to 79. Abdominal CT scan revealed a large left sided retroperitoneal hematoma. His INR on admission was 15. He responded to transfusions and reversal of his anticoagulation with vitamin K. On final review, the Zithromax would appear to be the culprit for the significant change in this patient's INR and the substantial bleed he developed. I have reviewed other antibiotics commonly used to treat community aquired respiratory tract infections. Many drug monographs state that no significant effects on INR have been noted clinically, but then they give a disclaimer that patients should have their INR's followed during therapy. From now on in my practice, every patient on Coumadin who requires antibiotic therapy will have to put up with repeated INR checks. I'm not going to risk the type of reaction I saw in this described patient again.