AIN't That the Flu Med? A Rare Case of Tamiflu-Induced Acute Interstitial Nephritis (AIN)

Introduction: Tamiflu is commonly prescribed for the treatment of influenza. It is considered a safe drug for short-term use; however, rarely, its metabolite, oseltamivir carboxylate, can accumulate and cause acute kidney injury in patients with known renal disease. We report a case of a 74-year-old male with biopsy-proven AIN following Tamiflu use. Case Description: A 74-year-old male, with chronic kidney disease (CKD stage 3b with baseline creatinine-1.5) and hypertension, presented with anuria for two days. The patient stated that he had had a week of myalgias and fevers preceding the development of anuria, for which his primary care office had prescribed Tamiflu 75 mg twice a day, of which he took three doses, which he stopped taking due to anuria. He denied any hematuria, dysuria, other new medication usage, back, or abdominal pain. Labs showed a creatinine of 9.5 and a BUN of 83. Urinalysis was significant for moderate proteinuria, small hematuria, no casts, no pyuria, and no eosinophiluria. A Foley catheter was inserted over 24 hours, confirming anuria. Renal ultrasound revealed no evidence of obstruction. Serum creatinine continued to worsen over the next 3 days, reaching a peak of 13.5, at which point the patient was started on hemodialysis. Due to the unclear etiology of acute renal failure, a renal biopsy was obtained, which revealed AIN. Prednisone therapy was initiated. Over the next two weeks, urine output and serum creatinine improved to 3.5, and hemodialysis was stopped and the patient was discharged. On follow-up one month later, his kidney function had returned to baseline. Discussion: AIN is a rarely reported adverse event associated with Tamiflu use. Recognizing this potential adverse effect is crucial for prompt discontinuation of the drug and corticosteroid initiation to prevent permanent renal damage. This case also emphasizes the importance of renal dosing of Tamiflu in patients with CKD.Renal biopsy showing peritubular neutrophils