Acute tubulointerstitial nephritis caused by fosfomycin: diagnosis confirmed by second drug-induced lymphocyte transformation test

Numerous drugs have been reported as causes of drug-induced acute tubulointerstitial nephritis (ATIN); however, cases due to fosfomycin have rarely been documented. We herein report a case involving a 3-year-old boy with no medical history who presented with acute renal injury characterized by fosfomycin-induced ATIN. The patient had been treated with fosfomycin for gastroenteritis and developed a rash 24 hours after beginning therapy. Even after fosfomycin was switched to tosufloxacin, he was brought to our hospital because of frequent vomiting and worsening malaise. Although intravenous rehydration therapy was performed, his urine output did not improve, and his blood test showed an elevated serum creatinine concentration. He was admitted to our hospital for treatment of acute kidney injury and underwent renal biopsy on day 5, which showed features of ATIN. A drug-induced lymphocyte transformation test was positive for both fosfomycin and tosufloxacin 8 weeks from symptom onset; 1 year later, however, the test was positive only for fosfomycin. ATIN caused by fosfomycin is very rare. The findings in the present case may indicate the usefulness of a second lymphocyte transformation test when the first test shows positivity for multiple drugs.