Acute interstitial nephritis due to acute pyelonephritis in a young woman: diagnostic utility of CK7 and CD10 immunostaining.

Acute Interstitial Nephritis (AIN) is a common cause of acute kidney injury (AKI) and may lead to irreversible renal scarring if the diagnosis is delayed. Although most cases are drug-induced, AIN can also occur in association with acute pyelonephritis (APN). Nevertheless, given that antibiotics are routinely administered for APN, cases of APN-associated AIN may be erroneously classified as drug-induced AIN. Distinguishing these two entities histologically remains challenging; however, their therapeutic strategies differ. We describe a case involving a previously healthy 21-year-old woman who presented with a fever and rapidly progressive renal impairment. Laboratory evaluation revealed serum creatinine and C-reactive protein concentrations of 3.54 mg/dL and 25.06 mg/dL, respectively, prompting an urgent renal biopsy. Histopathological examination by light microscopy demonstrated a diffuse interstitial and tubular inflammatory infiltrate predominantly composed of neutrophils, consistent with AIN. Based on these pathological findings and the clinical course, AIN secondary to APN was suspected. The patient was managed with an intensified antibiotic regimen without corticosteroid therapy, resulting in complete renal recovery. To further refine the diagnosis after hospital discharge, we examined immunostaining patterns across tubular segments. Immunostaining demonstrated a neutrophil-predominant infiltrate in the distal tubules and collecting ducts, which showed a segmental pattern characteristic of APN-related injury. This case highlights the potential utility of immunostaining in distinguishing APN-associated AIN from drug-induced AIN and supports its incorporation into diagnostic evaluation.