Acute Pyelonephritis

This renal biopsy from a 65-year-old male with a history of E. Coli urinary tract infection and bacteremia shows severe neutrophil-rich interstitial inflammation with neutrophilic rimming of the tubules, neutrophilic tubulitis and neutrophilic casts (Figures 1-3).  These features are diagnostic of acute pyelonephritis. In addition to the aforementioned morphologic findings, a characteristic feature of acute pyelonephritis is the patchy nature of the infiltrate (Fig 4). Relatively spared areas of renal parenchyma are often intermixed with areas displaying severe tubulointerstitial inflammation.  While an ascending urinary tract infection is the main mechanism for the development of acute pyelonephritis, hematogenous infections may also occur in a patient with bacteremia/ septicemia.