A Case of Acute Focal Bacterial Nephritis With Negative Pyuria and Urine Culture Test Results

Acute focal bacterial nephritis (AFBN) is a radiologically diagnosed acute localized kidney infection that appears in the continuum between a perinephric abscess and renal abscess. We report an unusual case of AFBN presenting without pyuria or positive urine cultures. A 42-year-old woman with chronic lower back pain who regularly used nonsteroidal anti-inflammatory drugs was admitted to our hospital with right-sided abdominal distention, fever, chills, and pain extending from the right lower abdomen to the back since two days. The physical examination revealed no abdominal or costovertebral angle tenderness. Urinalysis was negative. Abdominal ultrasound was notable for an indistinct nodular shadow (32 × 25 mm) on the upper pole of the right kidney. Abdominal contrast-enhanced computed tomography revealed a wedge-shaped area with a minimal uptake of the contrast in the right kidney. The patient was admitted to the hospital, and antimicrobial therapy was started for AFBN. Antibiotics were administered intravenously for one week and orally for two weeks. No relapse of symptoms was observed during the four-month follow-up period. This case report suggests the importance of considering AFBN as a differential diagnosis for cases of idiopathic fever and lateral pain or back pain, even when pyuria and urine culture test results are negative.