Anatomic Pathology: Renal Pathology

• Acute pyelonephritis is an acute suppurative infection of the kidney and renal pelvis associated with urinary tract infection. Greater than 85% of infections are due to gram-negative bacilli, most commonly E. coli.

• The most common pathway of renal infection is ascension of bacteria up the urinary stream from the bladder. Organisms reach the bladder most commonly in the setting of obstruction to urinary flow (e.g., prostatic hypertrophy, lower urinary tract tumor, pregnancy, incomplete voiding of the bladder owing to neurogenic bladder in patients with diabetes), urethral instrumentation (e.g., catheterization, cystoscopy), and vesicoureteral reflux (particularly prevalent in infants and young children with pyelonephritis).

• The alternative, less frequent pathway of renal infection is via the hematogenous route, typically occurring in patients with bacteremia or a source of septic emboli (e.g., endocarditis).

• Histologic findings in acute bacterial pyelonephritis include neutrophil-rich interstitial inflammation with neutrophilic tubulitis and intratubular neutrophil casts.

• Clinically, patients with acute pyelonephritis present with fever, flank pain, and dysuria. The diagnosis is usually made on clinical grounds with urinalysis and urine culture, and renal biopsy is generally not required.