White blood cell casts in urine most commonly indicate which pathology?

• A. Nephrotic syndrome
• B. Acute interstitial nephritis or pyelonephritis
• C. Minimal change disease
• D. Renal cell carcinoma

Answer: (B)

White blood cell casts indicate inflammation within the tubulointerstitial compartment or infection of the upper urinary tract. They form when leukocytes enter the tubular lumen and mix with Tamm-Horsfall protein to create casts that are then excreted in urine. This finding is most commonly seen with acute interstitial nephritis (often drug-induced) and pyelonephritis, which is why that option is the best match. In nephrotic syndrome and minimal change disease, the hallmark is heavy protein loss with little to no WBC casts, while renal cell carcinoma usually presents with hematuria or a mass rather than this urinary finding.