What is the most common component of urine casts and what does it indicate?

• A. RBC casts; glomerular bleeding
• B. WBC casts; pyelonephritis
• C. Granular casts; tubular degeneration
• D. Hyaline casts; nonspecific and often seen with concentrated urine or dehydration

Answer: (D)

Casts are cylindrical structures formed in the renal tubules when a protein-rich matrix precipitates and molds to the tubule lumen. The most common cast type is the hyaline cast, which is mainly composed of Tamm-Horsfall protein and appears as a clear, colorless, refractile cylinder. Its presence is nonspecific and can be seen even in healthy individuals, especially when urine is concentrated or during dehydration, exercise, or fever. So, while hyaline casts are the most frequent cast type, they do not point to a particular disease. In contrast, red blood cell casts suggest glomerular bleeding, white blood cell casts point to tubulointerstitial inflammation or pyelonephritis, and granular casts reflect tubular degeneration or acute tubular necrosis.