Presence of red blood cell casts in urine indicates which source of bleeding?

• A. Bladder/urethral bleeding
• B. Ureteral or renal pelvis bleeding
• C. Glomerular (renal) bleeding
• D. Non-glomerular urinary tract bleeding

Answer: (C)

Red blood cell casts form when red blood cells leak through a damaged glomerular filtration barrier, flow through the tubules, and become trapped in a proteinaceous cast matrix. This process ties the appearance of RBC casts directly to glomerular (renal) bleeding, since the RBCs have to cross the glomerular filtration barrier before entering the tubules. In non-glomerular sources like the bladder, ureter, or renal pelvis, bleeding typically yields whole red cells in the urine rather than casts, so RBC casts are not expected. The presence of RBC casts, often with dysmorphic red cells and protein in the urine, therefore points to a glomerular origin of bleeding.