Red blood cell casts in urine are most indicative of which process?

• A. Tubulointerstitial inflammation
• B. Glomerular bleeding (nephritic processes)
• C. Acute tubular necrosis
• D. Urinary tract obstruction

Answer: (B)

RBC casts indicate bleeding from the glomerulus. When the glomerular capillary walls are damaged, red blood cells escape into the filtrate. As these cells travel through the tubules, they get trapped in casts formed by Tamm-Horsfall protein secreted by tubular cells, creating red blood cell–containing casts in the urine. This pattern is characteristic of glomerular inflammation, such as nephritic processes, where the origin of the problem is intrarenal rather than in the tubules or lower urinary tract.

Tubulointerstitial inflammation tends to produce white blood cell casts rather than red cell casts, reflecting inflammation within the tubules. Acute tubular necrosis causes granular, muddy-brown casts made of tubular cell debris, not red cell casts. Urinary tract obstruction can alter flow and cause other findings like crystals or sterile pyuria, but red blood cell casts are not a typical feature of obstruction.