What urinary finding is indicative of acute tubular necrosis?

The presence of muddy brown casts in the urine is a classic indicator of acute tubular necrosis (ATN). In ATN, the renal tubules are injured, and this can lead to the sloughing off of necrotic epithelial cells into the urine. When these cells aggregate, they form casts that appear muddy brown in color. This finding is significant because it reflects the underlying pathology of tubular injury rather than issues related to glomerular function.

Clear urine generally does not provide information about tubular damage and is not characteristic of ATN. Red cell casts are associated with glomerulonephritis and indicate glomerular bleeding or inflammation, which is distinct from the tubular injury seen in ATN. Fatty casts are typically seen in conditions involving nephrotic syndrome, reflecting lipiduria and damage primarily affecting the glomeruli rather than the tubular system.

Thus, muddy brown casts are highly suggestive of ATN, as they directly relate to the morphological changes occurring within the renal tubules during injury.