What type of casts is pathognomonic for ischemic acute tubular necrosis (ATN)?

Muddy brown (granular) casts are considered pathognomonic for ischemic acute tubular necrosis (ATN). This type of cast occurs due to the degeneration and sloughing off of renal tubular epithelial cells in the setting of renal ischemia. In cases of ATN, the tubular epithelium becomes injured, often due to hypoperfusion of the kidneys, leading to a characteristic appearance that is described as "muddy brown." The presence of these granular casts suggests ongoing tubular injury and is a significant finding when evaluating a patient for acute kidney injury caused by ischemic events.

Other casts, such as hyaline, red blood cell, and white blood cell casts, indicate different pathological processes. While hyaline casts can be seen in various kidney conditions, they are nonspecific and do not provide a distinct indication of ATN. Red blood cell casts are typically associated with glomerular injury, indicating conditions such as glomerulonephritis. White blood cell casts suggest the presence of inflammation or infection, such as pyelonephritis. Thus, muddy brown casts are uniquely linked to the cellular changes seen in ischemic ATN, making them a critical diagnostic marker.