Test to differentiate intrarenal from postrenal AKI?

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Focus on how the urine looks reflects the injury pattern in the kidney. In intrinsic (intrarenal) AKI, the damaged tubules shed cells and proteins into the urine, producing granular ("muddy brown") casts and tubular epithelial cells on urinalysis. This sediment pattern points to tubular injury from conditions like acute tubular necrosis or interstitial nephritis. In contrast, postrenal (obstructive) AKI typically yields a bland sediment without those characteristic casts, because the primary issue is downstream blockage rather than tubular injury. So analyzing the urinary sediment directly tests the underlying mechanism and helps distinguish intrinsic renal injury from obstruction. Imaging, such as abdominal/renal ultrasound, is still used to confirm obstruction when a postrenal cause is suspected, but the urine sediment provides a rapid, mechanism-oriented clue. Serum BUN, serum creatinine, and their ratio indicate renal function but do not localize the cause as effectively, and ultrasound, while definitive for obstruction, is not the quickest biochemical clue to intrinsic vs obstructive injury.