In acute kidney injury, which features help distinguish prerenal from intrinsic AKI?

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Distinguishing prerenal from intrinsic AKI relies on how the kidney handles sodium and what the urine sediment shows in the setting of injury.

In prerenal AKI, reduced kidney perfusion triggers the tubules to conserve sodium and water. This results in very low urine sodium and a high BUN to creatinine ratio, reflecting concentrated filtrate and preserved kidney function despite reduced flow. Clinically, you often see signs of hypoperfusion and oliguria, with the urine appearing concentrated and sodium reabsorption intact.

In intrinsic AKI, tubular damage disrupts reabsorption, so the kidney loses the ability to hold onto sodium. Urine sodium becomes higher, and the BUN:creatinine ratio is not as elevated as in prerenal. The urine sediment often reveals casts due to tubular injury—granular or muddy brown casts in acute tubular necrosis, for example—and may show an active inflammatory sediment in other intrinsic causes.

The described pattern matches these physiologic responses: prerenal AKI shows low urine sodium, a high BUN:Cr ratio, and signs of hypoperfusion; intrinsic AKI shows higher urine sodium and casts (active sediment). This combination is what helps differentiate the two.