What does a high BUN/creatinine ratio indicate and what tests help differentiate prerenal from intrinsic kidney injury?

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A high BUN/creatinine ratio points to prerenal azotemia from reduced renal perfusion. In prerenal states the kidney conserves sodium and water, so the urine sodium is low and the fractional excretion of sodium is low (typically urine Na <20 mEq/L and FE Na <1%). This contrasts with intrinsic renal injury, where tubular damage impairs reabsorption, leading to higher urine sodium and a higher FE Na (often >2%). To differentiate these states, measuring urine sodium and calculating FE Na are most informative. Supporting data include a high urine osmolality (often >500 mOsm/kg) and a BUN/Cr ratio >20 for prerenal cases, versus lower osmolality and FE Na >2% for intrinsic injury. Renal ultrasound can help exclude obstruction (postrenal) if hydronephrosis is present, but the key differentiators here are urine sodium and FE Na. Thus, low urine sodium with a low FE Na best aligns with prerenal azotemia.