MEDizzy - 73-year-old male undergoes abdominal aortic aneurysm repair.

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Nephrology

73-year-old male undergoes abdominal aortic aneurysm repair. Postoperatively, his blood pressure is 110/70, heart rate is 110, surgical wound is clean, and a Foley catheter is in place. His urine output drops to 40 cc/h, and creatinine rises from 1.5 to 2.2 mg/dL. Hemoglobin and hematocrit are stable, K 4.6, uric acid 8.2. Which initial diagnostic test is most useful for this patient?

ExplanationUrinalysis would be the best test because it is likely to show muddy brown granular casts, suggesting acute tubular necrosis. In oliguric acute renal failure (less than 20 mL urine per hour), a urine sodium less than 10 mEq/L (and a fractional excretion of sodium< 1) suggest prerenal azotemia, whereas a value > 20 mEq/L (FENa > 2) suggests acute tubular necrosis. However, the urine sodium is less useful in nonoliguric ARF. Obstructive uropathy is unlikely since a functioning urinary catheter is in place; a normal urinalysis would raise the index of suspicion both for obstruction and for hypovolemia. Despite the high serum uric acid, acute uric acid nephropathy occurs with chemotherapy of aggressive tumors (eg, Burkitt lymphoma) and rarely in the postoperative setting. A urine uric acid— creatinine ratio > 1 is helpful in diagnosing uric acid nephropathy in the appro- priate setting. Any diagnostic study that uses IV radiocontract agents should be avoided if possible in the setting of acute renal insufficiency.