Which diagnostic test should be ordered first in the case of an elderly female whose status is posted abdominal aortic aneurysm repair with a foley catheter in place, a blood pressure of 110/75 and pulse of 115 whose urine output drops to 40 cc/hour and creatinine rises to 2.5 mg/dL?
Urine sodium and creatinine
• A urinalysis should be ordered first to look for signs of acute tubular necrosis such as muddy brown casts and if they are absent then hypovolemia causing pre-renal failure should be suspected (post-renal failure is not likely given the patient had a foley catheter in place).
• This is a non oliguric renal failure because the urine output hasn’t dropped below 20 cc/hour in this case, calculated fractional excretion of sodium is less useful.
• If the patient had oliguric renal failure, urine output < 20 cc/hour, then the FENa could help differentiate between prerenal failure (FENa<1) or acute tubular necrosis (FENa>2).