In an oliguric patient, what is the first step in differentiating between prerenal and intrinsic renal causes?
The fractional excretion of sodium (FENa) can help differentiate prerenal from intrinsic renal causes in patient with oliguria. If the FENa is < 1%, it suggests prerenal and if it is > 2%, it suggests intrinsic renal causes. There are many limitations to using this calculation, but it is a good first step in determining the cause of a patient’s oliguria. Other laboratory studies, such as BUN/Cr, can be useful as well.