Fractional excretion of sodium — separates prerenal from intrinsic AKI.

What it is and when to use it

The fractional excretion of sodium (FENa) is an index that estimates the percentage of filtered sodium that is ultimately excreted in the urine, calculated from plasma and urine sodium and creatinine. It is used in the workup of oliguric acute kidney injury (AKI) to help distinguish prerenal azotemia (hypoperfusion, with avid sodium reabsorption) from acute tubular necrosis (ATN). It is a classic diagnostic aid that complements the KDIGO classification of AKI rather than a formally guideline-endorsed score.

How to interpret it

A FENa below 1% suggests a prerenal cause, with intact tubules avidly reabsorbing sodium to preserve volume (also seen in low-flow states such as hepatorenal syndrome and heart failure). A FENa above 2% is typical of acute tubular necrosis, where the injured tubule cannot conserve sodium. Values between 1% and 2% are indeterminate and require clinical interpretation. The result should always be read together with the clinical context, urinalysis, and volume status.

Limitations and when not to use it

FENa was validated in patients with oliguric AKI and should not be used in isolation. It loses reliability with diuretic use (which increases sodium excretion and can yield a high FENa despite a prerenal cause); in this setting the fractional excretion of urea is preferred. It may also be below 1% in non-prerenal conditions such as contrast nephropathy, acute glomerulonephritis, rhabdomyolysis, or early sepsis. It does not identify the exact cause of AKI, does not replace examination of the urine sediment, and is not useful in chronic kidney disease.

Frequently asked questions

How is FENa calculated?

FENa (%) = (urine sodium × plasma creatinine) / (plasma sodium × urine creatinine) × 100. Simultaneous plasma and urine sodium and creatinine values are required.

What does a FENa below 1% mean?

It suggests a prerenal cause of acute kidney injury, with intact tubules avidly reabsorbing sodium because of hypoperfusion. It can also occur in hepatorenal syndrome and contrast nephropathy.

Why is FENa unreliable with diuretics?

Diuretics increase renal sodium excretion and can falsely raise FENa even when the cause is prerenal; in patients on diuretics the fractional excretion of urea (FEUrea) is recommended instead.