FENa estimates the percentage of filtered sodium that is excreted in the urine. It is used as one adjunct in the evaluation of acute kidney injury (AKI), particularly when considering prerenal (haemodynamic) causes versus intrinsic tubular injury.

Early hospital series in adults with oliguric AKI reported FENa <1% in most prerenal azotaemia and higher values in acute tubular necrosis.
Subsequent work and nephrology reviews support pragmatic cut-offs of FENa <1% suggesting a prerenal pattern and FENa >2% suggesting intrinsic tubular injury, with 1–2% an indeterminate range where mixed mechanisms, diuretics or chronic kidney disease (CKD) are common.
A 2022 systematic review and meta-analysis of 19 studies (1287 adults) found that, in 15 studies (872 patients) using a 1% threshold, pooled sensitivity and specificity for distinguishing intrinsic from prerenal AKI were 90% and 82% overall, improving to 95% and 91% in oliguric patients without CKD or diuretics, but falling to 83% and 66% when CKD or diuretics were present, and 80% and 54% among patients on diuretics alone.
The 2026 public-review KDIGO AKI guideline advises that urine chemistries, including FENa and fractional excretion of urea, be reserved for differentiating haemodynamic-mediated (prerenal) AKI from acute tubular injury in selected scenarios because of their limited overall clinical utility.
In cirrhosis-associated AKI, very low FENa values (often <0.1–0.2%) are more consistent with hepatorenal physiology, whereas higher values favour tubular injury, but interpretation still depends on the broader clinical picture.

In practice FENa is most informative in adults with new, oliguric AKI, measured early in the admission and, where possible, before diuretics. It should be interpreted alongside urine microscopy, volume status, haemodynamics, medication exposure and imaging, and not used alone to decide escalation or renal replacement therapy.

Fractional excretion of sodium (FENa) in acute kidney injury