Calculate FENa to differentiate prerenal from intrinsic renal AKI. Includes FENa formula, AKI diagnostic indices comparison, and clinical interpretation with diuretic use guidance.
The Fractional Excretion of Sodium (FENa) calculator estimates the percentage of filtered sodium that is being excreted in the urine. It is commonly used as one clue in the acute kidney injury workup because low values often fit sodium-avid states such as volume depletion, while higher values can fit tubular injury or other intrinsic renal processes.
The useful part of FENa is that it puts urine sodium and creatinine together with plasma sodium and creatinine in one ratio, so a spot urine sample can be used without a timed collection. The less useful part is that the number is easy to overinterpret. Diuretics, chronic kidney disease, glomerulonephritis, contrast nephropathy, obstruction, sepsis, and other settings can all make the usual thresholds less reliable. This page is therefore meant as a structured AKI clue, not a stand-alone sorter.
FENa is most useful when it is read beside volume status, medication exposure, and urine sediment rather than by itself. This page keeps the ratio, the usual threshold bands, and the common failure modes together so the result supports the AKI differential instead of pretending to settle it.
FENa (%) = (UNa × PCr) / (PNa × UCr) × 100 Where: • UNa = Urine sodium (mEq/L) • PNa = Plasma sodium (mEq/L) • UCr = Urine creatinine (mg/dL) • PCr = Plasma creatinine (mg/dL) Common bedside ranges: • FENa < 1%: Pattern often compatible with sodium-avid / prerenal physiology • FENa 1–2%: Indeterminate • FENa > 2%: Pattern more compatible with impaired tubular sodium reabsorption
Result: FENa = 0.12% — Pattern compatible with prerenal physiology
FENa = (10 × 2.0) / (140 × 120) × 100 = 20/16800 × 100 = 0.12%. This very low FENa (<1%) fits avid sodium reabsorption, which can be seen in sodium-avid states such as volume depletion or reduced effective circulating volume. The high urine creatinine relative to plasma creatinine (ratio 60) also fits preserved tubular concentrating ability.
FENa is one piece of the AKI evaluation, not the whole answer. The ratio is most helpful when it is read together with the urine sediment, medication exposure, volume status, ultrasound findings, and the broader hemodynamic picture.
Diuretics, chronic kidney disease, contrast-associated injury, rhabdomyolysis, sepsis, glomerular disease, and obstruction can all make the classic FENa thresholds less dependable. That is why the page keeps the usual exceptions visible instead of treating the cutoffs as absolute.
When the result is ambiguous, FEUrea, urine microscopy, repeat chemistry, and the clinical course often do more to sort out the cause of AKI than one FENa value alone.
Last updated:
This page calculates fractional excretion of sodium from urine sodium, plasma sodium, urine creatinine, and plasma creatinine using the standard spot-sample equation. It then places the result next to the familiar low, intermediate, and higher FENa bands and keeps common failure modes visible, especially the effect of diuretics and the possibility of low FENa values in some intrinsic renal disorders.
The result is a bedside AKI clue, not a stand-alone diagnosis. The ratio should be interpreted with urine sediment, volume status, medication exposure, FEUrea when appropriate, and the broader kidney workup rather than used as a single yes-or-no sorter.
FENa estimates the percentage of filtered sodium that ends up in the urine. Low values often fit sodium retention, while higher values can fit impaired tubular sodium reabsorption. The main value of the number is as one part of the AKI picture rather than as a diagnosis by itself.
Diuretics directly increase sodium excretion, so they can push FENa upward even when the patient is actually sodium avid. That is why FEUrea is often preferred when diuretics have already been given.
Several intrinsic renal problems can still show a low FENa early on, including contrast-associated AKI, rhabdomyolysis, some glomerular diseases, early sepsis, and early obstruction. A low value therefore does not automatically rule out intrinsic disease.
Urine microscopy, FEUrea, urine osmolality, BUN/creatinine ratio, urine sediment, and the overall hemodynamic picture often add more context than FENa alone. In many cases the urine microscopy and the clinical course are more informative than the ratio by itself.
In the AKI context, values below about 1% are commonly discussed as sodium-avid, values above about 2% are more compatible with impaired tubular reabsorption, and the 1–2% range is a gray zone. Those cutoffs are useful shorthand, not a perfect rule.
Its value is more limited in CKD because baseline tubular handling may already be altered. In acute-on-chronic kidney injury, the ratio should be interpreted cautiously and always alongside the broader renal workup.