FENa Calculator

FENa Calculator for Nephrology. The classic cutoff is 1%: FENa below 1% suggests prerenal azotemia (the tubules are working hard to hold onto sodium in response to hypoperfusion), while FENa above 2% suggests ATN, where injured tubules can no longer reabsorb sodium. Values between 1% and 2% are indeterminate and overlap both diagnoses. Espinel's original oliguric series showed prerenal patients clustering below 1% and ATN patients above 3%, cleanly separating the two. Note that a low FENa is not synonymous with prerenal disease: it also occurs in contrast nephropathy, rhabdomyolysis, early obstruction, acute glomerulonephritis, and hepatorenal syndrome, so the result must be read against the clinical picture.

How this calculator works

FENa expresses the percentage of sodium filtered by the glomerulus that ends up in the urine, computed from paired serum and spot-urine samples as FENa (%) = (urine sodium x serum creatinine) / (serum sodium x urine creatinine) x 100. Using creatinine as a surrogate for filtered volume cancels out the need to measure or time urine flow, so a single random urine and simultaneous blood draw suffice. A low value means the tubules are avidly reclaiming sodium (intact tubular function plus a stimulus to retain salt), while a high value means sodium reabsorption has failed.

When to use this calculator

Use FENa in oliguric acute kidney injury to separate prerenal azotemia from established acute tubular necrosis (ATN), most usefully when the history and urine sediment are ambiguous. It is a supplement to, not a replacement for, clinical assessment of volume status and microscopy. Do not rely on it in patients on loop or thiazide diuretics (use FEUrea instead), in chronic kidney disease with a high baseline sodium excretion, in non-oliguric AKI, or in glomerulonephritis and contrast/pigment nephropathy, where FENa can be low despite intrinsic injury.

Inputs used

  • Urine sodium
  • Serum sodium
  • Urine creatinine
  • Serum creatinine

Clinical interpretation

The classic cutoff is 1%: FENa below 1% suggests prerenal azotemia (the tubules are working hard to hold onto sodium in response to hypoperfusion), while FENa above 2% suggests ATN, where injured tubules can no longer reabsorb sodium. Values between 1% and 2% are indeterminate and overlap both diagnoses. Espinel's original oliguric series showed prerenal patients clustering below 1% and ATN patients above 3%, cleanly separating the two. Note that a low FENa is not synonymous with prerenal disease: it also occurs in contrast nephropathy, rhabdomyolysis, early obstruction, acute glomerulonephritis, and hepatorenal syndrome, so the result must be read against the clinical picture.

Worked example

A 72-year-old with oliguria after gastroenteritis: urine Na 12 mmol/L, urine creatinine 90 mg/dL, serum Na 140 mmol/L, serum creatinine 3.0 mg/dL. FENa = (12 x 3.0) / (140 x 90) x 100 = 36 / 12,600 x 100 = 0.29%. A value well under 1% points to a prerenal, sodium-avid state, supporting a fluid challenge rather than an ATN diagnosis.

Limitations and safety notes

The 1% cutoff was derived in oliguric AKI and loses discrimination in non-oliguric injury, so a normal-output patient can have a misleading FENa. Diuretics force natriuresis and drive FENa above 1% even in true prerenal states, making the number uninterpretable for hours to days after dosing; substitute fractional excretion of urea (FEUrea below roughly 35% favors prerenal). Chronic kidney disease raises baseline FENa because fewer nephrons each excrete more sodium, and conversely FENa stays low in several intrinsic causes (radiocontrast, myoglobinuric/hemoglobinuric injury, sepsis, acute GN, hepatorenal syndrome), so a low value never rules ATN out on its own.

Frequently asked questions

What is a normal or reassuring FENa in acute kidney injury?

There is no single normal value; interpretation depends on context. In oliguric AKI, FENa below 1% points toward a prerenal cause, 1-2% is indeterminate, and above 2% points toward acute tubular necrosis. In a patient with normal kidney function and no AKI, FENa is typically well below 1% simply because the kidney tightly regulates sodium.

Why use FENa instead of just spot urine sodium?

Spot urine sodium alone is confounded by urine concentration; a concentrated urine can show a high sodium even when total sodium excretion is low. FENa normalizes urine sodium to filtered load using creatinine, removing the effect of water reabsorption, which is why Espinel found it more discriminating than urine sodium, urine osmolality, or the urine-to-plasma creatinine ratio.

My patient is on furosemide. Can I still trust the FENa?

No. Loop and thiazide diuretics block tubular sodium reabsorption and push FENa above 1% regardless of volume status, so a high value cannot be attributed to ATN. Use fractional excretion of urea instead, since urea handling is largely independent of these agents; FEUrea under about 35% supports a prerenal state.

Can FENa be low in intrinsic (non-prerenal) kidney injury?

Yes. A FENa below 1% is well described in radiocontrast nephropathy, rhabdomyolysis and hemoglobinuria, acute glomerulonephritis, early urinary obstruction, sepsis, and hepatorenal syndrome. A low FENa therefore supports but does not prove a prerenal cause and must always be read alongside history, exam, and urine microscopy.

What units do I enter?

FENa is a ratio, so units cancel as long as serum and urine sodium share the same units (mmol/L) and serum and urine creatinine share the same units (both mg/dL or both micromol/L). Mixing creatinine units between serum and urine will give a wrong result.

References

  • Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976. PMID: 947239.
  • Espinel CH, Gregory AW. Differential diagnosis of acute renal failure. Clin Nephrol. 1980. PMID: 7363517.
  • Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984. PMID: 6486145.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: March 18, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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