Fractional Excretion of Sodium (FENa) Calculator
Fractional Excretion of Sodium (FENa) Calculator
Distinguish a pre-renal pattern from intrinsic renal injury by comparing sodium clearance with creatinine clearance — only valid before diuretics have been given.
Fractional Excretion of Sodium (FENa)
Na clearance ratioUrine Na 20 mmol/L, serum Na 138 mmol/L, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
Formula
- urine Na, serum Na
- mmol/L, from a single paired urine and blood sample
- urine creatinine, serum creatinine
- mg/dL
- validity
- the test is meaningless once loop or thiazide diuretics have been given — use FEUrea instead
Worked example
Urine Na 20 mmol/L, serum Na 138 mmol/L, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
20 × 2.4 = 48
138 × 90 = 12,420
48 ÷ 12,420 × 100 = 0.39%
Below 1% → pre-renal pattern
Interpretation
| FENa | Pattern | Typical causes |
|---|---|---|
| < 1% | Pre-renal | Volume depletion, heart failure, cirrhosis, sepsis with intact tubules |
| 1 – 2% | Indeterminate | Overlap zone; correlate clinically |
| > 2% | Intrinsic renal | Acute tubular necrosis; also raised by diuretics regardless of volume status |
What FENa can and cannot tell you
The fractional excretion of sodium compares how much of the filtered sodium load actually appears in the urine against how much of the filtered creatinine does. In pre-renal states the tubules are structurally intact and respond to reduced perfusion by reabsorbing sodium avidly, so FENa stays low. Once tubular injury sets in, that reabsorptive capacity is lost and sodium excretion rises.
The single most important caveat is diuretics. Loop and thiazide diuretics force sodium excretion by blocking tubular reabsorption directly, so a patient on either will show an elevated FENa regardless of true volume status. If diuretics have been given in the preceding hours, use the fractional excretion of urea instead, since urea reabsorption is largely unaffected by them.
A low FENa is also not synonymous with volume depletion even without diuretics. Hepatorenal syndrome, early urinary tract obstruction, contrast-induced nephropathy and some forms of acute glomerulonephritis can all present with an avidly sodium-reabsorbing kidney and a FENa below 1%, despite being intrinsic renal processes. The test also requires oliguria to be interpretable — in a patient making a normal urine volume, FENa adds little beyond the clinical assessment already in hand.
Frequently asked questions
What does a FENa below 1% mean?
It suggests a pre-renal pattern — the tubules are intact and conserving sodium appropriately in response to reduced renal perfusion, as in volume depletion, heart failure or cirrhosis.
Why is FENa invalid after diuretics?
Loop and thiazide diuretics force sodium excretion by blocking tubular reabsorption directly, raising FENa regardless of the patient’s true volume status. Use the fractional excretion of urea instead in that setting.
Can FENa be low in intrinsic kidney disease?
Yes. Hepatorenal syndrome, contrast nephropathy, early obstruction and some forms of glomerulonephritis can all produce a FENa below 1% despite being intrinsic renal processes, not volume depletion.
Does FENa work in a patient who isn't oliguric?
It requires oliguria to be clinically meaningful. In a patient with a normal urine output, FENa adds little useful information over the history and examination.
Related calculators
References
- Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–81.
- Diamond JR, Yoburn DC. Nonoliguric acute renal failure associated with a low fractional excretion of sodium. Ann Intern Med. 1982;96(5):597–600.