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Fractional Excretion Of Sodium (Fe Na) Calculator

Calculate fractional excretion of sodium (FENa) using urine and serum sodium and creatinine values to distinguish prerenal azotemia from acute tubular necrosis.

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Fractional Excretion of Sodium

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What Is the FENa Calculator?

The Fractional Excretion of Sodium (FENa) calculator computes the percentage of glomerularly filtered sodium that is ultimately excreted in the urine. Emergency physicians and nephrologists rely on the FENa calculator as a rapid, first-line bedside tool when evaluating acute kidney injury (AKI), because it distinguishes prerenal azotemia—where intact tubules aggressively conserve sodium in response to low perfusion—from intrinsic renal failure caused by acute tubular necrosis (ATN), where damaged tubules can no longer reclaim filtered sodium.

The FENa Formula

The calculation requires four values drawn simultaneously: two from a spot urine specimen and two from serum.

FENa (%) = (UNa × PCr) ÷ (PNa × UCr) × 100

Variable Definitions

  • UNa — Urine Sodium: Sodium concentration in a fresh spot urine specimen, expressed in mEq/L. In prerenal states, healthy tubules reabsorb the majority of filtered sodium, typically yielding a UNa below 20 mEq/L.
  • PNa — Serum Sodium: Plasma sodium concentration, with a physiologic normal range of 135–145 mEq/L.
  • UCr — Urine Creatinine: Creatinine concentration in the same spot urine specimen, reported in mg/dL. Higher values reflect a more concentrated urine.
  • PCr — Serum Creatinine: Plasma creatinine concentration, with a normal range of approximately 0.6–1.3 mg/dL. Elevated levels indicate impaired glomerular filtration.

Physiological Basis and Derivation

The kidneys filter roughly 180 liters of plasma daily. Creatinine is freely filtered at the glomerulus with negligible tubular reabsorption or secretion under normal conditions, making it an ideal filtration marker. The fraction of filtered sodium that is excreted equals the excreted sodium divided by the filtered sodium load. When both ratios are expressed using spot samples and creatinine clearance is substituted for GFR, the equation simplifies algebraically to the FENa formula—eliminating the burden of a 24-hour urine collection and allowing near-instantaneous bedside calculation.

How to Interpret FENa Results

  • FENa < 1%: Consistent with prerenal azotemia. Intact renal tubules are retaining over 99% of filtered sodium, a compensatory response to decreased effective circulating volume. Common etiologies include dehydration, hemorrhage, congestive heart failure, cirrhosis, and sepsis-related hypoperfusion. Volume resuscitation typically restores kidney function.
  • FENa 1%–2%: Indeterminate. Both prerenal and intrinsic causes may produce values in this range; clinical correlation with urine osmolality, urine microscopy, and hemodynamic status is essential.
  • FENa > 2%: Consistent with intrinsic renal failure, most commonly ATN. Tubular damage from ischemia, nephrotoxins (contrast agents, aminoglycosides, myoglobin), or sepsis impairs sodium reabsorption, allowing a greater fraction to reach the final urine.

Worked Clinical Example

A 67-year-old patient presents with 12 hours of oliguria following a hypotensive episode. Laboratory results: UNa = 14 mEq/L, PNa = 140 mEq/L, UCr = 105 mg/dL, PCr = 2.8 mg/dL.

FENa = (14 × 2.8) ÷ (140 × 105) × 100 = 39.2 ÷ 14,700 × 100 = 0.27%

A FENa of 0.27% strongly supports prerenal azotemia. Intravenous fluid resuscitation is initiated, and serum creatinine trends downward within 24 hours—confirming the diagnosis and avoiding unnecessary dialysis workup.

Known Limitations of FENa

Research published at the National Library of Medicine (PMC, 2022) emphasizes that FENa is an imperfect diagnostic tool with well-documented pitfalls. Loop diuretics such as furosemide block tubular sodium reabsorption, artificially elevating urinary sodium excretion and producing falsely high FENa values even when the underlying cause is prerenal. Similarly, radiocontrast nephropathy, myoglobinuric ATN, and early urinary obstruction can produce paradoxically low FENa values mimicking a prerenal picture. Metabolic alkalosis obligates bicarbonate excretion with sodium cotransport, also raising FENa without true tubular injury.

In diuretic-confounded clinical contexts, the Fractional Excretion of Urea (FEUrea, cutoff <35% for prerenal) offers a more reliable alternative. Additionally, clinical protocols for preterm neonates note that physiologically immature tubular function produces baseline FENa values far exceeding adult thresholds, requiring adjusted reference ranges for neonatal interpretation.

Best Practices for Accurate Measurement

Collect urine and blood specimens simultaneously—ideally within 30 minutes—to ensure both samples reflect the same hemodynamic state. Document any loop or thiazide diuretic use within the prior 6–12 hours, as this renders FENa unreliable. Enter urine and serum sodium in matching units (mEq/L) and urine and serum creatinine in matching units (mg/dL). Always interpret FENa alongside urine osmolality, urine sediment microscopy, clinical history, and volume status for the most accurate and actionable diagnosis.

Reference

Frequently asked questions

What is a normal FENa value and what does it indicate?
A FENa below 1% is considered the normal threshold in the setting of acute kidney injury, indicating that the kidneys are conserving sodium appropriately—a hallmark of prerenal azotemia. Intact tubules retain over 99% of filtered sodium. A FENa above 2% signals tubular dysfunction consistent with ATN, while values between 1% and 2% are indeterminate and always require correlation with clinical findings such as urine osmolality, blood pressure, and urine microscopy.
What does a FENa less than 1% mean clinically?
A FENa below 1% means the renal tubules are intact and actively reabsorbing sodium in response to decreased effective circulating blood volume. This pattern occurs with true dehydration, hemorrhage, congestive heart failure, hepatorenal syndrome, and early sepsis. The primary treatment is correcting the underlying perfusion deficit—usually with intravenous fluids or diuresis for heart failure—rather than treating the kidney itself, since tubular function is preserved.
When is FENa unreliable or misleading?
FENa is unreliable after recent administration of loop diuretics (e.g., furosemide) or thiazides, which block tubular sodium reabsorption and falsely elevate FENa even in volume-depleted patients. Additional confounders include contrast-induced nephropathy, myoglobinuric ATN, early urinary tract obstruction, and metabolic alkalosis—all of which can produce a low FENa despite intrinsic renal injury. In diuretic-treated patients, Fractional Excretion of Urea (FEUrea, cutoff below 35%) offers a substantially more accurate diagnostic alternative.
How do you collect samples for an FENa test?
Collect a random spot urine sample and a peripheral blood draw simultaneously—ideally within 30 minutes of each other—so both specimens reflect the same physiological state. The urine sample is analyzed for sodium and creatinine concentrations. The blood draw is sent for a basic metabolic panel to obtain serum sodium and serum creatinine. No 24-hour urine collection is required; the FENa formula algebraically corrects for urine concentration using the creatinine ratio.
What is the difference between FENa and FEUrea?
FENa expresses what percentage of filtered sodium is excreted, while FEUrea expresses what percentage of filtered urea is excreted—with a prerenal cutoff of below 35%. FEUrea is preferred when loop or thiazide diuretics have been administered within the preceding 6–12 hours, because those drugs impair tubular sodium reabsorption without significantly affecting urea handling. Published studies confirm that FEUrea retains its diagnostic accuracy in diuretic-treated patients where FENa becomes unreliable.
Can the FENa calculator be used for patients with chronic kidney disease?
Interpreting FENa in chronic kidney disease (CKD) requires significant caution. Advanced CKD patients have baseline tubular dysfunction and may chronically excrete a higher fraction of filtered sodium, producing elevated FENa values even in the absence of new ATN. The standard 1% and 2% thresholds may not apply. Clinicians should compare the current FENa against the patient's known stable baseline and evaluate trends in serum creatinine over time rather than relying on single-point absolute cutoffs.