π§ͺ FENa Calculator
Prerenal or tubular? The urine sodium ratio decides.
For informational purposes only, not medical advice. Consult a healthcare professional.
Fractional excretion of sodium answers the core AKI question: under 1% points to prerenal causes, over 2% to acute tubular necrosis. FENa = (urine Na Γ plasma creatinine) / (plasma Na Γ urine creatinine) Γ 100. Urine Na 10, plasma Na 140, urine Cr 100, plasma Cr 2.0 gives 0.14%, a classic prerenal pattern. Enter your labs.
How to Use This Calculator
FENa needs simultaneous blood and spot urine measurements: sodium and creatinine in both. No 24-hour collection required.
Enter the urine sodium (mEq/L) and plasma sodium from the same time period. Low urine sodium is the prerenal signature.
Enter urine and plasma creatinine (mg/dL). The creatinine ratio corrects for how concentrated the urine is.
Read the FENa percent and band. Under 1%: resuscitate perfusion. Over 2%: think tubular injury. Between: use clinical context and consider FeUrea.
The Formula
The formula is a clearance ratio: sodium clearance divided by creatinine clearance. Creatinine clearance approximates GFR, so the ratio is the fraction of filtered sodium that escapes into the urine. Healthy tubules reabsorb over 99% of filtered sodium, especially when perfusion drops and aldosterone surges, which is why prerenal states show FENa under 1%. Injured tubules cannot reabsorb, so sodium spills and FENa exceeds 2%. The creatinine terms cancel out urine concentration, which is why a spot sample works: it does not matter whether the urine is dilute or concentrated, only the ratios.
Worked Example
Prerenal case: UNa 10, PNa 140, UCr 100, PCr 2.0. FENa = (10 Γ 2.0)/(140 Γ 100) Γ 100 = 20/14000 Γ 100 = 0.14%. The tubules are avidly conserving sodium; give fluids (if appropriate) and find why perfusion fell. ATN case: UNa 60, PNa 140, UCr 40, PCr 3.0. FENa = (60 Γ 3.0)/(140 Γ 40) Γ 100 = 180/5600 Γ 100 = 3.21%. The tubules are wasting sodium; this is intrinsic injury, and fluids will not fix dead tubules. I verified both against the formula. Note the urine sodium alone hints the answer (10 vs 60), but the creatinine correction makes it rigorous.
Reading the Bands
| FENa | Pattern | Usual causes |
|---|---|---|
| Under 1% | Prerenal | Volume depletion, hypotension, heart failure, cirrhosis, NSAIDs, obstruction |
| 1 - 2% | Gray zone | Diuretics, early ATN, mixed, contrast injury evolving |
| Over 2% | Intrinsic (ATN) | Ischemia, nephrotoxins (aminoglycosides, contrast), sepsis, rhabdomyolysis |
The bands guide the first move: prerenal gets perfusion fixed (fluids, pressors, hold the ACE inhibitor), ATN gets the toxin removed and supportive care while tubules recover over days to weeks. But FENa is one data point in the AKI workup, alongside the history (hypotension? new drugs?), the urine sediment (muddy brown casts in ATN, bland in prerenal), and ultrasound (obstruction). A FENa of 0.3% in a patient on diuretics means nothing, which brings us to the caveats. For related kidney math, our GFR calculator stages the chronic side, our FeUrea calculator is the diuretic-proof alternative, and our HCT/Hgb converter helps read the anemia that often accompanies AKI.
When FENa Lies
Diuretics are the big one: loop and thiazide diuretics force sodium excretion, pushing FENa up even in prerenal states. If diuretics were given, use FeUrea instead (urea handling resists diuretics better). Adrenal insufficiency and salt-wasting states raise FENa without tubular injury. Bicarbonaturia (vomiting, NG suction) drags sodium with it. Chronic kidney disease blunts the test's discrimination. And early obstruction can look prerenal before becoming intrinsic. The rule: FENa interprets the tubules only when nothing else is forcing sodium out.
Beyond the Number: The AKI Workup
Order of operations in suspected AKI: review medications (NSAIDs, ACE-I, diuretics, aminoglycosides, contrast), assess volume status (orthostatics, mucous membranes, daily weights), check bladder (retention is the reversible cause people miss; bladder scan), get the urinalysis with microscopy, and send the spot urine lytes for FENa/FeUrea. Ultrasound rules out obstruction. Then act on the pattern: prerenal gets volume, obstruction gets drained, ATN gets time and toxin removal. Nephrology gets consulted for severe, unclear, or non-recovering AKI, or any hint of glomerulonephritis (RBC casts, proteinuria) which is a different emergency entirely.
FENa in Heart Failure and Cirrhosis
Two common scenarios produce very low FENa without volume depletion: decompensated heart failure and cirrhosis with ascites. In both, arterial underfilling from poor cardiac output or splanchnic vasodilation triggers the same aldosterone-driven sodium avidity as bleeding, so FENa reads prerenal while the patient is visibly volume-overloaded. Treating these as simple dehydration with aggressive fluids causes harm; the "prerenal" pattern here means the kidneys perceive underperfusion despite total-body volume excess. Diuretics, the actual treatment, then invalidate the FENa, which is why FeUrea and the clinical picture take over. Context always outranks the number.
Draw the blood and urine samples close together in time. A FENa built from morning blood and afternoon urine after fluids and diuretics have been given can mislead; simultaneity is part of the test.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is FENa?
Fractional excretion of sodium: the percentage of filtered sodium excreted in urine, calculated from spot urine and plasma sodium and creatinine. It distinguishes prerenal AKI (<1%) from acute tubular necrosis (>2%).
How do you calculate FENa?
FENa = (urine Na Γ plasma creatinine) / (plasma Na Γ urine creatinine) Γ 100. All from simultaneously drawn blood and spot urine samples; no 24-hour collection needed.
What does FENa under 1% mean?
A prerenal pattern: working tubules conserving sodium avidly because perfusion is down. Causes include volume depletion, hypotension, heart failure, and obstruction. Treatment targets perfusion, not the kidney itself.
What does FENa over 2% mean?
An intrinsic pattern, usually acute tubular necrosis: injured tubules cannot reabsorb sodium. Causes include ischemia, nephrotoxins, sepsis, and rhabdomyolysis. Management is supportive while tubules recover.
Can you use FENa if the patient took diuretics?
No, not reliably: diuretics force sodium excretion and falsely raise FENa. Use fractional excretion of urea (FeUrea) instead, which stays interpretable after diuretics.
What is the difference between FENa and FeUrea?
Both are clearance ratios distinguishing prerenal from ATN. FENa uses sodium; FeUrea uses urea. FeUrea (<35% prerenal, >50% ATN) remains valid after diuretics, making it the choice in diuretic-treated patients.
Do you need a 24-hour urine for FENa?
No. A spot urine sample drawn near the blood sample suffices, because the creatinine ratio corrects for urine concentration. That bedside convenience is the test's main virtue.
What urine sodium alone suggests prerenal?
Urine Na under 20 mEq/L suggests prerenal; over 40 suggests ATN. But urine sodium alone is fooled by concentration, which is why FENa's creatinine correction is preferred.
Can FENa diagnose glomerulonephritis?
No. FENa addresses the prerenal vs ATN question. Glomerulonephritis is suggested by RBC casts, proteinuria, and active urine sediment, and needs urgent nephrology input regardless of FENa.
How fast do ATN tubules recover?
Typically days to weeks, sometimes requiring temporary dialysis support. The recovery phase often includes a diuretic phase with large urine volumes; creatinine lags behind tubular recovery.