FENa Calculator

Use the FENa Calculator

Free FENa calculator to find the fractional excretion of sodium from urine and serum sodium and creatinine — separate prerenal AKI from ATN, with FEUrea mode.

How to Use FENa Calculator

1. Step 1: Enter serum values

   Type the serum sodium and serum creatinine from the chemistry panel, using the current AKI creatinine rather than the patient’s baseline.

2. Step 2: Enter spot urine values

   Add urine sodium and urine creatinine from the same spot urine sample, ideally collected before fluids or diuretics were given.

3. Step 3: Pick your lab units

   Choose US (mg/dL) or SI (µmol/L serum, mmol/L urine) — the calculator converts creatinine automatically so the ratio stays valid.

4. Step 4: Flag diuretic use

   Check the loop diuretic box if furosemide or similar was given in the last 24–48 hours, then enter BUN and urine urea to compute FEUrea instead.

5. Step 5: Read the classification

   FENa under 1% suggests a prerenal pattern and over 2% suggests ATN — review the false-low and false-high pitfalls panel before acting on it.

Key Features

• FENa from spot urine and serum sodium and creatinine
• Instant classification at the 1% and 2% AKI cutoffs
• FEUrea mode for patients on loop diuretics (35%/50% cutoffs)
• US and SI units with automatic creatinine conversion
• Supportive indices: urine/serum creatinine and BUN:creatinine ratios
• Built-in false-low and false-high pitfall reference

Understanding Results

Formula

FENa = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100. It is the ratio of sodium clearance to creatinine clearance, expressed as the percentage of filtered sodium that ends up in the urine. The creatinine terms cancel out water reabsorption, which is why FENa beats a raw urine sodium. In diuretic mode the tool applies the same structure to urea: FEUrea = (urine urea × serum creatinine) ÷ (serum urea × urine creatinine) × 100.

Reference Ranges & Interpretation

In oliguric AKI, FENa below 1% indicates a prerenal pattern (intact, sodium-avid tubules), 1–2% is indeterminate, and above 2% indicates intrinsic injury, classically acute tubular necrosis — thresholds from Espinel's original JAMA 1976 validation. On loop diuretics, use FEUrea instead: below 35% suggests prerenal azotemia and above 50% suggests ATN, with 85% sensitivity and 92% specificity in the Carvounis 2002 series of 102 patients.

Assumptions & Limitations

The cutoffs assume oliguric AKI, no diuretics within 24–48 hours, and previously normal kidneys. FENa reads falsely low (under 1% despite intrinsic injury) in contrast nephropathy, rhabdomyolysis, acute glomerulonephritis, hepatorenal syndrome, and early sepsis; it reads falsely high in CKD, bicarbonaturia from vomiting, and adrenal insufficiency. This is a clinical decision-support and education tool — interpretation and treatment belong with the treating clinician.

Complete Guide: FENa Calculator

Every FENa calculatorin clinical use is built on a study of exactly 17 patients. Carlos Espinel published the fractional excretion of sodium test in JAMA in 1976, and in his small oliguric cohort it separated prerenal azotemia from acute tubular necrosis almost perfectly: below 1% meant the kidneys were underperfused but intact, above 3% meant the tubules themselves were damaged. Fifty years later, that cutoff gets applied to patients Espinel deliberately excluded — people on furosemide, people with CKD, people making plenty of urine — and that's where FENa earns its reputation as the most misused number in nephrology. This guide works the formula, shows how one diuretic dose flips the result, and maps every major false positive and false negative so you know exactly when the percentage means what it claims to mean.

The 1% rule rests on 17 patients

Espinel's original series enrolled 17 oliguric patients — urine output under 500 mL/day — with a clear clinical diagnosis of either prerenal failure or acute tubular necrosis. Nobody was on diuretics. Nobody had chronic kidney disease. In that narrow population the separation was striking: every prerenal patient had FENa under 1%, every ATN patient over 3%. Miller and colleagues confirmed the pattern two years later in a 102-patient series, and the test entered textbooks as near-gospel.

The catch is selection. Modern AKI rarely arrives that cleanly — roughly half of hospital-acquired cases involve diuretics, sepsis, contrast exposure, or pre-existing CKD, and each of those breaks one of Espinel's entry criteria. The number still works, but only inside the box it was validated in: oliguric AKI, no diuretics, previously normal kidneys. Outside the box it doesn't degrade gracefully — it points confidently in the wrong direction.

What does a FENa calculator actually measure?

FENa answers one physiologic question: of all the sodium the glomeruli filter, what percentage escapes into the final urine? A healthy adult filters roughly 25,000 mmol of sodium a day and excretes 100–200 mmol of it — a fractional excretion of about 0.5–1%. The tubules reclaim more than 99% of the filtered load, and how hard they work at it is under direct hormonal control. When perfusion drops — hemorrhage, vomiting, heart failure — angiotensin II and aldosterone push reabsorption even harder, and FENa falls below 1%. When the tubular cells themselves are injured, they physically cannot reclaim sodium, and FENa climbs above 2% no matter what the hormones demand.

The formula is a ratio of two clearances: FENa = (urine Na × serum creatinine) ÷ (serum Na × urine creatinine) × 100. The creatinine terms are the clever part. A raw urine sodium of 25 mmol/L could mean sodium-avid kidneys making concentrated urine or leaky tubules making dilute urine — you can't tell from the sodium alone. Dividing by the urine-to-serum creatinine ratio corrects for how much water the kidney reabsorbed, which is why FENa outperforms urine sodium read in isolation. It also makes the test immune to units: because everything enters as a ratio, the result is identical whether your lab reports creatinine in mg/dL or µmol/L, so long as serum and urine use consistent units — our calculator handles that conversion automatically.

One dose of furosemide, two different answers

Here's the sensitivity problem in numbers. A 68-year-old arrives after three days of vomiting. Creatinine has risen from a baseline of 1.0 to 2.4 mg/dL, serum sodium is 134 mmol/L, spot urine sodium is 12 mmol/L, urine creatinine is 88 mg/dL:

Now suppose the admitting team gave 80 mg of IV furosemide an hour before the urine was collected. Furosemide blocks the Na-K-2Cl transporter in the loop of Henle, forcing sodium out regardless of volume status. Urine sodium jumps to 65 mmol/L and urine creatinine falls to 45 mg/dL as the urine dilutes:

Nothing about the kidneys changed between those two calculations. One drug dose moved the result from 0.24% to 2.59% — a tenfold shift across both diagnostic thresholds. This is why the calculator asks about loop diuretics up front, and why FEUrea exists as the fallback: in the same scenario, a BUN of 42 mg/dL and urine urea nitrogen of 260 mg/dL give FEUrea = (260 × 2.4) ÷ (42 × 45) × 100 = 33% — still correctly under the 35% prerenal cutoff, because furosemide doesn't touch urea handling.

Five conditions that fake a prerenal FENa

A FENa under 1% is routinely read as "give fluid," but several intrinsic kidney diseases produce exactly the same number. The common thread is intense renal vasoconstriction or an intact-tubule injury pattern — the sodium-retaining machinery still runs even though the kidney is genuinely damaged. Up to 15% of ATN cases, particularly non-oliguric ones, present with FENa below 1%.

The practical consequence: a low FENa in a patient with a compatible story — recent angiography, a CK in the tens of thousands, an active urine sediment, decompensated cirrhosis — should never close the case. In hepatorenal syndrome especially, chasing a FENa of 0.1% with saline delays the albumin challenge and vasoconstrictor therapy the patient actually needs.

Four ways FENa reads high without tubular injury

The reverse error matters just as much, because a "high" FENa can talk a team out of fluid a dry patient needs. Diuretics are the classic culprit — the effect lingers 24–48 hours after the last dose. Chronic kidney disease is subtler. A person with a GFR of 20 mL/min has lost 80% of their nephrons but eats the same 100–200 mmol of sodium daily; each surviving nephron must excrete five times its usual share, so the baseline FENa in advanced CKD often sits above 2% before anything acute happens. Staging that baseline properly is what our CKD calculatoris for — and it's why FENa thresholds simply don't transfer to acute-on-chronic kidney injury.

Two rarer offenders round out the list. Bicarbonaturia — typically from vomiting with metabolic alkalosis — forces sodium into the urine as the obligate cation accompanying bicarbonate, so a genuinely volume-depleted vomiting patient can post a FENa above 2%. (Urine chloride under 20 mmol/L gives the game away, and if that same patient is hyponatremic, correct the sodium slowly — our sodium correction calculatorwalks through the rate limits.) Adrenal insufficiency does it hormonally: without aldosterone, the distal nephron can't conserve sodium regardless of volume status.

FENa vs. FEUrea: which index when?

FEUrea applies the same fractional-excretion logic to urea: (urine urea × serum creatinine) ÷ (serum urea × urine creatinine) × 100. Urea is reabsorbed mainly in the proximal tubule — upstream of the thick ascending limb where loop diuretics act — so furosemide leaves it largely untouched. In the Carvounis series of 102 ICU patients, FEUrea below 35% identified prerenal azotemia with 85% sensitivity and 92% specificity, and it kept discriminating in the diuretic-treated group where FENa failed outright.

Neither index is a tiebreaker for the other — they fail in different places. FEUrea misleads during osmotic diuresis (uncontrolled hyperglycemia, mannitol) and in patients on high-protein feeds, where urea delivery overwhelms reabsorption. When the two disagree, the answer is almost never "average them" — it's to go back to the urine sediment and the volume exam.

When to trust the number — and when to ignore it

A usable bedside rule: FENa deserves weight only when all four of Espinel's conditions still hold. First, true AKI by KDIGO criteria — a creatinine rise of at least 0.3 mg/dL within 48 hours or 1.5× baseline within 7 days. Second, oliguria — the cutoffs were never validated in patients making 2 L of urine a day. Third, no diuretic within 48 hours (otherwise switch to FEUrea). Fourth, no significant baseline CKD. Check those boxes and a FENa of 0.3% or 4% is genuinely informative; miss two of them and the number is closer to noise.

Even inside the box, FENa is one vote among several. Muddy brown granular casts on microscopy push hard toward ATN whatever the percentage says. A urine-to-serum creatinine ratio above 40:1 supports prerenal physiology; below 20:1 supports tubular injury. And the response to a 500 mL crystalloid challenge over 30–60 minutes remains the most honest test of volume responsiveness ever devised. In practice, most nephrologists treat FENa as a way to quantify suspicion, not settle it.

What to do with the result

A prerenal pattern earns a perfusion fix: hold diuretics and nephrotoxins (NSAIDs, ACE inhibitors, ARBs), give measured fluid, and expect creatinine to start falling within 24–72 hours if the read was right. An ATN pattern shifts the goal to protection — strict avoidance of further insults, daily weights and electrolytes, and renal dosing of every cleared drug. That last step is where the companion tools come in: dose adjustments key off estimated clearance, so run the numbers through our creatinine clearance calculator — remembering that during unsteady-state AKI, any equation based on a single creatinine (including the CKD-EPI formula behind our eGFR calculator) overestimates true function while creatinine is still climbing.

Most ATN recovers: tubular cells regenerate over 1–3 weeks if the kidney is kept out of harm's way. The number to watch isn't the FENa trending back down — it's urine output climbing and creatinine turning the corner. If neither happens within a week, or potassium and acidosis become unmanageable, that's the point where dialysis conversations start, and a percentage from a spot urine no longer has anything to add.

References

1. Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581
2. Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223–2229
3. Kidney Disease: Improving Global Outcomes (KDIGO). Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2(1):1–138
4. Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47–50

Frequently Asked Questions