FENa calculator (fractional excretion of sodium)
Work out the fractional excretion of sodium in acute kidney injury: enter urine and plasma sodium and urine and plasma creatinine, in whichever units your lab reports, and see the FENa with Espinel's 1% and 3% cut-offs for prerenal injury and acute tubular necrosis.
Guides
- FENa vs FEUrea: which to use when the patient is on diureticsWhen the fractional excretion of urea is used instead of FENa in patients on diuretics: the formula, the 35% cut-off, the studies and two worked cases.
How this FENa calculator works
This FENa calculator works out what percentage of the sodium filtered by the glomeruli ends up in the urine. Creatinine stands in for the glomerular filtration rate, so the formula needs only four spot values: urine sodium, plasma sodium, urine creatinine and plasma creatinine, ideally from blood and urine collected at about the same time. Carlos Espinel described the test in JAMA in 1976 in patients in the oliguric phase of acute renal failure: those with prerenal azotemia had a FENa below 1%, and those with acute tubular necrosis had a FENa above 3%. In 1978, Miller and colleagues reported in the Annals of Internal Medicine that the FENa and the renal failure index reliably separated reversible prerenal azotemia from acute tubular necrosis when simpler urine tests fell in their grey zones. The idea is simple: a kidney that is underperfused but intact holds on to sodium, while injured tubules let it through.
FENa (%) = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100
Sodium in mmol/L, which is the same number as mEq/L. The two creatinines must be in the same unit before they cancel, so this calculator converts both to µmol/L first (1 mg/dL = 88.4 µmol/L, 1 mmol/L = 1,000 µmol/L). That matters because many labs report urine creatinine in mmol/L and plasma creatinine in µmol/L.
Worked example, the default values above: urine sodium 15 mmol/L, plasma sodium 138 mmol/L, urine creatinine 140 mg/dL and plasma creatinine 2.1 mg/dL give (15 × 2.1) ÷ (138 × 140) × 100 = 31.5 ÷ 19,320 × 100 = 0.16%. The same patient in SI units, urine creatinine 12.4 mmol/L and plasma creatinine 186 µmol/L, gives (15 × 186) ÷ (138 × 12,400) × 100 = 0.16%. The result is shown and compared to two decimals, so 1.00% and 3.00% fall in the middle band.
FENa results and what they mean
A FENa result falls into one of three bands, set by Espinel's 1976 data and repeated in the 2012 Cleveland Clinic Journal of Medicine review by Gotfried and colleagues. A FENa below 1% suggests a prerenal cause, meaning a fall in effective circulating volume or kidney perfusion with tubules that still reabsorb sodium normally. A FENa above 3% suggests an intrinsic cause, most often acute tubular necrosis, in which damaged tubules can no longer reclaim filtered sodium. Values from 1% to 3% sit between the two groups and do not point either way on their own. Some references quote 2% as the upper cut-off, but the original data used 3%, and Gotfried notes that cut-off values differ between studies. A euvolemic person with normal kidney function on a moderate salt intake has a FENa of about 1%, so the test only makes sense in a patient who already has acute kidney injury.
| FENa | Suggests | What it means |
|---|---|---|
| Below 1% | Prerenal AKI | Sodium is being held back, as in volume depletion or low kidney perfusion with intact tubules. |
| 1% to 3% | Indeterminate | Between the published cut-offs; weigh the urine sediment, urine sodium and clinical course. |
| Above 3% | Acute tubular necrosis | Damaged tubules fail to reabsorb filtered sodium, as in ischemic or toxic tubular injury. |
The calculator also shows the urine to plasma creatinine ratio. In Miller's 1978 study of acute oliguria, a ratio above 40 pointed to prerenal azotemia and one below 20 to acute tubular necrosis, alongside a urine sodium below 20 or above 40 mmol/L. Because the kidney concentrates creatinine in the urine, a ratio below 1 almost always means one creatinine was entered in the wrong unit, so the calculator stops and asks you to check.
When the FENa is less reliable
The FENa is less reliable whenever something other than tubular injury changes how much sodium the kidney excretes. Diuretics are the most common problem: they block sodium reabsorption directly, so a patient with prerenal injury can have a FENa above 1%. In Carvounis's 2002 study, 92% of patients with prerenal azotemia had a FENa below 1%, but only 48% of those who had received diuretics did. Steiner's 1984 review adds chronic kidney disease and a high salt intake as causes of a raised FENa. The opposite error, a low FENa in an intrinsic injury, has been reported in contrast nephropathy, rhabdomyolysis and other pigment nephropathy, acute glomerulonephritis, acute urinary obstruction, hepatorenal syndrome, heart failure, sepsis, renal transplant rejection and some nonoliguric acute tubular necrosis, as Steiner and Zarich and colleagues list. Both reviews conclude that the FENa must be read with the clinical setting and other tests, never on its own.
The 2022 meta-analysis by Abdelhafez and colleagues pooled 19 studies with 1,287 patients and concluded that the FENa has a limited role in patients with chronic kidney disease or on diuretics and is most valuable when oliguria is present. For patients on diuretics, the fractional excretion of urea is often suggested instead; the guide to FENa vs FEUrea explains when it helps and what the newer evidence shows.
Questions
What is the FENa formula?
FENa (%) = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. It compares the sodium cleared by the kidney with the creatinine cleared, which stands in for the glomerular filtration rate.
Can I use mg/dL for one creatinine and µmol/L for the other?
Yes, with this calculator. Pick each creatinine's unit and it converts both to µmol/L before dividing. Done by hand, both creatinines must be in the same unit, and both sodiums too.
Does the FENa work if the patient is on diuretics?
Less well. Diuretics raise urinary sodium excretion, so a FENa above 1% does not rule out a prerenal cause. In Carvounis's 2002 study, only 48% of prerenal patients on diuretics had a FENa below 1%. See FENa vs FEUrea for the urea alternative.
What is a normal FENa?
About 1% in a euvolemic person with normal kidney function on a moderate salt intake, and higher after a salt load. The 1% and 3% cut-offs apply to patients with acute kidney injury, not to healthy people.
Is my data sent anywhere?
No. The calculation runs in your browser, and the values you enter are not sent or stored.
Sources
- Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581. doi:10.1001/jama.236.6.579
- 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. doi:10.7326/0003-4819-89-1-47
- Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77(4):699–702. doi:10.1016/0002-9343(84)90368-1
- Zarich S, Fang LS, Diamond JR. Fractional excretion of sodium: exceptions to its diagnostic value. Arch Intern Med. 1985;145(1):108–112. PubMed 3970621
- 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. doi:10.1046/j.1523-1755.2002.00683.x
- Gotfried J, Wiesen J, Raina R, Nally JV Jr. Finding the cause of acute kidney injury: which index of fractional excretion is better? Cleve Clin J Med. 2012;79(2):121–126. doi:10.3949/ccjm.79a.11030
- Abdelhafez M, Nayfeh T, Atieh A, et al. Diagnostic performance of fractional excretion of sodium for the differential diagnosis of acute kidney injury: a systematic review and meta-analysis. Clin J Am Soc Nephrol. 2022;17(6):785–797. doi:10.2215/CJN.14561121