For healthcare professionals
FENa Calculator (Fractional Excretion of Sodium)
The fractional excretion of sodium (FENa) is the share of filtered sodium that ends up in the urine. In acute kidney injury, a value under 1% traditionally points to a prerenal (transient) cause and a value over 1% to an intrinsic (persistent) cause such as acute tubular necrosis. This tool is written for clinicians and is not a diagnosis.
Laboratory values
Use a spot urine sample and a blood sample taken at about the same time. Nothing you enter leaves this page.
What you will see
The FENa as a percentage, the working, and how the value reads against the 1% cut point.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
The formula as printed by Vanmassenhove J and colleagues, Critical Care 2013, and the 1% reference value from the review by Lima C and Macedo E, Disease Markers 2018, which traces it to Espinel 1976. Both papers are open access (CC BY). The tests check the formula, the 1% cut point and that the creatinine unit cancels out.
Validated in
Adults with acute kidney injury. The cut point comes from early studies of acute renal failure (Espinel 1976, as described by Lima and Macedo 2018). Vanmassenhove 2013 studied 107 consecutive adults with sepsis in one intensive care unit in Belgium.
Limits
FENa is a ratio, not a validated risk score. Diuretics raise urinary sodium and can make FENa high without tubular damage (Vanmassenhove 2013). In early sepsis most patients had a FENa under 1%, so a low value does not rule out kidney injury there (Vanmassenhove 2013). FENa has not been consistently shown to match clinical and biopsy findings (Ostermann 2016). A low FENa is also described in acute glomerulonephritis (Lima 2018).
The formula
FENa (%) = (urine sodium x plasma creatinine) / (plasma sodium x urine creatinine) x 100 (Vanmassenhove 2013) (Lima 2018). Both creatinine values must be in the same unit; the unit then cancels out, so mg/dL and umol/L give the same result.
Worked example: urine sodium 20 mmol/L, plasma creatinine 2.0 mg/dL, plasma sodium 140 mmol/L and urine creatinine 100 mg/dL give (20 x 2.0) / (140 x 100) x 100 = 0.29%, under 1%.
Reading the result
- Under 1%: the traditional pattern of transient (prerenal) acute kidney injury, where working tubules reabsorb sodium in response to reduced kidney perfusion (Lima 2018).
- Over 1%: the traditional pattern of persistent (intrinsic) acute kidney injury, consistent with sodium loss from tubular damage (Lima 2018).
Lima and Macedo describe early work by Espinel and others in which FENa was over 1% in oliguric and nonoliguric acute tubular necrosis and in urinary tract obstruction, and under 1% in prerenal azotemia and acute glomerulonephritis (Lima 2018) (Espinel 1976).
What the studies found
In 107 adults with sepsis, 75% had a FENa under 1% and half had a value under 0.5%, so the authors suggest the 1% cut point may be too high in sepsis (Vanmassenhove 2013). They also note that a high FENa can reflect diuretic use as well as tubular damage, and so cannot be read on its own (Vanmassenhove 2013). A review notes that urinary electrolytes and FENa have not been consistently shown to correlate with clinical and biopsy findings, and that low FENa values are seen in the first hours of sepsis (Ostermann 2016). Lima and Macedo describe the fractional excretion of urea as possibly more useful than FENa for this question (Lima 2018).
Related tools
For patients on diuretics, the FEUrea calculator uses urea in place of sodium. For a high potassium, the TTKG calculator estimates collecting duct potassium secretion. The BUN to creatinine ratio calculator needs only blood values. To estimate kidney function from creatinine, see the eGFR calculator and the creatinine clearance calculator. For a metabolic acidosis, the anion gap calculator also gives the albumin-corrected gap. The serum osmolality calculator uses sodium, glucose and BUN. Other bedside scores are listed under For healthcare professionals.
Sources
- Vanmassenhove J, Glorieux G, Hoste E, Dhondt A, Vanholder R, Van Biesen W. Urinary output and fractional excretion of sodium and urea as indicators of transient versus intrinsic acute kidney injury during early sepsis. Crit Care. 2013;17(5):R234. doi:10.1186/cc13057. PMID 24119730. Open access (CC BY). Methods (formula; 107 consecutive sepsis patients) and Discussion (75% under 1%; diuretics). pmc.ncbi.nlm.nih.gov/articles/PMC4057160/
- Lima C, Macedo E. Urinary biochemistry in the diagnosis of acute kidney injury. Dis Markers. 2018;2018:4907024. doi:10.1155/2018/4907024. PMID 30008975. Open access (CC BY). Table 1 (formula; under 1% transient, over 1% persistent AKI) and text (Espinel; glomerulonephritis; obstruction; fractional excretion of urea). pmc.ncbi.nlm.nih.gov/articles/PMC6020498/
- Ostermann M, Joannidis M. Acute kidney injury 2016: diagnosis and diagnostic workup. Crit Care. 2016;20(1):299. doi:10.1186/s13054-016-1478-z. PMID 27670788. Open access (CC BY). Text (urinary electrolytes and FENa; low FENa in early sepsis). pmc.ncbi.nlm.nih.gov/articles/PMC5037640/
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.