Espinel CH, JAMA 1976
In patients in the oliguric phase of acute renal failure, FENa was below 1% in prerenal azotemia and above 3% in acute tubular necrosis, clearly separating the two groups.
- Nephrology
- Acute kidney injury
FENa Calculator
Calculates the fractional excretion of sodium, to help tell prerenal acute kidney injury from tubular injury.
Result
Enter plasma and urine sodium and creatinine.
Sources
5 studies
5
studies
256
citations
1,990
PubMed papers
Evidence
The original studies, the meta-analysis of its accuracy and the studies of FEUrea after diuretics.
Known limitations
- Best studied in oliguric acute kidney injury; much less reliable without oliguria.
- Diuretics and chronic kidney disease raise FENa whatever the cause.
- A low FENa occurs in several intrinsic injuries, so it does not prove a prerenal cause.
- A single spot measurement; the clinical course and urine sediment matter more.
Key papers
Miller TR et al., Ann Intern Med 1978
In a prospective study of acute renal failure, many oliguric patients had urine sodium and osmolality in the overlap zone; the renal failure index and FENa reliably separated prerenal azotemia from tubular necrosis in that setting.
Ann Intern Med: Journal Metrics Score 4.6, Q1 · cited 427 times
Abdelhafez M et al., Clin J Am Soc Nephrol 2022
Of 19 studies (1,287 patients), the 15 that used a 1% cut-off (872 patients) gave a pooled sensitivity of 90% and specificity of 82%. In patients with chronic kidney disease or on diuretics specificity fell to 66%, and on diuretics alone to 54%; in oliguric patients without them it reached 95% and 91%.
Clin J Am Soc Nephrol: Journal Metrics Score 2.8, Q1 · cited 31 times
PubMed · Free full text · DOI
Carvounis CP et al., Kidney Int 2002
In 102 episodes of acute renal failure, only 48% of prerenal patients given diuretics had a FENa below 1%, while 89% had a FEUrea of 35% or less.
Kidney Int: Journal Metrics Score 12.3, Q1 · cited 337 times
Pépin MN et al., Am J Kidney Dis 2007
In 99 hospital patients with acute kidney injury, FENa (1% or less) separated transient from persistent injury better than FEUrea without diuretics (sensitivity 78%, specificity 75%); with diuretics neither test was accurate.
Am J Kidney Dis: Journal Metrics Score 1.2, Q3 · cited 143 times
The original paper has been cited 256 times, and 1,990 PubMed articles mention the fractional excretion of sodium in their title or abstract. Citation counts from OpenAlex and article counts from PubMed, retrieved November 11, 2026. Journal Metrics Scores and quartiles are from this site’s current release; follow the journal name for details.
After the result
Below 1% · Above 2% · Any result
- Below 1%
- Above 2%
- Any result
Look for reduced kidney perfusion. Look for tubular or other intrinsic injury. Rule out obstruction.
Next steps
Below 1%
- Assess volume status, blood pressure and recent losses, and review drugs that reduce kidney perfusion (NSAIDs, ACE inhibitors, angiotensin receptor blockers).
- Consider heart failure and cirrhosis, where the effective circulating volume is low despite fluid overload.
- Watch the creatinine after the cause is corrected: a prerenal injury usually improves within a day or two.
Above 2%
- First ask whether diuretics, chronic kidney disease or a salt-wasting state explain the high value.
- Check the urine sediment for muddy brown granular casts (tubular necrosis), red cell casts (glomerulonephritis) or white cell casts (interstitial nephritis).
- Review nephrotoxins and recent contrast, and check creatine kinase if rhabdomyolysis is possible.
Any result
- A bladder scan or kidney ultrasound to exclude urinary obstruction is part of the work-up whatever the FENa.
Avoid common errors
5 common errors to avoid
- 1Diuretics
- 2Chronic kidney disease
and 3 more
When FENa misleads
1Diuretics
A loop or thiazide diuretic in the last day or so raises FENa even when the cause is prerenal. In a meta-analysis, FENa specificity fell to 54% on diuretics.
2Chronic kidney disease
With fewer nephrons, each excretes more sodium, so baseline FENa is often above 1% in chronic kidney disease.
3Non-oliguric injury
The cut-offs were set in oliguric patients. In patients passing normal amounts of urine, FENa separates causes less well.
4Low FENa without volume depletion
Contrast nephropathy, rhabdomyolysis, sepsis, early obstruction, glomerulonephritis and the hepatorenal syndrome can all give a FENa below 1%.
5Mismatched samples
Urine and blood should be taken close together, and both creatinines must be in the same unit; this calculator converts if you enter them in different units.
Formula
(Urine Na × plasma creatinine) / (plasma Na × urine creatinine) × 100; below 1% prerenal, above 2% tubular.
(UNa × PCr) / (PNa × UCr)
<1%1–2%>2%
(Urine Na × plasma creatinine) / (plasma Na × urine creatinine) × 100; below 1% prerenal, above 2% tubular.
Formula and ranges
| Value | Formula |
|---|---|
| FENa (%) | (urine Na × plasma creatinine) / (plasma Na × urine creatinine) × 100 |
| FEUrea (%), for comparison | (urine urea × plasma creatinine) / (plasma urea × urine creatinine) × 100; 35% or less suggests a prerenal cause |
| Units | sodium in mmol/L (mEq/L); creatinine 1 mg/dL = 88.4 µmol/L, same unit for urine and plasma |
| Result | FENa | Meaning |
|---|---|---|
| Below 1%: prerenal pattern | below 1% | Below 1%: the tubules are holding on to sodium, the pattern of reduced kidney perfusion (prerenal acute kidney injury) when it is measured in acute kidney injury, especially with oliguria. |
| 1% to 2%: indeterminate | 1% to 2% | 1% to 2%: does not separate prerenal injury from tubular injury; weigh the history, urine sediment and response to fluids. |
| Above 2%: suggests tubular injury | above 2% | Above 2%: the tubules are losing sodium, as in acute tubular necrosis, unless diuretics, chronic kidney disease or another cause of salt wasting explains it. |
Espinel 1976
In oliguric acute renal failure, prerenal azotemia gave a FENa below 1% and acute tubular necrosis above 3%.
Why 2%
Espinel’s groups fell below 1% or above 3%, so the original study says nothing about values in between. The pooled accuracy figures use a single 1% cut-off. 2% is a teaching convention, not a validated threshold, so 1% to 2% is shown as indeterminate and values just above 2% need the same caution.
Fractional excretion of urea
Urea is reabsorbed passively, so FEUrea is less changed by diuretics. Carvounis found 35% or less in most prerenal patients given diuretics; Pépin found it lacked specificity.
Test cases
3 published cases, each also a test case
- 2.55%
- 0.13%
- 0.80%
3 published cases; the calculator reproduces every result.
Worked examples
Each example is also a test case: the calculator must reproduce the result the source reports before any change to it can be published.
Acute tubular necrosis after carfilzomib
2.55%A 60-year-old man with relapsed multiple myeloma developed acute kidney injury (creatinine 3.4 mg/dL from 0.8) after carfilzomib. Plasma sodium was 131 mmol/L, urine sodium 49 mmol/L and urine creatinine 49.8 mg/dL. The report gives a FENa of 2.55%; biopsy showed acute tubular necrosis.
Source and detailsHide details
The patient took furosemide as an outpatient. The authors judged a prerenal state unlikely because he had no signs of volume depletion and his FENa was above 2% about 24 hours after he last received it. The biopsy confirmed tubular injury.
Entered as: Plasma sodium: 131 mmol/L (mEq/L); Plasma creatinine: 3.4 mg/dL; Urine sodium: 49 mmol/L (mEq/L); Urine creatinine: 49.8 mg/dL; Diuretics or chronic kidney disease: Diuretic in the last 24 hours or so.
Source: Liberman V et al., Kidney Int Rep 2016 (PubMed · Free full text · DOI)
Prerenal injury and hyponatremia after biliary drainage
0.13%A 78-year-old man lost about 680 mL of bile a day through an external biliary drain and presented with weakness, a plasma sodium of 118 mmol/L and a creatinine of 1.29 mg/dL. Urine sodium was 8 mmol/L and urine creatinine 69 mg/dL. The report gives a FENa of 0.1% and attributes the kidney injury to a prerenal cause.
Source and detailsHide details
“with a fractional excretion of sodium (FENa) of 0.1%, was attributed to prerenal causes”
The calculator gives 0.127%, shown as 0.13%; the report rounds to 0.1%.
Entered as: Plasma sodium: 118 mmol/L (mEq/L); Plasma creatinine: 1.29 mg/dL; Urine sodium: 8 mmol/L (mEq/L); Urine creatinine: 69 mg/dL; Diuretics or chronic kidney disease: Neither.
Source: Galanis FC et al., Cureus 2026 (PubMed · Free full text · DOI)
Tea and toast (porridge) hyponatremia, low solute intake
0.80%A 68-year-old woman who for five days of vomiting had taken only unsalted rice porridge and black tea, and was taking hydrochlorothiazide, presented with a plasma sodium of 109.8 mmol/L and a creatinine of 0.59 mg/dL. Urine sodium was 14 mmol/L and urine creatinine 9.4 mg/dL, a very dilute urine. The report’s table gives a FENa of 0.8%.
Source and detailsHide details
Here FENa was part of a hyponatremia work-up, not acute kidney injury. The authors based the low-solute diagnosis on a urine osmolality of 71.9 mOsm/kg and did not interpret the FENa, which the thiazide may have raised.
Entered as: Plasma sodium: 109.8 mmol/L (mEq/L); Plasma creatinine: 0.59 mg/dL; Urine sodium: 14 mmol/L (mEq/L); Urine creatinine: 9.4 mg/dL; Diuretics or chronic kidney disease: Diuretic in the last 24 hours or so.
Source: Anandan S et al., Cureus 2025 (PubMed · Free full text · DOI)
FAQ
6 questions and answers
- QWhat is the FENa formula?
- QWhat does a FENa below 1% mean?
and 4 more answers
Questions
What is the FENa formula?
+
FENa (%) = (urine sodium × plasma creatinine) / (plasma sodium × urine creatinine) × 100. Both creatinines must be in the same unit. A plasma sodium of 131, plasma creatinine of 3.4 mg/dL, urine sodium of 49 and urine creatinine of 49.8 mg/dL give (49 × 3.4) / (131 × 49.8) × 100 = 2.55%.
What does a FENa below 1% mean?
+
The tubules are reabsorbing almost all filtered sodium, as they should when kidney perfusion is low. In acute kidney injury this points to a prerenal cause such as volume depletion, heart failure or cirrhosis. It is not specific: contrast nephropathy, rhabdomyolysis, sepsis, early obstruction and glomerulonephritis can also give a low FENa.
What does a FENa above 2% mean?
+
The tubules are losing sodium, which fits acute tubular necrosis or another intrinsic kidney injury. Espinel found values above 3% in oliguric patients with tubular necrosis. Diuretics, chronic kidney disease, salt-wasting states and adrenal insufficiency also raise FENa without tubular injury.
Can I use FENa after furosemide?
+
Not reliably. Loop and thiazide diuretics block sodium reabsorption and raise FENa for many hours. In a meta-analysis, specificity fell to 54% in patients on diuretics. The fractional excretion of urea (35% or less suggests a prerenal cause) is less affected, though studies disagree on how well it performs.
How accurate is FENa?
+
In a 2022 meta-analysis, the 15 studies that used a 1% cut-off (872 patients) gave a pooled sensitivity of 90% and specificity of 82% for intrinsic versus prerenal injury. It worked best in oliguric patients without chronic kidney disease or diuretics (95% and 91%) and poorly in those with them.
Should both samples be taken at the same time?
+
Yes, as close together as possible: a spot urine and a blood sample from the same hour. Ideally collect the urine before diuretics or a fluid bolus change the picture.
Further reading
8 references with PubMed and DOI links
8
references
4
free full text
From Espinel CH, JAMA 1976 to the newest case reports.
References
- Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581. PubMed · DOI
- Miller TR, Anderson RJ, Linas SL, Henrich WL, Berns AS, Gabow PA, Schrier RW. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50. PubMed · DOI
- Abdelhafez M, Nayfeh T, Atieh A, AbuShamma O, Babaa B, Baniowda M, 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. PubMed · Free full text · DOI
- 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. PubMed · DOI
- Pépin MN, Bouchard J, Legault L, Ethier J. Diagnostic performance of fractional excretion of urea and fractional excretion of sodium in the evaluations of patients with acute kidney injury with or without diuretic treatment. Am J Kidney Dis. 2007;50(4):566-573. PubMed · DOI
- Liberman V, D’Agati VD, Masani NN, Drakakis J, Mattana J. Acute tubular necrosis in a patient with myeloma treated with carfilzomib. Kidney Int Rep. 2016;1(2):89-92. PubMed · Free full text · DOI
- Galanis FC, Nikolaidis CG, Kyvetos AD, Boulmetis G, Vrettos I. Severe hypovolemic hyponatremia following percutaneous external biliary drainage in a patient with metastatic lung adenocarcinoma: a case report. Cureus. 2026;18(7):e113269. PubMed · Free full text · DOI
- Anandan S, Rajendran S, Rajan SS, Suresh A. Tea and porridge syndrome: a rare cause of severe hyponatremia. Cureus. 2025;17(8):e90051. PubMed · Free full text · DOI
MZ
About this page
Contributed by Dr. Meng Zhao (MD · Former Neurosurgeon · Medical AI Researcher), from the original publications listed here. Last updated November 11, 2026. LinkedIn
The calculator code is tested against every worked example on this page before each release.
This calculator supports, and does not replace, clinical judgment. It is meant for healthcare professionals and is not medical advice for patients. Report an error through the contact page.
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