FENa Calculator

Determines the fractional excretion of sodium which tells whether renal failure is of pre, post or intrinsic pathology.

In the text below the tool you can find more information about how the FENa value is interpreted.

The FENa calculator determines the fractional excretion of sodium, which is a determination based on urine and plasma sodium and creatinine values.

FENa offers information about the kidney pathology that is likely to have caused or to cause renal failure, be it pre-, post- or intrinsic renal.

Fractionary excretion of sodium can be calculated as follows:

FENa = (UNa x PCr / PNa x UCr) x 100

The three types of possible results are summarized below:

FENa level Interpretation
<1% Pre-renal pathology
1-4% Intrinsic renal pathology
>4% Post-renal pathology

Urine sodium [UNa]:*
Urine creatinine [UCr]:*
Plasma sodium [PNa]:*
Plasma creatinine [PCr]:*
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


Variables and formula

The FENa calculator provides information on the possible renal pathology (pre-, post-, intrinsic) that caused or might cause renal failure based on sodium and creatinine from blood and urine.

The sodium values are input in mEq/L whilst the creatinine values are input in mg/dL.

The following formula is then employed:

FENa = (UNa x PCr / PNa x UCr) x 100

The four parameters are collected through a blood chemistry test and urine sample that are collected at the same time and examined straight away.

There are no specific dietary requests before providing the samples, however, patients who are under medication (especially diuretics) may be required to interrupt medication temporarily.

For most patients on diuretics, FEUrea may be a more suitable determination.

The determination may not be indicated for patients with known:

■ Chronic kidney disease;

■ Urinary tract obstruction;

■ Acute glomerular disease.


FENa interpretation

FENa represents the percentage of sodium that is filtered by the kidneys and then excreted in urine.

Because it is dependent on the level of filtration, it offers indirect information about the renal function.

The specific cut offs for FENa according to the three possible causes of renal failure are summarized in the table below:

FENa level Interpretation Conditions
<1% Pre-renal pathology Hypovolemia, heart failure, renal artery stenosis, sepsis
1-4% Intrinsic renal pathology Acute tubular necrosis, AIN, glomerulonephritides
>4% Post-renal pathology Obstructions such as bladder stones, bilateral ureteral obstruction, benign prostatic hyperplasia

In the original study by Espinel, the determination of excreted fraction of the filtered sodium, was performed in patients in the oliguric phase of acute renal failure. The following were found:

■ Patients with prerenal azotemia had an FENa of less than 1;

■ Patients with acute tubular necrosis had an FENa of more than 3.

Please note that the calculation provided is of informational purpose and should not replace specialist diagnostic.


Original source

Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976; 236(6):579-81.

Other references

1. 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.

2. Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984; 77(4):699-702.

3. Pahwa AK, Sperati CJ. Urinary fractional excretion indices in the evaluation of acute kidney injury. J Hosp Med. 2016; 11(1):77-80.

4. Zarich S, Fang LS, Diamond JR. Fractional excretion of sodium. Exceptions to its diagnostic value. Arch Intern Med. 1985; 145(1):108-12.

Specialty: Nephrology

System: Urinary

Objective: Determination

Type: Calculator

No. Of Variables: 4

Year Of Study: 1976

Abbreviation: FENa

Article By: Denise Nedea

Published On: June 16, 2017

Last Checked: June 16, 2017

Next Review: June 16, 2023