This FENa calculator estimates the fractional excretion of sodium to determine the cause of renal failure, be it pre, post or intrinsic renal pathology. Discover more on this subject below the form.


Urine sodium [UNa]:*
Urine creatinine [UCr]:*
Plasma sodium [PNa]:*
Plasma creatinine [PCr]:*

How does this FENa calculator work?

This is a quick tool designed to calculate the fractional excretion of sodium. The fractional excretion of sodium is an indicator if renal failure is caused by a pre-renal, post-renal or intrinsic renal pathology. The calculator takes account of the urine sodium in mEq/L, urine creatinine in mg/dL, plasma sodium in mEq/L and plasma creatinine in mg/dL that should be provided.

The algorithm behind this FENa calculator uses the following formula:

How Much do I...

Please note that the calculations this tool produces are approximate and they cannot replace any professional medical investigation.

Example calculation:

Urine sodium [UNa]: 110 mEq/L

Urine creatinine [UCr]: 35 mg/dl

Plasma sodium [PNa]: 140 mEq/L

Plasma creatinine [PCr]: 1.5 mg/dl

The result is FENa =3.4% and the interpretation is intrinsic renal pathology

Definition and FENa levels interpretation

This is the acronym for fractional excretion of sodium which represents a percentage of the sodium filtered by the kidney which is excreted in the urine. FENa indicator is used in clinical investigations when evaluating the renal failure.

The levels of the FENa and what they suggest are presented below:

FENa level



Pre-renal pathology


Intrinsic renal pathology


Post-renal pathology

How is the test performed?

FENa itself is not a test but it based on urine and blood chemistry test results, the four values that are exemplified above. Both samples are collected at the same time and examined and there is no special preparation required for them, usually you will be able to eat as you normally do in the previous 24h.

However, if you are under treatment with any medication that might alter the test result (especially diuretics), as advised by your medical professional you will need to interrupt it temporarily.

The blood test is the usual blood draw in which you will feel the minor discomfort or stinging. Urine probes are usually taken when the amount has dropped to 500mL per day.


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

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

3) Carmody JB. (2011) Urine electrolytes Pediatr Rev; 32(2):65-8.

20 Jan, 2015