FENa formula: how to calculate fractional excretion of sodium

Fractional excretion of sodium (FENa) is the share of filtered sodium that ends up in the urine, as a percent. With plasma sodium 140 mmol/L, plasma creatinine 2.0 mg/dL, urine sodium 20 mmol/L and urine creatinine 100 mg/dL, FENa = 100 × (20 × 2.0) ÷ (140 × 100) = 0.29%.

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Kidney calculators
Espinel CH · 1976
The FENa formula
Worked examples
Frequently asked questions

What is FENa?

FENa compares how much sodium the kidneys clear with how much creatinine they clear, from a blood sample and a urine sample. Creatinine stands in for filtration. Most filtered sodium is reabsorbed, so the number is usually small.

Magent's calculator follows C. H. Espinel, who described the test in JAMA in 1976. It returns the FENa to 2 decimal places and a label from Espinel's cutoffs: prerenal below 1%, atn (acute tubular necrosis) above 3%, and indeterminate in between.

Those cutoffs are Espinel's, from patients in the oliguric phase of acute renal failure. They aren't a universal rule, and they aren't a full system for classifying kidney injury.

The FENa formula

FENa (%) = 100 × (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine)

Exact expression the calculator uses:

FENa_percent = 100 * (urine_sodium_mmol_l * plasma_creatinine_mg_dl) / (plasma_sodium_mmol_l * urine_creatinine_mg_dl)

In words: multiply urine sodium by plasma creatinine, divide by plasma sodium times urine creatinine, then multiply by 100.

What each part means

Term	Plain-English meaning	Unit	Accepted range
`sodium_mmol_l`	Plasma (serum) sodium	mmol/L	110–160
`creatinine_mg_dl`	Plasma (serum) creatinine	mg/dL	0.1–20; give this or `creatinine_umol_l`
`creatinine_umol_l`	Plasma creatinine in SI units	µmol/L	9–1768, converted ÷ 88.42
`urine_sodium_mmol_l`	Urine sodium	mmol/L	1–300
`urine_creatinine_mg_dl`	Urine creatinine	mg/dL	1–500; give this or `urine_creatinine_umol_l`
`urine_creatinine_umol_l`	Urine creatinine in SI units	µmol/L	88–44210, converted ÷ 88.42
`fena_percent`	Fractional excretion of sodium	%	2 decimal places
`espinel_stratum`	Espinel label	text	`prerenal`, `indeterminate` or `atn`

How the formula works

Top: urine sodium × plasma creatinine.
Bottom: plasma sodium × urine creatinine.
Divide the top by the bottom and multiply by 100.
Round to 2 decimal places.
Label the rounded value: below 1 is prerenal, above 3 is atn, anything from 1 to 3 is indeterminate.

The label is set from the rounded number the calculator reports.

Units and conversions

Sodium goes in as mmol/L for both samples. Creatinine can go in as mg/dL or µmol/L for each sample; the calculator converts µmol/L to mg/dL by dividing by 88.42. Because creatinine sits on the top and the bottom of the fraction, the conversion cancels out when both creatinine values use the same unit.

A plasma creatinine of 176.84 µmol/L converts to 176.84 ÷ 88.42 = 2.0 mg/dL.
A urine creatinine of 8842 µmol/L converts to 8842 ÷ 88.42 = 100 mg/dL.

Worked examples

These use Magent's own expected values for the calculator. Plasma sodium is 140 mmol/L and plasma creatinine 2.0 mg/dL in each.

Example 1: urine sodium 20, urine creatinine 100

Top: 20 × 2.0 = 40
Bottom: 140 × 100 = 14,000
40 ÷ 14,000 × 100 = 0.2857…
FENa: 0.29%, label prerenal (below 1)

Example 2: urine sodium 60, urine creatinine 20

Top: 60 × 2.0 = 120
Bottom: 140 × 20 = 2,800
120 ÷ 2,800 × 100 = 4.2857…
FENa: 4.29%, label atn (above 3)

Example 3: urine sodium 40, urine creatinine 40

Top: 40 × 2.0 = 80
Bottom: 140 × 40 = 5,600
80 ÷ 5,600 × 100 = 1.4285…
FENa: 1.43%, label indeterminate (from 1 to 3)

Example 4: Example 1 with plasma creatinine in µmol/L

Plasma creatinine: 176.84 ÷ 88.42 = 2.0 mg/dL
The rest is unchanged: FENa = 0.29%, label prerenal

Example 5: Example 1 with urine creatinine in µmol/L

Urine creatinine: 8842 ÷ 88.42 = 100 mg/dL
The rest is unchanged: FENa = 0.29%, label prerenal

History of the formula

The test comes from C. H. Espinel, The FENa test. Use in the differential diagnosis of acute renal failure, JAMA, 1976 (volume 236, issue 6, pages 579–581). The abstract describes patients in the oliguric phase of acute renal failure, with FENa below 1 in prerenal azotemia and above 3 in acute tubular necrosis.

Miller and colleagues reported a prospective study of urinary indices, including the fractional excretion of filtered sodium, in acute renal failure in Annals of Internal Medicine in 1978.

Steiner wrote on interpreting FENa in the American Journal of Medicine in 1984. The abstract notes a low FENa in several states besides volume depletion, notes values above 1% with diuretics or chronic renal insufficiency even when volume depletion is present, and says the result has to be read in the clinical setting.

Pépin and colleagues compared FENa with the fractional excretion of urea in acute kidney injury, with and without diuretics, in the American Journal of Kidney Diseases in 2007.

When it's used

Espinel's cutoffs come from patients with oliguric acute renal failure. FENa is a ratio built from blood and urine values.

Where it's used

It needs four lab values: sodium and creatinine in plasma, and sodium and creatinine in urine.

Who uses it

Anyone with the four lab values can do the arithmetic. Reading the result depends on the clinical picture.

Limits of the calculation

The prerenal, indeterminate and atn labels use Espinel's 1976 cutoffs from oliguric acute renal failure. They aren't a classification of kidney injury.
Diuretics, chronic kidney disease and non-oliguric injury can change FENa. The calculator doesn't adjust for any of them.
The calculator uses Espinel's cutoffs only.
It needs blood and urine values from the same person, entered together.
This page doesn't say what a value means for a particular person.

Clinical decisions belong to a qualified clinician who knows the patient.

FEUrea uses the same structure with urea nitrogen in place of sodium.
Creatinine clearance estimates filtration from creatinine, age, weight and sex, not a sodium ratio.
eGFR estimates filtration rate from blood creatinine, not urine values.

Glossary

Prerenal: reduced kidney blood flow, upstream of the kidney itself.
ATN: acute tubular necrosis, injury to the kidney tubules.
Oliguric: passing a low volume of urine.
Fractional excretion: the percent of a filtered substance that leaves in the urine.
mmol/L, mg/dL, µmol/L: concentration units; creatinine converts µmol/L ÷ 88.42 to mg/dL.

Common mistakes

Swapping plasma and urine values. Urine sodium and plasma creatinine go on top.
Mixing creatinine units. Give each creatinine in mg/dL or µmol/L; the calculator converts µmol/L.
Reading the cutoffs as universal. Below 1% and above 3% are Espinel's numbers, from oliguric acute renal failure.
Ignoring diuretics. They change FENa, and the calculator doesn't adjust for them.
Forgetting the 100. Without multiplying by 100, the ratio isn't a percent.

Source

Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581. DOI: 10.1001/jama.1976.03270060029022. PMID: 947239

Frequently asked questions

How do you calculate FENa?

Multiply urine sodium by plasma creatinine, divide by plasma sodium times urine creatinine, and multiply by 100. Example 1 gives 0.29%.

What FENa means prerenal?

In Espinel's 1976 paper, below 1% went with prerenal azotemia and above 3% with acute tubular necrosis. The calculator uses those cutoffs for its labels. They come from oliguric acute renal failure and aren't universal.

Do diuretics affect FENa?

Yes. Diuretics can raise FENa, and the calculator doesn't adjust for them. FEUrea is a related calculation.

Can I enter creatinine in µmol/L?

Yes. The calculator divides µmol/L by 88.42 to get mg/dL, for plasma or urine creatinine.

How many decimals does the calculator report?

The label is set from that rounded value.