Fractional Excretion of Sodium (FENa)

What FENa measures and when it is used

FENa measures what part of the sodium filtered by the glomeruli the kidney lets out into the urine, as a percentage. Creatinine serves as the reference for filtration; for that reason the calculation needs no 24-hour collection and uses only blood and urine samples collected at the same time.

The test was described by Espinel in 1976, in JAMA, in patients in the oliguric phase of acute kidney injury, then called acute renal failure. The logic is physiological: when the kidney is poorly perfused but the tubules are preserved, it retains sodium and FENa falls; when there is acute tubular necrosis, reabsorption fails and FENa rises.

FENa is usually analyzed alongside the fractional excretion of urea (FEUrea), more useful when the patient has received a diuretic, and other urinary indices described below. The serum sodium used in the calculation can be read together with the calculated serum osmolality.

How to use the calculator: inputs and result

The calculator asks for four values from samples collected at the same time:

• Serum sodium (PNa), in mEq/L, between 90 and 200. Example: 140.
• Urine sodium (UNa), in mEq/L, from 0 to 500. Example: 40.
• Serum creatinine (PCr), in mg/dL, greater than zero and up to 30. Example: 1.0.
• Urine creatinine (UCr), in mg/dL, greater than zero and up to 1,000. Example: 100.

Values outside these ranges produce a message saying which field to correct. The output is the FENa, in %, with two decimal places, and the corresponding range, read from the already rounded value:

• below 1%: range described in prerenal azotemia by Espinel (1976) and Miller (1978);
• 1% to 3%: intermediate zone, between the groups described by Espinel;
• above 3%: range described by Espinel in oliguric acute tubular necrosis.

A note reminds you that the classic studies evaluated oliguric patients not on diuretics. With the field example values, the FENa is 0.29%.

FENa formula and worked example

FENa compares the clearance of sodium with that of creatinine. Because urine volume and time cancel out, the four concentrations are enough:

FENa (%) = (UNa × PCr) ÷ (PNa × UCr) × 100

UNa and PNa are urine and serum sodium, in mEq/L; UCr and PCr are urine and serum creatinine, in mg/dL. The essential point is that each pair be in the same unit: sodium with sodium, creatinine with creatinine.

Teaching example (not the interpretation of a patient): PNa 135 mEq/L, UNa 20 mEq/L, PCr 2.0 mg/dL and UCr 100 mg/dL.

1. Numerator: 20 × 2.0 = 40.
2. Denominator: 135 × 100 = 13,500.
3. FENa: 40 ÷ 13,500 × 100 = 0.30%.
4. Range: below 1%.

In a second case, with PNa 138 mEq/L, UNa 60 mEq/L, PCr 3.0 mg/dL and UCr 50 mg/dL, the FENa is (60 × 3.0) ÷ (138 × 50) × 100 = 2.61%, and the tool shows the intermediate zone of 1% to 3%. With PNa 140, UNa 70, PCr 4.0 and UCr 40, the FENa is (70 × 4.0) ÷ (140 × 40) × 100 = 5.00%, in the above 3% range.

How to understand FENa: what the studies say

FENa has no single normal value and only makes sense within a clinical context. Steiner, in the American Journal of Medicine in 1984, observed that in healthy people it usually stays below 1%, but rises with higher salt intake.

In oliguric acute kidney injury, Espinel found FENa below 1 in prerenal azotemia and above 3 in acute tubular necrosis, in 17 patients without diuretics, obstruction, glomerulonephritis or chronic kidney disease. Miller and colleagues, in the Annals of Internal Medicine in 1978, confirmed the 1% cutoff in a prospective study: 90% of prerenal patients and only 4% of patients with oliguric tubular necrosis were below it.

The calculator uses exactly these published limits: below 1%, 1% to 3% and above 3%. A 2% cutoff, common in summaries, was not adopted because we did not find a primary source for it. The range is a suggestion for clinical reasoning: the decision belongs to the physician, with the history, physical examination and urine output.

Common errors, units and checklist

The most common error with FENa is using samples from different times. Blood and urine should be collected at the same time, because the formula assumes the concentrations represent the same moment.

• Mixed-up units: serum creatinine in µmol/L and urine creatinine in mg/dL distort the ratio. Convert to the same unit; with the KDIGO factor, mg/dL × 88.4 = µmol/L.
• mEq/L and mmol/L: for sodium, both units have the same numerical value, because the ion is monovalent.
• Swapped fields: swapping serum sodium for urine sodium completely changes the result.
• Impossible values: negative urine sodium, zero creatinine or serum sodium outside 90 to 200 mEq/L produce an error message; check the value entered.
• Rounding: the range is read from the displayed value, with two decimal places. A FENa of 0.996% appears as 1.00% and falls in the intermediate zone.

Checklist before using the result:

• Are the blood and urine samples simultaneous?
• Is sodium in mEq/L or mmol/L in both samples?
• Are both creatinines in the same unit?
• Was a diuretic used before collection?
• Is the patient oliguric? The classic studies were done in that setting.

When FENa can mislead

FENa can mislead in several situations described by Steiner in 1984, and knowing them is part of using it correctly. A low FENa, below 1%, also appears in:

• liver or heart failure;
• acute glomerulonephritis;
• pigment nephropathy and contrast nephrotoxicity;
• polyuric renal failure associated with burns, acute obstruction, transplant rejection and, sometimes, nonoliguric acute kidney injury.

On the other hand, a FENa above 1% can occur with volume depletion in people who have received a diuretic and in some patients with chronic kidney disease. In the study by Carvounis and colleagues, published in Kidney International in 2002, 92% of patients with prerenal azotemia not on diuretics had FENa below 1%, compared with only 48% of those who had received diuretics.

The classic studies by Espinel and Miller evaluated oliguric patients. Outside that setting, the separation between causes is less clear, and the calculator's note reminds you that, after a diuretic, the fractional excretion of urea is usually more informative. See also the FEUrea calculator.

FENa, FEUrea and other urinary indices

FENa is one of several urinary indices used in acute kidney injury. The table summarizes the main ones and the finding associated with a prerenal cause in each study:

Miller and colleagues observed that many oliguric patients do not fit the urine sodium and osmolality cutoffs, and that in these cases FENa and the renal failure index discriminate better. For those who have received a diuretic, the FEUrea calculator follows the same logic with urea. In hyperglycemia, glucose-corrected sodium helps read the serum sodium.

Frequently asked questions about the fractional excretion of sodium (FENa)

References

1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-81. doi:10.1001/jama.236.6.579. View
2. Miller TR, Anderson RJ, Linas SL, Henrich WL, Berns AS, Gabow PA, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50. doi:10.7326/0003-4819-89-1-47. View
3. Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77(4):699-702. doi:10.1016/0002-9343(84)90368-1. View
4. 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):2223-9. doi:10.1046/j.1523-1755.2002.00683.x. View