Hong Life CalculatorsUnderstanding the FENa calculator — concept and principle
The fractional excretion of sodium (FENa) is the percentage of sodium filtered by the kidneys that ends up in the urine. In a patient with acute kidney injury and low urine output, it helps distinguish a "prerenal" cause, where the kidneys are healthy but underperfused, from intrinsic kidney damage such as acute tubular necrosis (ATN).
How it is calculated
FENa (%) = (urine Na × plasma Cr) ÷ (plasma Na × urine Cr) × 100
- Urine and plasma sodium in mmol/L.
- Urine and plasma creatinine in the same unit (mg/dL here); the units cancel.
- Both samples should be taken at about the same time, before diuretics.
The principle
Creatinine is filtered at the glomerulus and is hardly reabsorbed, so the ratio of urine to plasma creatinine shows how concentrated the filtrate has become. Comparing that with the urine-to-plasma ratio for sodium tells you what fraction of filtered sodium escaped reabsorption. When the kidney is underperfused but the tubules work, it avidly holds on to sodium and FENa is low. Damaged tubules cannot reabsorb sodium well, so more is lost and FENa is high.
Worked example
- Numerator: urine Na × plasma Cr = 20 × 2 = 40.
- Denominator: plasma Na × urine Cr = 140 × 100 = 14,000.
- 40 ÷ 14,000 = 0.002857.
- × 100 = 0.2857, shown as 0.29%.
- This is below 1%, so the pattern suggests prerenal kidney injury.
| Plasma sodium (mmol/L) | FENa |
|---|---|
| 35 | 1.14% |
| 70 | 0.57% |
| 105 | 0.38% |
| 140 | 0.29% |
| 175 | 0.23% |
| 210 | 0.19% |
| 280 | 0.14% |
How to read the result
A FENa below 1% in oliguric acute kidney injury suggests a prerenal state such as dehydration, bleeding, heart failure or cirrhosis. Above 2% suggests intrinsic tubular damage, most commonly ATN. Between 1% and 2% is indeterminate. In people with normal kidney function, FENa is usually under 1% and depends heavily on dietary salt.
Common mistakes
- Using FENa after loop or thiazide diuretics, which raise it artificially; fractional excretion of urea is often used instead.
- Applying it to non-oliguric patients or chronic kidney disease, where baseline values are higher.
- Forgetting exceptions: contrast nephropathy, rhabdomyolysis, early obstruction and glomerulonephritis can give a low FENa without a prerenal cause.
Frequently asked questions
What is a normal FENa?
In healthy people it is usually below 1%, varying with salt intake.
What is FEUrea?
The same calculation with urea; values below about 35% suggest a prerenal cause and are less affected by diuretics.
Do the units of creatinine matter?
Only that urine and plasma creatinine use the same unit, because they cancel.
Sources: Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA 1976.