FENa Calculator — Fractional Excretion of Sodium

Evaluate kidney function and differentiate causes of acute kidney injury using laboratory values.

🩸 Plasma (Serum) Values
Normal: 0.7–1.3 mg/dL
Normal: 136–145 mEq/L
🧪 Urine Values
Normal: 20–320 mg/dL
Normal: 20–220 mEq/L
—
%
Fractional Excretion of Sodium
< 1%
Prerenal
1 – 4%
Intrinsic Renal
> 4%
Postrenal
FENa = (PCr × UNa) / (PNa × UCr) × 100

What is FENa?

The Fractional Excretion of Sodium (FENa) is a measure used in clinical medicine to help differentiate the causes of acute kidney injury (AKI). It compares the amount of sodium filtered by the kidneys to the amount actually excreted in the urine.

FENa is expressed as a percentage and provides insight into whether the kidneys are conserving sodium (suggesting the problem is before the kidneys — prerenal) or losing sodium inappropriately (suggesting intrinsic kidney damage).

FENa Formula

The formula for FENa is:

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

Where:

  • PCr = Plasma (serum) creatinine concentration (mg/dL)
  • UNa = Urine sodium concentration (mEq/L)
  • PNa = Plasma (serum) sodium concentration (mEq/L)
  • UCr = Urine creatinine concentration (mg/dL)

FENa Interpretation

FENa Value Classification Common Causes
< 1% Prerenal Dehydration, heart failure, hemorrhage, sepsis (early), liver cirrhosis
1 – 4% Intrinsic Renal Acute tubular necrosis (ATN), glomerulonephritis, acute interstitial nephritis
> 4% Postrenal Urinary tract obstruction, bladder outlet obstruction

Prerenal (FENa < 1%)

A low FENa indicates that the kidneys are avidly reabsorbing sodium, which is appropriate when the body senses decreased blood flow. The kidneys themselves are functioning normally — they're responding correctly to reduced perfusion.

Intrinsic Renal (FENa 1–4%)

An intermediate FENa suggests damage to the kidney itself. The tubules are unable to properly reabsorb sodium, leading to sodium wasting in the urine. The most common cause is acute tubular necrosis (ATN).

Postrenal (FENa > 4%)

A high FENa may indicate obstruction below the kidneys (ureter, bladder, urethra). Prolonged obstruction can lead to kidney damage and elevated sodium excretion.

Required Lab Tests

To calculate FENa, you need four laboratory values from simultaneous blood and urine samples:

  • Basic Metabolic Panel (BMP) — provides serum creatinine and sodium
  • Urinalysis with electrolytes — provides urine creatinine and sodium

It is important that blood and urine samples are collected at approximately the same time for accurate results.

Limitations

  • Diuretics — FENa may be unreliable in patients taking diuretics (consider using FEUrea instead)
  • Chronic kidney disease — Baseline sodium handling may be altered
  • Contrast nephropathy — May show prerenal values despite intrinsic damage
  • FENa should always be interpreted in the clinical context — it is a supportive tool, not a definitive diagnosis
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