Fractional Excretion of Sodium

\[FENa=100\times\frac{U_{Na}\times P_{Cr}}{P_{Na}\times U_{Cr}}\]

Variables

FENafractional excretion of sodium (%)

UNa[UNa]urine sodium concentration (mEq/L)

PNa[PNa]plasma sodium concentration (mEq/L)

UCr[UCr]urine creatinine concentration (mg/dL)

PCr[PCr]plasma creatinine concentration (mg/dL)

Description

What is this formula?

The Fractional Excretion of Sodium (FENa) estimates the percentage of filtered sodium that is excreted in the urine. It is widely used to help distinguish prerenal azotemia from intrinsic renal injury.

When to use it

Use this formula when evaluating acute kidney injury, oliguria, volume depletion, or suspected renal tubular dysfunction.

Example

Data:

UNa = 10 mEq/L

PNa = 140 mEq/L

UCr = 100 mg/dL

PCr = 2.0 mg/dL

Formula:

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

Substitution:

FENa = 100 × (10 × 2.0) / (140 × 100)

FENa = 2000 / 14000

Result:

FENa = 0.14 %

A very low FENa suggests sodium conservation, which is commonly seen in prerenal states.

Applications

- Acute kidney injury evaluation

- Differential diagnosis of azotemia

- Nephrology

- Critical care medicine

- Renal function assessment

Note

FENa is a clinical indicator rather than a definitive diagnostic test. Values below approximately 1% are classically associated with prerenal azotemia, whereas higher values may suggest intrinsic renal injury. Interpretation may be unreliable in patients receiving diuretics, with chronic kidney disease, or under certain clinical conditions.