Fractional excretion of sodium

Definition: percentage of the glomerular filtered sodium (Na_Filtered) that is eventually excreted in the urine (Na_Excreted)

Usage

Can help establish the cause of acute kidney injury
Can help distinguish between renal and extrarenal etiology in hypotonic hyponatremia (see “Diagnostic approach to hyponatremia”)

Calculation

F e_{N a} = N a_{E x c r e t e d} / N a_{F i l t e r e d}

F e_{N a} = \frac{N a_{U r i n e}}{C r_{U r i n e}} \div \frac{N a_{P l a s m a}}{C r_{P l a s m a}} = \frac{N a_{U r i n e} \times C r_{P l a s m a}}{N a_{P l a s m a} \times C r_{U r i n e}}

Interpretation

In acute kidney injury
- Low FeNa (< 1%): indicates a prerenal cause (renal hypoperfusion)
- High FeNa (> 2%): indicates an intrarenal etiology (e.g., acute tubular necrosis)
- Inconclusive FeNa (1–2%): can be seen with either disorder
In hypotonic hyponatremia
- Low FeNa (< 1%): extrarenal cause
- High FeNa (> 1%): renal cause