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

$F{e}_{Na}=N{a}_{Excreted}/N{a}_{Filtered}$ $F{e}_{Na}=\frac{N{a}_{Urine}}{C{r}_{Urine}}÷\frac{N{a}_{Plasma}}{C{r}_{Plasma}}=\frac{N{a}_{Urine}\times C{r}_{Plasma}}{N{a}_{Plasma}\times C{r}_{Urine}}$

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