From the Guidelines
The Fractional Excretion of Sodium (FeNa) should be interpreted as follows: a FeNa cutoff of <1% suggests prerenal causes, including hepatorenal syndrome (HRS), while a FeNa >1% is suggestive of structural causes of acute kidney injury (AKI) like acute tubular necrosis (ATN) 1. To interpret FeNa results:
- FeNa < 1%: Suggests prerenal AKI, such as volume depletion or decreased renal perfusion
- FeNa > 1%: Suggests intrinsic renal AKI, such as ATN However, it is essential to consider the limitations of FeNa in certain patient populations, such as those with cirrhosis, where an FeNa of <1% had a sensitivity of 100% but a specificity of only 14% in diagnosing prerenal causes of AKI 1. Important considerations:
- FeNa may be affected by water reabsorption and diuretic use
- Fractional excretion of urea (FEUrea) may be a more reliable diagnostic tool in certain cases, such as differentiating HRS from prerenal azotemia or ATN 1
- Clinical context and other diagnostic tools should be used alongside FeNa for a comprehensive assessment The FeNa test can help guide management by distinguishing between conditions requiring fluid resuscitation (prerenal) and those needing further investigation or specific treatments for kidney damage (intrinsic renal) 1.
From the Research
Interpretation of Fractional Excretion of Sodium (FeNa)
The interpretation of FeNa is based on its value, which can indicate the cause of acute renal failure.
- A high FeNa (greater than 3%) is typically seen in acute tubular necrosis 2, while a low FeNa (less than 1%) is seen in volume depletion 2.
- However, the FeNa test has limited value in diagnosing acute renal failure, as it cannot distinguish between acute tubulointerstitial nephritis and acute glomerulonephritis 3.
- A low FeNa in acute renal failure may be due to milder insults, earlier testing, or superimposed renal ischemia 4.
- FeNa depends on the glomerular filtration rate (GFR) and sodium intake, and tubular function cannot be assessed without knowledge of these factors 5.
- The utility of FeNa in differentiating between intrinsic and prerenal acute kidney injury is limited, and it does not provide a superior alternative to other tests, such as the fractional excretion of urea (FEUrea) 6.
Factors Affecting FeNa
Several factors can affect the interpretation of FeNa, including:
- Timing of the test: earlier testing may result in a lower FeNa 4.
- Renal ischemia: superimposed renal ischemia may result in a lower FeNa 4.
- GFR: FeNa depends on the GFR 5.
- Sodium intake: FeNa depends on sodium intake 5.
- Tubular function: FeNa cannot be used to assess tubular function without knowledge of GFR and sodium intake 5.
- Diuretic therapy: diuretic therapy may affect the utility of FeNa in differentiating between intrinsic and prerenal AKI 6.