From the Guidelines
The fractional excretion of sodium (FeNa) is calculated using the formula: (Urine sodium × Serum creatinine) ÷ (Serum sodium × Urine creatinine) × 100, with a cutoff of <1% suggesting prerenal causes and >1% suggesting structural causes of AKI, as reported in a recent study published in 2022 1. When interpreting FeNa results, it is essential to consider the clinical context, as diuretic use can affect sodium handling and FeNa values.
- A FeNa <1% typically indicates volume depletion or prerenal causes, such as hepatorenal syndrome (HRS) in patients with cirrhosis.
- A FeNa >1% suggests renal sodium wasting, as seen in acute tubular necrosis (ATN). However, the specificity of FeNa in diagnosing prerenal causes of AKI is limited, with a recent study finding a specificity of only 14% in patients with cirrhosis 1. In such cases, the fractional excretion of urea (FEUrea) may be a more useful diagnostic tool, with a cutoff of <28.16% having a sensitivity of 75% and specificity of 83% in separating HRS from non-HRS 1. To accurately measure FeNa, simultaneous blood and urine samples are required, and results should be interpreted with caution in patients taking diuretics, as these medications can artificially elevate FeNa values 1.
From the Research
Definition of Fractional Excretion of Sodium (FeNa)
- The fractional excretion of sodium (FeNa) is a measure used to distinguish between prerenal and intrinsic causes of acute kidney injury (AKI) 2, 3, 4, 5.
- FeNa is calculated as the ratio of sodium excretion to sodium filtration, and it is typically expressed as a percentage 2, 3, 4, 5.
Interpretation of FeNa
- A low FeNa (<1%) is often seen in prerenal azotemia, while a high FeNa (>2-3%) is typically associated with acute tubular necrosis (ATN) 2, 3, 4, 5.
- However, the use of diuretics can increase FeNa, making it less reliable in differentiating between prerenal and intrinsic causes of AKI 2, 3, 4.
- FeNa can also be influenced by other factors, such as glomerular filtration rate (GFR), sodium intake, and tubular function 6.
Clinical Applications of FeNa
- FeNa can be used to guide volume depletion during recovery from acute renal failure 5.
- However, its interpretation requires careful consideration of other factors that can influence its value, such as diuretic use and tubular function 2, 3, 4, 6.
- In some cases, the fractional excretion of urea nitrogen (FEUN) may be a more reliable index than FeNa in differentiating between prerenal and intrinsic causes of AKI 2, 3, 4.