Fractional Excretion of Sodium (FeNa) Calculation
The formula to calculate FeNa is: FeNa (%) = [(Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine)] × 100 1
Formula Components
You need four laboratory values to calculate FeNa 1:
- Urine sodium concentration (mmol/L or mEq/L)
- Serum sodium concentration (mmol/L or mEq/L)
- Urine creatinine concentration (mg/dL or μmol/L)
- Serum creatinine concentration (mg/dL or μmol/L)
Clinical Interpretation Framework
Standard Cutoff Values
- FeNa <1% suggests prerenal causes of acute kidney injury, including hypovolemia and hepatorenal syndrome, with 100% sensitivity but only 14% specificity for prerenal causes in cirrhosis 1
- FeNa >1% suggests structural causes of AKI like acute tubular necrosis 1
Target Ranges for Specific Conditions
In cystic fibrosis patients, maintain FeNa between 0.5-1.5% to assess sodium supplementation needs 2, 1
In cirrhosis with ascites, FeNa helps assess sodium avidity and response to diuretics 1
Critical Pitfalls and Limitations
When FeNa <1% Does NOT Indicate Prerenal Azotemia
A low FeNa can occur in intrinsic kidney injury in these settings 3:
- Patients receiving diuretics
- Hepatic or cardiac failure
- Acute glomerulonephritis
- Contrast nephrotoxicity
- Non-oliguric acute renal failure
- Acute obstruction
- Renal transplant rejection
When FeNa >1% Does NOT Indicate Intrinsic Injury
A high FeNa can occur in volume depletion when 3:
- Patients are on diuretics
- Patients have chronic kidney disease with impaired sodium reabsorption
The Muddy Brown Cast Paradox
38% of patients with muddy brown granular casts (diagnostic of acute tubular injury) have FeNa <1% 4. This means relying solely on low FeNa to exclude acute tubular injury is unreliable 4. When muddy brown casts are present on urinalysis, they have 100% specificity and 100% positive predictive value for acute tubular injury, regardless of FeNa 4.
Factors That Influence FeNa
FeNa depends heavily on 5:
- Glomerular filtration rate (GFR)
- Sodium intake (higher intake increases FeNa)
- Tubular function
Over 80% of FeNa variation is explained by GFR and sodium excretion alone 5. In normal subjects, FeNa is usually <1% but increases with higher salt intake 3.
Alternative Measurement in Cirrhosis
For patients with cirrhosis and ascites, a spot urine sodium/potassium ratio >1 indicates sodium excretion >78 mmol/day with 90-95% confidence 2, 1. This is easier than 24-hour urine collection and can be tested at any time of day 2.
Complementary Test: Fractional Excretion of Urea (FEUrea)
**FEUrea <28.16%** had 75% sensitivity and 83% specificity for hepatorenal syndrome versus other causes of AKI 1. FEUrea >31.5% combined with FENa >0.36% has a 92% negative predictive value for intrinsic AKI 6.