Fractional Excretion of Sodium
Enter serum and urine sodium and creatinine concentrations. The formula assumes values in consistent units (e.g., mmol/L for sodium, mg/dL for creatinine).
How to Use This Calculator
Collect simultaneous serum and urine samples
Draw blood and urine at roughly the same time to avoid dilutional changes that can skew the FENa.
Enter sodium and creatinine concentrations
Use consistent units (e.g., mmol/L for sodium, mg/dL for creatinine). Convert values if the lab reports in different units.
Interpret together with clinical context
FENa is most reliable in oliguric patients not on diuretics. Consider FEUrea if diuretics were recently administered.
Formula
FENa (%) = (Urine Na × Serum Cr ÷ (Serum Na × Urine Cr)) × 100
Values <1% suggest prerenal azotaemia, while values >2% favour intrinsic acute tubular injury.
Full Description
The fractional excretion of sodium quantifies the percentage of filtered sodium excreted in urine. It helps distinguish prerenal azotaemia (low FENa) from intrinsic Acute Kidney Injury such as acute tubular necrosis (high FENa). Diuretics, chronic kidney disease, and acute glomerulonephritis can alter FENa interpretation. Use alongside FEUrea, urine microscopy (casts), and haemodynamic assessment for a comprehensive evaluation.
Frequently Asked Questions
Does diuretic therapy affect FENa?
Yes. Diuretics increase urinary sodium, making FENa unreliable. In such cases, calculate fractional excretion of urea (FEUrea) instead.
Is FENa valid in chronic kidney disease?
Baseline tubular dysfunction may elevate FENa even in prerenal states. Interpret with caution in advanced CKD.
What about non-oliguric AKI?
In non-oliguric states the discriminatory power lessens. Combine with urinary sediment findings and clinical assessment.
How do I handle different units?
Ensure sodium values are both in mmol/L (or mEq/L) and creatinine values in the same units (e.g., mg/dL). Convert before entering to avoid errors.