FENa Calculator (Fractional Excretion of Sodium)

The Fractional Excretion of Sodium measures the percentage of filtered sodium that is excreted in the urine. The standard mathematical formula used is:

FENa = 100 × [(Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine)]

Expressed as a ratio or percentage, a result under 1% typically suggests a pre-renal etiology where the kidneys are holding onto sodium to conserve water. Conversely, a value exceeding 2% often points toward intrinsic renal damage, where the tubules have lost their reabsorptive capacity.

Consider a patient presenting with elevated creatinine levels. Their laboratory panel yields the following results:

• Serum Sodium (Serum Na): 140 mEq/L
• Serum Creatinine (Serum Cr): 2.0 mg/dL
• Urine Sodium (Urine Na): 20 mEq/L
• Urine Creatinine (Urine Cr): 100 mg/dL

Step 1: Multiply Urine Sodium by Serum Creatinine (20 × 2.0 = 40).

Step 2: Multiply Serum Sodium by Urine Creatinine (140 × 100 = 14,000).

Step 3: Divide the result of Step 1 by the result of Step 2 (40 / 14,000 = 0.00285).

Step 4: Convert to a percentage by multiplying by 100, yielding a FENa of 0.29% or roughly 0.3%. This low value strongly indicates a pre-renal azotemia state, such as dehydration or heart failure.

Always interpret FENa values within the complete clinical picture, keeping in mind that patients taking diuretic medications may yield falsely elevated FENa results due to natriuresis. In settings where diuretics confound the sodium excretion, consider utilizing the fractional excretion of urea (FEUrea) as an alternative diagnostic metric.