Fractional Excretion of Sodium

The Fractional Excretion of Sodium (FENa) is a crucial diagnostic parameter used in nephrology to evaluate acute kidney injury (AKI). This calculation represents the percentage of filtered sodium that is ultimately excreted in the urine, providing valuable insights into kidney function and the underlying cause of renal dysfunction.

FENa is particularly useful in differentiating between pre-renal azotemia (where the kidneys are functioning normally but receiving inadequate blood flow) and intrinsic renal disease such as acute tubular necrosis. The test requires simultaneous measurement of sodium and creatinine concentrations in both blood and urine samples.

Normal kidney function typically maintains FENa below 1% through efficient sodium reabsorption in the tubules. When this mechanism is impaired due to tubular damage or dysfunction, FENa values increase, indicating intrinsic renal pathology rather than pre-renal causes.

Step 1: Gather Required Laboratory Values

Collect the necessary laboratory results from both blood and urine samples. You will need urine sodium concentration, plasma sodium concentration, urine creatinine, and plasma creatinine values. Ensure all measurements are taken from samples collected within the same timeframe for accuracy.

Step 2: Enter Urine Sodium Concentration

Input the urine sodium value in the first field. This value is typically measured in mEq/L or mmol/L. Normal urine sodium levels can vary widely depending on dietary intake and clinical conditions, ranging from very low in volume-depleted states to over 100 mEq/L in normal conditions.

Step 3: Input Plasma Sodium and Creatinine Values

Enter the plasma sodium concentration (normally 135-145 mEq/L) and plasma creatinine level. Plasma creatinine is typically measured in mg/dL, with normal values ranging from 0.6-1.2 mg/dL in adult males and 0.5-1.1 mg/dL in adult females, though these may be elevated in kidney injury.

Step 4: Add Urine Creatinine and Calculate

Complete the calculation by entering the urine creatinine concentration and clicking the calculate button. The calculator will apply the FENa formula and provide both the numerical result and clinical interpretation. Values less than 1% typically suggest pre-renal causes, while values greater than 2% indicate intrinsic renal disease.

Step 5: Interpret Results in Clinical Context

Review the calculated FENa percentage along with the provided clinical interpretation. Consider patient factors such as diuretic use, chronic kidney disease, or other medications that may affect sodium handling. The result should be integrated with other clinical findings and laboratory values for comprehensive patient assessment.