The Fractional Excretion of Sodium (FENa) is a calculated measurement used by healthcare professionals to evaluate how the kidneys handle sodium. It provides a functional assessment of the renal tubules and offers insight into the body’s volume status and the underlying cause of impaired kidney function. Understanding FENa requires grasping the physiological concept it represents, knowing the precise mathematical formula for its calculation, and correctly interpreting the resulting percentage in a clinical context.
Defining the Fractional Excretion of Sodium
The kidneys continuously filter the blood, removing waste products and regulating electrolytes like sodium. This process is essential for maintaining overall fluid balance. Filtration begins in the glomerulus, where fluid and sodium are filtered into the renal tubules. The body must conserve most of this sodium to maintain blood volume and pressure, so the renal tubules actively reabsorb approximately 99% of the filtered sodium back into the bloodstream.
The FENa measures the small fraction of filtered sodium that the kidney fails to reabsorb and is excreted in the urine. By comparing the rate of sodium excretion to the rate of sodium filtration, the FENa provides a percentage reflecting tubular efficiency. This measurement is more informative than urine sodium concentration alone because it accounts for variable water reabsorption based on the body’s hydration level. A high FENa indicates the tubules cannot effectively hold sodium, while a low FENa suggests aggressive sodium conservation in response to the body’s needs.
The Calculation Formula and Required Variables
Calculating the Fractional Excretion of Sodium requires simultaneous measurements of four specific values from both the blood (serum/plasma) and a spot urine sample. The formula is designed to find the ratio of sodium clearance to creatinine clearance. This approach simplifies the calculation by canceling out the need to measure the variable urine flow rate. The precise formula is expressed as: $FENa = \frac{(Urine Na \times Serum Creatinine)}{(Serum Na \times Urine Creatinine)} \times 100$.
The calculation requires four variables: the concentration of sodium and creatinine in both the urine and the blood.
- Urine Na (concentration of sodium in the urine)
- Serum Creatinine (concentration of creatinine in the blood)
- Serum Na (concentration of sodium in the blood)
- Urine Creatinine (concentration of creatinine in the urine)
To ensure an accurate result, the units for sodium must be consistent (e.g., mEq/L), and the units for creatinine must also be consistent (e.g., mg/dL) across both serum and urine values. Multiplying the final ratio by 100 converts the result into the FENa percentage.
For example, consider a patient with lab results: Urine Na of 15 mEq/L, Serum Creatinine of 2.5 mg/dL, Serum Na of 140 mEq/L, and Urine Creatinine of 150 mg/dL. First, calculate the numerator ($15 \times 2.5 = 37.5$) and the denominator ($140 \times 150 = 21,000$). Dividing the numerator by the denominator yields a ratio of approximately 0.001785. Multiplying this ratio by 100 provides the final FENa of 0.18%. This low percentage indicates that the kidneys are aggressively conserving sodium, excreting less than one-fifth of one percent of the filtered amount.
Interpreting the FENa Results
The calculated FENa percentage is primarily used to differentiate between the causes of Acute Kidney Injury (AKI), especially in patients presenting with low urine output. Interpretation relies on the kidney’s physiological response to hypoperfusion, which is the mechanism behind pre-renal azotemia. When blood flow to the kidneys is low, healthy renal tubules aggressively conserve sodium to retain water and restore circulating volume, resulting in an FENa less than 1%.
Conversely, in intrinsic renal failure, such as Acute Tubular Necrosis (ATN), the kidney tubules themselves are damaged and cannot effectively reabsorb sodium. This tubular dysfunction leads to inappropriate sodium wasting in the urine, even when the body attempts conservation, resulting in an FENa greater than 2%. The range between 1% and 2% is considered an indeterminate or “gray zone,” which may represent a transitional phase of injury requiring further clinical investigation.
FENa is a diagnostic tool, not a definitive diagnosis, and its reliability is subject to several important caveats. Patients who have recently received diuretics will have a falsely elevated FENa because these medications directly block sodium reabsorption. FENa may also remain low despite tubular damage in patients with Chronic Kidney Disease (CKD) or specific conditions like acute glomerulonephritis. Therefore, the result must always be interpreted alongside the patient’s overall clinical picture and other laboratory findings.