Fractional Excretion of Sodium Calculator
Calculate FENa to differentiate prerenal from intrinsic renal failure. Enter values for instant results with step-by-step formulas.
Formula
FENa = (UNa x SCr) / (SNa x UCr) x 100
Where UNa = urine sodium (mEq/L), SCr = serum creatinine (mg/dL), SNa = serum sodium (mEq/L), UCr = urine creatinine (mg/dL). FENa < 1% suggests prerenal, > 2% suggests intrinsic renal disease.
Worked Examples
Example 1: Prerenal Azotemia Evaluation
Problem: A dehydrated patient presents with AKI. Labs show: Urine Na = 8 mEq/L, Serum Na = 140 mEq/L, Urine Creatinine = 120 mg/dL, Serum Creatinine = 2.5 mg/dL.
Solution: FENa = (UNa x SCr) / (SNa x UCr) x 100\nFENa = (8 x 2.5) / (140 x 120) x 100\nFENa = 20 / 16800 x 100\nFENa = 0.119%\nFENa < 1% indicates prerenal azotemia.
Result: FENa = 0.12% | Diagnosis: Prerenal azotemia | Treatment: Volume resuscitation
Example 2: Acute Tubular Necrosis Assessment
Problem: A patient with aminoglycoside exposure develops AKI. Labs: Urine Na = 45 mEq/L, Serum Na = 138 mEq/L, Urine Creatinine = 30 mg/dL, Serum Creatinine = 3.2 mg/dL.
Solution: FENa = (UNa x SCr) / (SNa x UCr) x 100\nFENa = (45 x 3.2) / (138 x 30) x 100\nFENa = 144 / 4140 x 100\nFENa = 3.48%\nFENa > 2% indicates intrinsic renal disease (ATN).
Result: FENa = 3.48% | Diagnosis: Intrinsic renal disease (ATN) | Stop nephrotoxins
Frequently Asked Questions
What is Fractional Excretion of Sodium (FENa) and what does it measure?
Fractional Excretion of Sodium (FENa) is a clinical laboratory calculation that measures the percentage of filtered sodium that is ultimately excreted in the urine. It serves as a key diagnostic tool to differentiate between prerenal azotemia and intrinsic renal disease in patients presenting with acute kidney injury. The test works because functioning kidneys respond to decreased perfusion by aggressively reabsorbing sodium to conserve volume, resulting in a very low FENa below 1 percent. When the renal tubules are damaged, as in acute tubular necrosis, they lose the ability to reabsorb sodium efficiently, and FENa rises above 2 percent. The calculation requires simultaneous measurement of sodium and creatinine in both urine and serum samples.
What formula does Fractional Excretion of Sodium Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
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How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Does Fractional Excretion of Sodium Calculator work offline?
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Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.