FENa Calculator

What is the FENa Calculator?

The FENa Calculator is a professional clinical tool for calculating the Fractional Excretion of Sodium with step-by-step formula breakdown and clinical interpretation. FENa is a valuable diagnostic test used to differentiate between prerenal azotemia and intrinsic renal disease in patients with acute kidney injury (AKI). This calculator provides instant results with detailed clinical guidance.

What is FENa (Fractional Excretion of Sodium)?

Fractional Excretion of Sodium (FENa) is the percentage of filtered sodium that is ultimately excreted in the urine. It measures how well the kidneys are handling sodium relative to their filtration of creatinine, providing insight into tubular function and the cause of acute kidney injury.

In a normally functioning kidney, less than 1% of filtered sodium is excreted because the tubules reabsorb most of the filtered sodium. When renal perfusion decreases (prerenal state), the kidneys attempt to conserve sodium even more aggressively, resulting in very low FENa values. However, when the tubules are damaged (intrinsic renal disease), they lose the ability to reabsorb sodium effectively, leading to higher FENa values.

FENa Formula

The FENa formula calculates the ratio of sodium excretion to creatinine excretion:

$$FE_{Na} = \frac{U_{Na} \times P_{Cr}}{U_{Cr} \times P_{Na}} \times 100\%$$

Where UNa is Urine Sodium concentration (mEq/L), PCr is Plasma Creatinine concentration, UCr is Urine Creatinine concentration, and PNa is Plasma Sodium concentration (mEq/L).

How to Use This Calculator

Collect simultaneous urine and blood samples for sodium and creatinine measurements. Enter the urine sodium concentration in mEq/L, the plasma sodium concentration in mEq/L, and both urine and plasma creatinine values in the same units (mg/dL or umol/L). The calculator will compute the FENa result along with a clinical interpretation.

Clinical Interpretation

• FENa less than 1%: Indicates prerenal azotemia. Kidneys are appropriately retaining sodium in response to decreased perfusion. Common causes include hypovolemia, heart failure, and hepatorenal syndrome.
• FENa 1 to 2%: Indeterminate range requiring clinical correlation and additional testing. Possible causes include early ATN or contrast nephropathy.
• FENa greater than 2%: Suggests intrinsic renal disease like acute tubular necrosis. Tubular damage prevents appropriate sodium reabsorption.

Limitations of FENa

FENa is unreliable in patients taking diuretics, which increase sodium excretion regardless of renal perfusion status. It also has limitations in patients with chronic kidney disease, non-oliguric AKI, recent contrast administration, or glycosuria. In patients on diuretics, consider using Fractional Excretion of Urea (FEUrea) instead.

Normal Reference Ranges

• Urine Sodium: 40-220 mEq/L (varies with diet and hydration)
• Plasma Sodium: 136-145 mEq/L
• Plasma Creatinine: 0.7-1.3 mg/dL (male), 0.6-1.1 mg/dL (female)

Related Medical Calculators

If you found this tool useful, check out our other medical calculators: BUN to Creatinine Ratio Calculator and GFR Calculator for comprehensive kidney function assessment.