FENa Calculator
Calculate the Fractional Excretion of Sodium (FENa) with step-by-step formula breakdown, clinical interpretation, and visual diagnosis guide for acute kidney injury assessment.
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About FENa Calculator
Welcome to the FENa Calculator, 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
Where:
- UNa = Urine Sodium concentration (mEq/L)
- PCr = Plasma (Serum) Creatinine concentration
- UCr = Urine Creatinine concentration
- PNa = Plasma (Serum) Sodium concentration (mEq/L)
How to Use This Calculator
- Collect samples: Obtain simultaneous urine and blood samples for sodium and creatinine measurements.
- Enter urine sodium: Input the urine sodium concentration in mEq/L (same as mmol/L).
- Enter plasma sodium: Input the serum/plasma sodium concentration in mEq/L.
- Enter creatinine values: Input both urine and plasma creatinine in the same units (mg/dL or μmol/L).
- Calculate and interpret: Review the FENa result and clinical interpretation for your patient.
Clinical Interpretation
| FENa Value | Interpretation | Common Causes |
|---|---|---|
| < 1% | Prerenal Azotemia Kidneys appropriately retaining sodium in response to decreased perfusion |
Hypovolemia, heart failure, hepatorenal syndrome, NSAID use, renal artery stenosis |
| 1 - 2% | Indeterminate Requires clinical correlation and additional testing |
Early ATN, contrast nephropathy, myoglobinuria, partial obstruction |
| > 2% | Intrinsic Renal Disease Tubular damage preventing appropriate sodium reabsorption |
Acute tubular necrosis, acute interstitial nephritis, glomerulonephritis |
Limitations of FENa
- Diuretics: FENa is unreliable in patients taking diuretics, which increase sodium excretion regardless of renal perfusion status
- Chronic Kidney Disease: Patients with CKD may have elevated baseline FENa
- Non-oliguric AKI: FENa may be less reliable in non-oliguric acute kidney injury
- Contrast agents: Recent contrast administration may affect results
- Glycosuria: Presence of glucose in urine may alter sodium handling
In patients on diuretics, consider using Fractional Excretion of Urea (FEUrea) instead, as urea handling is less affected by diuretics. FEUrea < 35% suggests prerenal disease.
Normal Reference Ranges
- Urine Sodium: 40-220 mEq/L (varies with diet and hydration)
- Plasma Sodium: 136-145 mEq/L
- Urine Creatinine: Variable (depends on muscle mass and hydration)
- Plasma Creatinine: 0.7-1.3 mg/dL (male), 0.6-1.1 mg/dL (female)
Frequently Asked Questions
What is FENa (Fractional Excretion of Sodium)?
FENa (Fractional Excretion of Sodium) is the percentage of filtered sodium that is excreted in urine. It helps differentiate between prerenal azotemia (FENa < 1%) and intrinsic renal disease like acute tubular necrosis (FENa > 2%). The formula is: FENa = (UNa × PCr) / (UCr × PNa) × 100%.
What does FENa less than 1% mean?
FENa less than 1% suggests prerenal azotemia, indicating that the kidneys are responding appropriately to decreased blood flow by retaining sodium. Common causes include hypovolemia, congestive heart failure, hepatorenal syndrome, and renal artery stenosis. Treatment focuses on restoring adequate renal perfusion.
What does FENa greater than 2% indicate?
FENa greater than 2% suggests intrinsic renal disease, typically acute tubular necrosis (ATN). This indicates that the kidney tubules are damaged and have lost the ability to appropriately reabsorb sodium. Common causes include ischemic ATN, nephrotoxic drugs, acute interstitial nephritis, and glomerulonephritis.
When should FENa not be used?
FENa has limitations and should not be used in patients taking diuretics (which increase sodium excretion regardless of cause), patients with chronic kidney disease (CKD), non-oliguric acute kidney injury, or in the presence of glycosuria or contrast agents. In these cases, FEUrea may be more reliable.
What is the FENa formula?
The FENa formula is: FENa (%) = (UNa × PCr) / (UCr × PNa) × 100. UNa = Urine Sodium (mEq/L), PCr = Plasma Creatinine, UCr = Urine Creatinine, PNa = Plasma Sodium (mEq/L). The creatinine values must be in the same units for accurate calculation.
Additional Resources
Reference this content, page, or tool as:
"FENa Calculator" at https://MiniWebtool.com/fena-calculator/ from MiniWebtool, https://MiniWebtool.com/
by miniwebtool team. Updated: Feb 01, 2026
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