fena calculator

What Is a FENa Calculator?

A FENa calculator estimates the Fractional Excretion of Sodium, a percentage that reflects how much filtered sodium is ultimately excreted in urine. In practical terms, clinicians often use FENa as one clue in evaluating causes of acute kidney injury (AKI), especially when trying to distinguish reduced kidney perfusion (often called a prerenal pattern) from intrinsic tubular damage.

This number should always be interpreted alongside history, exam, urine microscopy, medications, and overall hemodynamics. It is not a stand-alone diagnosis.

The Formula

FENa (%) = [(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100

• Urine Na = urine sodium concentration
• Serum Na = blood sodium concentration
• Urine Cr = urine creatinine concentration
• Serum Cr = blood creatinine concentration

Keep sodium units consistent with sodium units, and creatinine units consistent with creatinine units. Most labs report sodium in mEq/L and creatinine in mg/dL.

How to Use This Calculator

Step-by-step

• Enter urine sodium.
• Enter serum sodium.
• Enter urine creatinine.
• Enter serum creatinine.
• Click Calculate FENa to get percentage and interpretation.

The calculator validates entries to reduce obvious input errors such as zero or negative values.

How to Interpret FENa

Common bedside thresholds

• < 1%: often suggests a prerenal physiology (kidneys avidly reabsorbing sodium).
• 1% to 2%: gray zone; interpretation depends on context.
• > 2%: can suggest intrinsic tubular injury (e.g., acute tubular necrosis pattern).

Important caveats

Diuretic therapy can raise urine sodium and make FENa less reliable. In those scenarios, clinicians may consider FEUrea and additional clinical data.

• Recent diuretic use can distort the expected values.
• Chronic kidney disease may alter interpretation.
• Sepsis-associated AKI can produce mixed patterns.
• Early obstruction or contrast exposure may not follow simple cutoffs.

Worked Example

Suppose values are: Urine Na = 20 mEq/L, Serum Na = 140 mEq/L, Urine Cr = 80 mg/dL, Serum Cr = 2.0 mg/dL.

FENa = [(20 × 2.0) / (140 × 80)] × 100 = 0.36%

A value near 0.36% is generally consistent with a sodium-retentive (often prerenal) pattern, but this still requires full clinical correlation.

FENa vs FEUrea

FENa is often first-line because the variables are widely available. FEUrea can be useful when diuretics are on board, since urea handling may be less affected than sodium in certain settings. Neither metric replaces careful clinical assessment.

Best Practices for Clinical Use

Use trends, not isolated values

A single number is less informative than repeated measurements combined with changing creatinine, urine output, and response to treatment.

Pair with urine sediment

Microscopy findings (e.g., granular casts, tubular epithelial cells) can significantly improve diagnostic confidence when interpreted with FENa.

Integrate with patient context

Blood pressure, volume status, nephrotoxin exposure, heart failure, cirrhosis, and sepsis all influence how meaningful a FENa result will be.

Frequently Asked Questions

Is a low FENa always prerenal?

No. It is suggestive, not definitive. Mixed etiologies are common in real-world AKI.

Can I use SI units?

Yes, as long as sodium and creatinine pairs remain internally consistent.

Does this calculator provide medical advice?

No. It is an educational and computational aid, not a diagnostic decision-maker.

Bottom Line

A FENa calculator is a fast, practical bedside tool for renal physiology assessment in AKI. Use it to support, not replace, clinical judgment. The most accurate interpretation always comes from integrating labs, exam findings, and trajectory over time.