fena calculate

What is FENa?

FENa stands for Fractional Excretion of Sodium. It estimates the percentage of filtered sodium that is ultimately excreted in urine. Clinicians often use it as part of the evaluation of acute kidney injury (AKI), especially when trying to distinguish between:

• Prerenal physiology (low perfusion state, where kidneys conserve sodium), and
• Intrinsic renal injury (such as acute tubular injury, where sodium reabsorption is impaired).

Because sodium handling changes quickly with volume status, medications, and kidney pathology, FENa is most useful when interpreted with the full clinical picture—not as a stand-alone number.

How to calculate FENa

The calculator above applies the classic formula:

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

This equation adjusts urine sodium by creatinine concentration, helping account for urine concentration and flow differences.

Typical interpretation ranges

• < 1%: Often suggests a sodium-avid state (commonly prerenal physiology).
• 1% to 2%: Gray zone; mixed findings or evolving process may be present.
• > 2%: More consistent with impaired tubular sodium reabsorption, often intrinsic kidney injury.

Worked example

Suppose the labs are:

• Urine sodium = 40
• Serum sodium = 140
• Urine creatinine = 100
• Serum creatinine = 2.0

Then:

FENa = [(40 × 2.0) / (140 × 100)] × 100 = (80 / 14000) × 100 = 0.57%

A value around 0.57% would usually support a prerenal pattern, provided the clinical context matches.

When FENa is less reliable

FENa can be misleading in specific scenarios. Be careful in:

• Recent diuretic use (can increase urinary sodium and falsely elevate FENa).
• Chronic kidney disease (baseline tubular handling may already be altered).
• Sepsis-related AKI (physiology may be mixed and dynamic).
• Early obstruction or pigment nephropathy (values may not follow classic patterns).
• Very early AKI where tubular behavior is still changing.

If diuretics were recently administered, many clinicians also consider FEUrea (fractional excretion of urea) as an additional data point.

Practical clinical interpretation framework

1) Start with volume and hemodynamics

Hypotension, tachycardia, poor intake, GI losses, or hemorrhage can all drive a low FENa due to sodium conservation.

2) Review medications

Diuretics, ACE inhibitors, ARBs, NSAIDs, and nephrotoxic antibiotics can alter kidney physiology and interpretation.

3) Match with urinalysis and microscopy

Urine sediment findings (e.g., muddy brown casts) often provide stronger clues for acute tubular injury than a single FENa value.

4) Trend labs over time

Serial creatinine, urine output, and repeated urine chemistries frequently provide more insight than one isolated sample.

Quick FAQ

Can FENa be used in every AKI case?

No. It is a supportive index, not a definitive diagnostic test.

Does the sodium unit matter (mEq/L vs mmol/L)?

Not for the ratio, as long as urine and serum sodium use the same unit system.

Should I make treatment decisions from this calculator alone?

No. This page is educational. Clinical decisions should come from licensed professionals using complete patient data.

Bottom line

If you need to fena calculate quickly, this tool gives a fast and accurate percentage with immediate interpretation guidance. The most important step is what comes after the number: integrate it with history, exam, urine findings, imaging, and trajectory. FENa is best used as one piece of a broader kidney assessment strategy.