Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
FeNa
0.43
Interpretation
Prerenal azotemia pattern
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How to use this
- 1Enter urine sodium (mEq/L).
- 2Enter plasma sodium (mEq/L).
- 3Enter urine creatinine (mg/dL).
- 4Enter plasma creatinine (mg/dL).
- 5Enter recent diuretic use.
- 6Read your fena on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Fractional excretion of sodium compares how much filtered sodium the kidney excretes versus reabsorbs, and it helps distinguish prerenal azotemia (kidneys appropriately conserving sodium and water in response to poor perfusion) from intrinsic acute kidney injury like acute tubular necrosis (tubules damaged and unable to reabsorb sodium normally), in a patient with acute kidney injury and oliguria. A FeNa below 1% suggests prerenal physiology — the kidney is working correctly but underperfused — while a FeNa above 2% suggests intrinsic renal damage. Values between 1-2% are indeterminate. FeNa is unreliable after diuretic administration (diuretics force sodium excretion regardless of underlying cause), in chronic kidney disease, and in some intrinsic causes like contrast nephropathy or early sepsis-associated AKI that can present with a low FeNa despite intrinsic damage — in diuretic-exposed patients, fractional excretion of urea (FeUrea) is used instead since urea handling is less affected by diuretics. This tool is for education and workflow support only — not medical advice. Always verify results with a clinician and use clinical judgment alongside your institution's protocols.
Worked example
Using the values the calculator loads with:
Inputs
- Urine sodium: 15 mEq/L
- Plasma sodium: 138 mEq/L
- Urine creatinine: 60 mg/dL
- Plasma creatinine: 2.4 mg/dL
- Recent diuretic use: No
Results
- FeNa: 0.43
- Interpretation: Prerenal azotemia pattern
What each field means
Inputs
- Urine sodium (mEq/L)
- The urine sodium used in the calculation, measured in mEq/L. Starts at 15 mEq/L so you have a working example on load. Accepted range: 0–300 mEq/L.
- Plasma sodium (mEq/L)
- The plasma sodium used in the calculation, measured in mEq/L. Starts at 138 mEq/L so you have a working example on load. Accepted range: 100–170 mEq/L.
- Urine creatinine (mg/dL)
- The urine creatinine used in the calculation, measured in mg/dL. Starts at 60 mg/dL so you have a working example on load. Accepted range: 1–500 mg/dL.
- Plasma creatinine (mg/dL)
- The plasma creatinine used in the calculation, measured in mg/dL. Starts at 2.4 mg/dL so you have a working example on load. Accepted range: 0.2–20 mg/dL.
- Recent diuretic use
- Pick the option that matches your situation — the maths changes per option. Choices: Yes, No.
Results
- FeNa
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Interpretation
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why doesn't FeNa work after diuretics?
Diuretics act directly on renal sodium reabsorption, forcing sodium into the urine regardless of whether the underlying process is prerenal or intrinsic. This artificially raises FeNa in a patient who may actually have simple volume depletion, making the test misleading. Fractional excretion of urea is preferred in that setting because diuretics affect urea handling much less.
What FeNa values point to prerenal versus intrinsic AKI?
Below 1% suggests prerenal azotemia — the kidney's concentrating mechanism is intact and appropriately conserving sodium in response to reduced perfusion. Above 2% suggests intrinsic damage, most often acute tubular necrosis, where damaged tubules can't reabsorb sodium normally. The 1-2% zone is a gray area requiring clinical correlation.
Are there exceptions where a low FeNa doesn't mean prerenal?
Yes — early sepsis-associated AKI, contrast-induced nephropathy, and acute glomerulonephritis can present with a deceptively low FeNa despite being intrinsic processes, because these conditions can initially preserve tubular sodium handling. Clinical context, urinalysis, and urine microscopy for casts or eosinophils add important complementary information.
Do I need a 24-hour urine collection for this?
No — FeNa uses a single spot urine and simultaneous plasma sample, which is what makes it practical at the bedside compared to timed collections. Just be sure the urine and blood samples are drawn close together in time for an accurate ratio.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
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Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Fractional Excretion of Sodium (FeNa) Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/fena-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/fena-calculator" target="_blank" rel="noopener">Fractional Excretion of Sodium (FeNa) Calculator — RevenueLab</a> (2026).</p>
Source: [Fractional Excretion of Sodium (FeNa) Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/fena-calculator) (2026).