Fractional Excretion of Sodium (FENa)

What This Calculator Measures

Quantifies the fractional excretion of sodium to help distinguish prerenal azotemia from intrinsic renal injury (such as acute tubular necrosis) in acute kidney injury.

How to Use

1. Obtain a spot urine sodium and urine creatinine together with paired plasma sodium and plasma creatinine.
2. Enter all four values.
3. Review the FENa and its suggested category.
4. In patients receiving diuretics, consider FEUrea instead.

Interpretation Guide

FENa < 1% suggests prerenal azotemia (the kidneys are appropriately conserving sodium), 1–2% is indeterminate, and > 2% suggests intrinsic renal injury such as ATN. Interpretation is unreliable in the setting of diuretics, chronic kidney disease, and in the elderly.

Clinical Significance

FENa is a classic bedside test that helps clinicians identify potentially reversible prerenal causes of AKI, where prompt volume resuscitation may prevent progression to intrinsic injury.

When to Use

• Evaluation of acute kidney injury to differentiate prerenal from intrinsic causes
• When paired urine and plasma electrolytes are available
• Complementing the BUN/Cr ratio and FEUrea

When Not to Use

• In patients receiving diuretics — prefer FEUrea
• In chronic kidney disease or the elderly, where thresholds are less reliable
• In acute kidney injury with rapidly changing renal function
• As a standalone diagnostic test

Limitations

• Diuretics increase urinary sodium excretion and invalidate the test.
• Chronic kidney disease and older age reduce its discriminating ability.
• Requires spot urine and plasma samples from the same time point.
• Results may be variable with obstruction and other intrinsic causes.
• Does not, by itself, establish the cause of acute kidney injury.

Worked Example

A 55-year-old man with diarrhea-induced volume depletion has a urine sodium of 20 mmol/L, plasma sodium 140 mmol/L, urine creatinine 100 mg/dL, and plasma creatinine 1.0 mg/dL.

FENa = (20/140) ÷ (100/1.0) × 100 ≈ 0.14%. This low value suggests prerenal azotemia — the kidneys are conserving sodium appropriately.

Evidence

Source

Nephrology

Reference

Carvounis CP, et al. Significance of the fractional excretion of sodium in the diagnosis of acute renal failure. Kidney Int. 2002;62(3):1184-1191.

Reviewed By

MedCalcHub Clinical Team

Version

1.0

Updated

2026-08

1. Carvounis CP, et al. Kidney Int. 2002;62(3):1184-1191.

References

1. Carvounis CP, et al. Significance of the fractional excretion of sodium in the diagnosis of acute renal failure. Kidney Int. 2002;62(3):1184-1191. (Original Description)

Frequently Asked Questions