Fractional Excretion of Sodium (FENa)

Q: What does FENa < 1% mean?

A FENa < 1% suggests prerenal azotemia, meaning the kidneys are appropriately retaining sodium in response to decreased perfusion.

Q: When is FENa unreliable?

FENa is unreliable in patients on diuretics, in chronic kidney disease, and in the elderly. Consider FEUrea in these situations.

Calculates the fractional excretion of sodium to distinguish prerenal azotemia from acute tubular necrosis.

Nephrology Internal MedicineEvidence-BasedUpdated 2026-08-06

Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.

Clinical Notes

The fractional excretion of sodium (FENa) expresses the percentage of filtered sodium that is excreted in urine, calculated as (urine sodium / plasma sodium) divided by (urine creatinine / plasma creatinine), multiplied by 100. Because the healthy kidney normally resorbs most filtered sodium, a low FENa can reflect intact tubular sodium conservation, a pattern classically associated with prerenal azotemia, whereas a higher FENa can accompany intrinsic tubular damage such as acute tubular necrosis. FENa is an adjunctive index in the evaluation of acute kidney injury, not a stand-alone diagnostic test. A single value should never be interpreted in isolation; it is meaningful only alongside urine microscopy, urine osmolality, electrolyte values, and the overall clinical picture, and its interpretation relies on correct, contemporaneous urine and plasma samples. Reliability declines in settings that alter sodium handling independently of tubular health. Loop and thiazide diuretics blunt the physiologic low FENa, so this index is unreliable in patients on these agents or in early acute kidney injury (within the first hours), where values may not yet reflect the underlying process. Low FENa values can also be seen in, for example, contrast-associated acute kidney injury and in conditions with intense tubular sodium avidity, so the numerical range alone does not prove a prerenal mechanism. The result supports differential diagnosis of established kidney dysfunction and is not a diagnosis by itself. Interpretation requires the concurrent clinical context, and where diuretic use or an early time course confounds the result, alternatives such as the fractional excretion of urea may be more informative.

Calculator Information

Specialty

Internal Medicine

📋 Reference Ranges

Normal Range

< 1% in prerenal azotemia

Reference ranges for this calculator
Classification	Range
Prerenal azotemia	<1.1
Indeterminate	1–2
Intrinsic renal injury (ATN)	≥2

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

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

How the Calculation Works

Fractional excretion of sodium = (urine sodium x plasma creatinine) / (plasma sodium x urine creatinine) x 100, expressed as a percentage.

Variables Explained

Urine Sodium

Urine sodium in mmol/L, used in the numerator.

Plasma Sodium

Plasma sodium in mmol/L, used in the denominator.

Urine Creatinine

Urine creatinine in mg/dL, used in the denominator.

Plasma Creatinine

Plasma creatinine in mg/dL, used in the numerator.

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 It Is Useful

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 It Is Not Useful

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

Common Mistakes

Ordering FENa in a patient on loop or thiazide diuretics, which blunts the low urinary sodium pattern the test depends on.
Calling prerenal azotemia from a value below 1% in an older patient or one with chronic kidney disease, where thresholds lose reliability.
Interpreting FENa in the first hours of acute kidney injury, before urine sodium reflects the underlying process.

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.

Inputs:

urineNa: 20

plasmaNa: 140

urineCr: 100

plasmaCr: 1.0

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

Evidence & References

Source:: Nephrology
Content updated:: 2026-08

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

References

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

What does FENa < 1% mean?

A FENa < 1% suggests prerenal azotemia, meaning the kidneys are appropriately retaining sodium in response to decreased perfusion.

When is FENa unreliable?

FENa is unreliable in patients on diuretics, in chronic kidney disease, and in the elderly. Consider FEUrea in these situations.

Disclaimer: This calculator is intended for educational and clinical decision support purposes only. FENa must be interpreted with the clinical context and is unreliable in patients on diuretics or with chronic kidney disease.