FENa Calculator (Fractional Excretion of Sodium)
Calculate fractional excretion of sodium based on urine sodium, plasma sodium, urine creatinine, and plasma creatinine. FENa Calculator (Fractional Excretion of Sodium) helps clinicians assess kidney function and distinguish prerenal from intrinsic renal causes.
Highlights
- Calculates fractional excretion of sodium (FENa) as a percentage.
- Uses paired urine and plasma sodium measurements.
- Incorporates urine and plasma creatinine values.
- Supports mEq/L, mmol/L, mg/dL, and µmol/L units.
- Helps assess kidney function and distinguish prerenal from intrinsic renal causes.
Table of Contents
How FENa Calculator Works?
The FENa Calculator estimates the fractional excretion of sodium, which is the percentage of filtered sodium eliminated in the urine. It uses paired urine and plasma measurements of sodium and creatinine. Creatinine provides a reference for filtration and helps normalize urinary sodium excretion.
The result is reported as a percentage. For example, using urine sodium of 40 mEq/L, plasma sodium of 140 mEq/L, urine creatinine of 100 mg/dL, and plasma creatinine of 1 mg/dL:
Common clinical interpretation ranges are approximate and must be considered alongside the patient's condition:
| FENa result | Typical interpretation |
|---|---|
| Less than 1% | Often consistent with increased sodium retention, such as a prerenal pattern |
| 1% to 2% | Intermediate range requiring clinical correlation |
| Greater than 2% | May suggest impaired tubular sodium reabsorption, often seen with intrinsic renal injury |
These ranges are guides rather than diagnostic cutoffs. The calculator accepts urine sodium and plasma sodium in mEq/L or mmol/L, and urine and plasma creatinine in mg/dL or µmol/L; paired inputs should be entered using compatible reporting conventions.
Practical Use Cases
Nephrologists can use FENa to support the assessment of acute kidney injury and compare a sodium-retaining pattern with possible intrinsic tubular injury. The result should be reviewed with urinalysis, urine sediment, kidney imaging, medication history, and the clinical course.
Emergency physicians can use the calculator as a rapid bedside estimate when evaluating oliguria or a rising creatinine. It may help organize the initial differential diagnosis, although treatment decisions should not rely on FENa alone.
Intensive care clinicians can use serial FENa estimates to monitor renal sodium handling in critically ill patients. Trends may provide context during hemodynamic changes, fluid resuscitation, or suspected acute tubular injury, provided the measurements are collected and interpreted in clinical context.
Medical students and trainees can use the calculator to practice the relationship between urine sodium, plasma sodium, urine creatinine, and plasma creatinine. It demonstrates how creatinine normalization converts paired concentration measurements into an estimate of the percentage of filtered sodium excreted.
Accuracy and Limitations
The calculator accurately applies the standard FENa equation to the four entered concentrations and reports the mathematical result as a percentage, but its clinical interpretation is limited. FENa can be misleading after diuretic use, with chronic kidney disease, early or mixed kidney injury, urinary obstruction, sepsis, contrast-associated injury, or disorders that alter sodium handling; it may also be less useful when urine output is very low or samples are not closely paired. Values below 1% do not definitively prove a prerenal cause, and values above 2% do not independently diagnose intrinsic renal injury. The result should be interpreted with volume status, medications, urinalysis, kidney function trends, and other clinical findings.
Quick Examples
Example #1: Prerenal Azotemia Pattern
Demonstrates low fractional sodium excretion, commonly associated with sodium retention in prerenal kidney dysfunction.
Example #2: Intrinsic Kidney Injury Pattern
Illustrates an elevated fractional sodium excretion consistent with impaired tubular sodium reabsorption.
Example #3: Intermediate Sodium Excretion
Provides a moderate FENa example for evaluating a patient with an uncertain or mixed cause of kidney dysfunction.
