Introduction
The FENa calculator (Fractional Excretion of Sodium) helps doctors figure out why a patient's kidneys are not working well. When someone has acute kidney injury (AKI), one of the first questions is whether the problem is caused by low blood flow to the kidneys (prerenal) or by damage inside the kidneys themselves (intrinsic renal). The FENa test answers this question by looking at how much sodium the kidneys are holding on to versus how much they let pass into the urine. Clinicians often pair it with a GFR calculator to get a fuller picture of overall kidney function.
To use this calculator, you need four lab values: serum sodium, serum creatinine, urine sodium, and urine creatinine. The first two come from a standard blood test like a Basic Metabolic Panel (BMP). The other two come from a spot urine sample. Enter your numbers, pick your units, and the tool does the math for you instantly.
A FENa below 1% usually points to prerenal AKI, meaning the kidneys are healthy but not getting enough blood. A FENa above 2% suggests intrinsic kidney damage, such as acute tubular necrosis (ATN). Values between 1% and 2% are less clear and need further clinical judgment. This calculator shows your result, a step-by-step solution, and a visual chart so you can see exactly where the value falls.
How to Use Our FENa Calculator
Enter four lab values from your blood and urine tests below. The calculator will give you your Fractional Excretion of Sodium (FENa) as a percentage and tell you if the result points to prerenal or intrinsic kidney injury.
Serum Sodium — Enter your blood sodium level. This number comes from a Basic Metabolic Panel (BMP) or similar blood test. The normal range is 136 to 145 mmol/L. You can switch the unit to mEq/L if needed. Serum sodium is also a key input for the serum osmolality calculator and the anion gap calculator, two other tools commonly used alongside FENa when evaluating kidney and metabolic status.
Serum Creatinine — Enter your blood creatinine level. This also comes from a BMP or blood panel. The normal range is 0.7 to 1.3 mg/dL. You can switch the unit to µmol/L if your lab report uses it.
Urine Sodium — Enter the sodium level from a spot urine sample. This comes from a urine electrolytes test. The normal range is 40 to 220 mmol/L. You can switch the unit to mEq/L if needed.
Urine Creatinine — Enter the creatinine level from the same spot urine sample. The normal range is 20 to 370 mg/dL. You can switch the unit to µmol/L or mmol/L if your lab uses a different unit. Tracking urine output alongside these lab values gives clinicians additional insight into how well the kidneys are functioning.
Once all four fields are filled in, click Calculate to see your FENa result, a color-coded interpretation, a step-by-step breakdown of the math, and a chart that shows where your value falls. Click Reset / Clear All to start over with new values.
What Is FENa (Fractional Excretion of Sodium)?
FENa stands for Fractional Excretion of Sodium. It is a simple test that helps doctors figure out why a person's kidneys are not working well. It measures how much sodium (salt) the kidneys are letting out compared to how much they filter. This tells doctors whether the problem is happening before the kidney, inside the kidney, or both.
How FENa Is Calculated
The FENa formula uses four lab values: urine sodium, serum sodium, urine creatinine, and serum creatinine. Two come from a blood test and two come from a urine sample. The formula is:
FENa = (Urine Sodium × Serum Creatinine) ÷ (Serum Sodium × Urine Creatinine) × 100%
The result is shown as a percentage. A low number means the kidneys are holding on to sodium. A high number means the kidneys are letting too much sodium escape.
What the Results Mean
- FENa below 1% — This points to prerenal acute kidney injury (AKI). The kidneys themselves are healthy, but they are not getting enough blood flow. Common causes include dehydration, blood loss, or heart failure. The kidneys respond by saving as much sodium as they can. In cases where heart failure is suspected, a cardiac index calculator can help assess how well the heart is pumping.
- FENa between 1% and 2% — This is an indeterminate range. The cause could be prerenal or intrinsic. More tests and clinical judgment are needed. The BUN creatinine ratio calculator is another tool that can help distinguish prerenal from intrinsic causes in this gray zone.
- FENa above 2% — This suggests intrinsic renal AKI, such as acute tubular necrosis (ATN). The kidney's filtering tubes are damaged and can no longer hold on to sodium properly.
When Is FENa Used?
Doctors use FENa when a patient shows signs of acute kidney injury — a sudden drop in kidney function. It is one of the first tools used to tell the difference between prerenal and intrinsic causes. Knowing the cause helps doctors choose the right treatment fast. The GFR calculator is typically used alongside FENa to quantify how much kidney function has been lost, while the corrected calcium calculator helps monitor calcium levels that can become abnormal when kidneys are impaired.
Limitations to Keep in Mind
FENa works best in patients who are not taking diuretics (water pills). Diuretics force the kidneys to push out extra sodium, which can make the FENa falsely high. In patients on diuretics, doctors often use FEUrea (fractional excretion of urea) instead. The URR calculator is another urea-based kidney metric used in dialysis settings. FENa may also be less reliable in chronic kidney disease, contrast dye exposure, or certain types of glomerulonephritis.