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.
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 calculates your result instantly.
A FENa below 1% usually points to prerenal AKI, meaning the kidneys are healthy but not getting enough blood.2 A FENa above 2% suggests intrinsic kidney damage, such as acute tubular necrosis (ATN). The original study put acute tubular necrosis above 3%,2 and cutoff values differ between studies, so the 2% line here is a middle setting rather than a fixed rule.1 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. A typical reference range is 136 to 144 mmol/L.4 You can switch the unit to mEq/L if needed.
Serum Creatinine: Enter your blood creatinine level. This also comes from a BMP or blood panel. Typical reference ranges are 0.7 to 1.3 mg/dL for men and 0.5 to 0.95 mg/dL for women.5 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. Adult urine sodium is generally above 20 mEq/L in a random sample, and 40 to 220 mEq per day over 24 hours; the value depends on how much fluid and salt you take in.6 You can switch the unit to mEq/L if needed.
Urine Creatinine: Enter the creatinine level from the same spot urine sample. Spot urine creatinine varies widely with hydration; over 24 hours, adult excretion runs about 500 to 2000 mg per day.7 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).2 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.
- FENa between 1% and 2%: This is an indeterminate range. The cause could be prerenal or intrinsic. More tests and clinical judgment are needed.
- 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.
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.1 In patients on diuretics, doctors often use FEUrea (fractional excretion of urea) instead.3 FENa can also read falsely low in several intrinsic conditions, including contrast-induced nephropathy, rhabdomyolysis, and acute glomerulonephritis.1