FENa Calculator (Fractional Excretion of Sodium)
The Fractional Excretion of Sodium (FENa) is one of the most useful calculations in nephrology for sorting out why a patient's kidneys are failing. It compares how much sodium the kidneys are filtering versus how much they are excreting, which tells you whether the tubules are working properly.
The FENa formula is: FENa = (Urine Na x Serum Cr) / (Serum Na x Urine Cr) x 100
This calculation requires a simultaneous serum and spot urine sample. You do not need a 24-hour urine collection.
How to interpret FENa:
- FENa < 1%: The kidneys are holding onto sodium tightly, which is normal when blood flow to the kidneys is reduced (prerenal azotemia). Causes include dehydration, hemorrhage, heart failure, liver cirrhosis, and sepsis. The tubules are intact and doing their job.
- FENa 1-2%: Gray zone. Could be either prerenal or intrinsic renal. Needs clinical correlation.
- FENa > 2%: The tubules have lost their ability to reabsorb sodium, suggesting intrinsic kidney damage. Acute tubular necrosis (ATN) is by far the most common cause, usually from ischemia or nephrotoxins.
Important limitations:
- Diuretics invalidate FENa by forcing sodium excretion regardless of renal perfusion. In patients on diuretics, use Fractional Excretion of Urea (FEUrea) instead.
- FENa can be misleadingly low (below 1%) in contrast nephropathy, rhabdomyolysis, early obstruction, and acute glomerulonephritis, even though these are intrinsic renal processes.
- In chronic kidney disease, baseline FENa may already be elevated, reducing its diagnostic utility.
Disclaimer: This calculator is for educational purposes only. Always interpret lab values in the full clinical context with your healthcare team.