Any unit works as long as the serum creatinine below uses the same one.
The equation, and where it comes from
FENa = [(urine Na x serum creatinine) / (serum Na x urine creatinine)] x 100
Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.
How to read the result
Below 1 percent classically points to a prerenal picture and above 2 percent to acute tubular necrosis, but the test is an adjunct: contrast nephropathy, early obstruction and glomerulonephritis can all give a low FENa despite intrinsic kidney injury.
Your patient's numbers never leave this device
The fena arithmetic runs in JavaScript inside your own browser. There is no server call when you type, nothing is written to a database, nothing is logged, and no account is involved. Once the page has loaded you can disconnect from the network and it still works.
That matters more here than on an ordinary calculator. Values entered on a hospital workstation come off a real chart, and this tool deliberately does not remember them: close the tab and they are gone, including for the next person who uses that machine.
Frequently asked questions
- What is the FENa formula?
- FENa = [(urine sodium x serum creatinine) / (serum sodium x urine creatinine)] x 100. It is the proportion of filtered sodium that ends up in the urine, and creatinine appears on both sides as a stand-in for the filtered volume.
- Do the units have to match?
- Each pair has to match, but the pairs are independent. Both sodium values in the same unit and both creatinine values in the same unit is enough, because the units cancel in the ratio. You can use mg/dL or µmol/L creatinine without converting, as long as you do not mix them.
- Why is FENa useless on diuretics?
- A diuretic works by forcing sodium excretion, which is precisely the quantity FENa measures. A patient on furosemide can be profoundly volume-depleted and still spill sodium, producing a high FENa that says nothing about their volume state. Fractional excretion of urea is the usual substitute, with a threshold near 35 percent.
- Can FENa be low in intrinsic kidney injury?
- Yes, and this is the trap. Contrast-induced nephropathy, rhabdomyolysis, early obstruction, acute glomerulonephritis and hepatorenal syndrome can all produce a FENa below 1 percent despite the injury being intrinsic. A low FENa narrows the differential; it does not close it.
- What sample timing does this need?
- A spot urine taken at roughly the same time as the serum sample. A timed collection is unnecessary because the equation uses a ratio rather than a rate, which is the practical advantage of a fractional excretion over a measured clearance.