FENa (%) = (urine sodium × blood creatinine) ÷ (blood sodium × urine creatinine) × 100, with the urine and blood samples taken at about the same time.
In Espinel's 1976 study of people in the low urine phase of sudden kidney failure, FENa was below 1% when reduced blood flow to the kidneys was the cause and above 3% with acute tubular necrosis, a form of injury to the kidney tubules. Values between 1% and 3% fall in a gray zone.
A prospective study two years later found that simpler urine measures often fell outside their guideline cut-offs in people with low urine output. In that setting, FENa and the related renal failure index separated reversible reduced blood flow from tubular injury more reliably.
In most healthy people FENa is usually below 1%, and a salty diet can raise it. A value below 1% in someone with normal kidney function says little, so FENa only carries meaning when kidney function has dropped suddenly. That is why this calculator is best seen as a way to understand a hospital measure rather than as a wellness score.
Diuretics block sodium reabsorption and push FENa up. In one study, 92% of people with reduced kidney blood flow had a FENa below 1%, but only 48% of those taking diuretics did. A high salt intake and intravenous fluids containing sodium can raise it too.
Several conditions can produce a low FENa without low fluid volume being the cause. A 1984 review lists heart and liver failure, acute glomerulonephritis, contrast injury, muscle pigment injury, acute obstruction and transplant rejection among them, alongside some cases of kidney failure with normal urine output.
Long standing kidney disease can keep FENa above 1% even when the current problem is reduced blood flow, the same review notes. Timing matters as well, because the urine and blood samples need to be taken close together for the formula to hold.
FENa is a snapshot of one moment in an acute problem, so the care team often repeats it as the situation changes, for example after fluids are given or a medicine is paused. A single value early in an illness can look quite different a day later.
Outside a hospital there is rarely a reason to calculate FENa on routine labs. A sudden rise in creatinine, a big drop in urine output, or swelling with breathlessness is worth raising with a clinician promptly, and FENa would be one of several tests they might use.
If you are looking back at a FENa from a hospital stay, useful questions for the care team are what else was drawn at the same time, whether diuretics or contrast had been given, and how the result changed over the following days.
The classic cut-offs come from small groups of hospital patients with low urine output. They were not built for people still passing normal volumes of urine or for people with long standing kidney disease, and they do not apply to wellness testing.
FENa also assumes creatinine and sodium are stable around the time of collection. Contrast dye, recent diuretics and a high salt load can all distort it, and many real situations mix more than one cause, which no single ratio can separate. The 1984 review concludes it must be read in light of the specific clinical setting and other lab data.
The renal failure index from the 1978 study works in a similar way, dividing urine sodium by the urine to blood creatinine ratio. It shares the same dependence on timing and diuretics, so it offers a cross-check rather than a way around those limits.
Take a urine sodium of 20 mEq/L, a blood sodium of 140 mEq/L, a urine creatinine of 100 mg/dL and a blood creatinine of 1.5 mg/dL, collected at about the same time. The top line is urine sodium times blood creatinine, 20 times 1.5, which is 30. The bottom line is blood sodium times urine creatinine, 140 times 100, which is 14,000.
Dividing 30 by 14,000 gives 0.00214, and multiplying by 100 gives a FENa of 0.21%. In the setting it was studied, sudden kidney failure with low urine output and no diuretics, a value this far below 1% pointed toward reduced blood flow to the kidneys. A clinician would still read it with the history, fluid status and other tests, since heart or liver failure can also produce a low value.
Change one input and the picture can shift. If the same person's urine sodium were 120 mEq/L with everything else unchanged, the top line would be 180 and FENa would be 1.29%, inside the gray zone, which is the kind of result a recent diuretic can produce.
Fractional excretion of urea follows the same logic but uses urea nitrogen in place of sodium, and it is less affected by diuretics. One study found a value of 35% or less separated reduced blood flow from tubular injury better than FENa, and 89% of people on diuretics with reduced blood flow had a low urea fraction.
Urine sodium alone is simpler. In the 1978 study, values below 20 mEq/L suggested reduced blood flow and values above 40 suggested tubular injury, but many patients fell in between. FENa brings in creatinine to correct for how concentrated the urine is.
The BUN to creatinine ratio is the blood only cousin, with a traditional line at 20. A large hospital study found that line did not reliably mark reduced blood flow, so it adds context rather than replacing a urine based measure like FENa. It is still a useful first look, since it needs no urine sample.
The Kidney Function panel measures the values this calculator uses. Related: sodium, creatinine, bun.
Yes. FENa needs urine sodium and urine creatinine from a urine sample, plus blood sodium and creatinine taken at about the same time.
In the setting it was studied, sudden kidney failure with low urine output, a value below 1% pointed toward reduced blood flow to the kidneys rather than kidney injury. Outside that setting it is hard to interpret.
Yes. Diuretics change how the kidneys handle sodium, so FENa can read higher than the underlying problem would suggest.