CalculatorsKidneys and electrolytesFeNa calculator
FeNa calculator
Work out FeNa from four numbers, two from the urine and two from the blood, and see what it says about why the kidneys are struggling.
0.18%
Under 1%
In someone with acute kidney injury, under 1% points to too little blood reaching the kidneys (prerenal), such as from dehydration or heart failure, rather than damage inside the kidney.
See the working
FeNa = (urine Na × blood creatinine) ÷ (blood Na × urine creatinine) × 100(15 × 2.0) ÷ (140 × 120) × 100 = 0.18%FeNa only means something in acute kidney injury. In people with healthy kidneys it's normally under 1%, and diuretics push it up whatever the cause.
What FeNa measures
FeNa is the share of the sodium filtered by the kidneys that ends up in the urine. Healthy kidneys hold on to almost all of it.
When creatinine rises suddenly, FeNa helps tell two kinds of problem apart: kidneys getting too little blood (they still hold on to sodium, so FeNa is low) and kidneys that are damaged themselves (they let sodium through, so FeNa is high).
When it misleads
Diuretics make the kidneys lose sodium, so FeNa reads high whatever the cause; doctors often use the fractional excretion of urea instead. Long-standing kidney disease, contrast dye and some early kidney injuries can also blur it.
It's a test for acute kidney injury. On a routine panel with normal kidney function it has nothing to say.
Questions
What's a normal FeNa?
In people with healthy kidneys it's usually under 1%. The 1% and 2% lines only apply in acute kidney injury.
What does FeNa under 1% mean?
In acute kidney injury, the kidneys are holding on to sodium, which points to too little blood reaching them, such as from dehydration, bleeding or heart failure.
What does FeNa over 2% mean?
In acute kidney injury, the kidneys are losing sodium they would normally keep, which points to damage inside the kidney, most often acute tubular necrosis.
Do the creatinine units matter?
Only that both creatinines are in the same unit. They cancel out, so mg/dL with mg/dL or µmol/L with µmol/L both work.
Sources
Written by LifeFrom from the sources above, and not yet reviewed by a clinician. An estimate for your own understanding: it doesn't diagnose or treat anything, and it doesn't replace your doctor.
