FENa Calculator
Fractional excretion of sodium from urine and serum labs, with the prerenal or intrinsic reading.
What it does
When creatinine climbs, the first question is whether the kidneys are underperfused or injured. FENa helps answer it by comparing how much sodium is filtered to how much actually reaches the urine. A fraction under 1 % usually means the tubules are working hard to keep sodium, a classic prerenal picture.
The tool takes four lab numbers: urine sodium, serum sodium, urine creatinine, and serum creatinine. The sodiums set the concentration side and the creatinines correct for water reabsorption, so the ratio reflects tubular handling rather than urine dilution.
Results land in one of three zones. Under 1 % is prerenal, over 2 % is intrinsic, and 1 to 2 % is the gray zone where neither label fits. Some references draw the upper line at 3 %; cutoffs vary by textbook while this tool follows the common 1 % and 2 % bands.
Diuretics, chronic kidney disease, contrast, and sample timing all bend the number, so read FENa next to the exam, the medication list, and the creatinine trend. In patients on diuretics, the fractional excretion of urea often tells the story better.
How to use
- Pick the creatinine unit your lab uses: mg/dL or µmol/L.
- Enter urine sodium and serum sodium in mmol/L from a matched pair of samples.
- Enter both creatinines in the selected unit. Serum creatinine is the small number, often near 1.0 mg/dL.
- Read FENa as a percent.
- Read the interpretation line for the prerenal, indeterminate, or intrinsic band.
- Sanity check the pairing: a spot urine and a serum draw from the same window.
Formula
- UNa
- Urine sodium, usually reported in mmol/L or mEq/L.
- PNa
- Serum sodium from the paired blood draw.
- UCr
- Urine creatinine in mg/dL or µmol/L.
- PCr
- Serum creatinine in the same unit as the urine value.
Creatinine appears top and bottom, so its unit cancels as long as the urine and serum entries use the same one; the µmol/L selector keeps the fields labeled correctly without rescaling. The classic bands are under 1 % prerenal and over 2 % intrinsic with 1 to 2 % indeterminate, though some sources put the upper line at 3 %. Diuretics, CKD, contrast exposure, and mismatched sample timing all blur the reading.
Worked example
Example: urine Na 10, serum Na 140
Urine sodium 10 mmol/L, serum sodium 140 mmol/L, urine creatinine 100 mg/dL, serum creatinine 1.0 mg/dL.
- Numerator: 10 x 1.0 = 10.
- Denominator: 140 x 100 = 14,000.
- Divide and scale: 10 / 14,000 x 100 = 0.071, shown as 0.07 %.
- Under 1 % reads as a prerenal pattern.
- The µmol/L mirror (urine 8840, serum 88.4) cancels to the same 0.07 %.
Reference table
| FENa | Reading | Sample case |
|---|---|---|
| 0.07 % | Prerenal pattern (under 1 %) | Default example: urine Na 10, serum Cr 1.0 |
| 1.07 % | Indeterminate range (1 to 2 %) | Urine Na 150 on the same serum numbers |
| 2.29 % | Intrinsic pattern (over 2 %) | Urine Na 80, urine Cr 50, serum Cr 2.0 |
Limits
- Diuretics raise urine sodium on their own and can push a prerenal picture over 1 %.
- Neonates and premature infants normally excrete a higher fraction, up to 2 to 5 %.
- Chronic kidney disease, contrast, and obstruction can make the picture read against type.
- Samples must be paired; values from different hours blend two pictures.
- One number does not diagnose anything; clinicians read it with the exam and the trend.
Common mistakes
- Treating the 1 to 2 % band as a yes or no answer instead of an indeterminate zone.
- Using urine from one collection and serum from a draw hours apart.
- Reading a single value in a patient on diuretics as proof of the cause.
- Forgetting that neonates run a higher baseline FENa.
- Mixing creatinine units between the urine and serum fields.
Try it yourself
Quick checks. Answers sit tucked under each question.
Urine Na 80, serum Na 140, urine Cr 50, serum Cr 2.0. What does FENa read?
(80 x 2.0) / (140 x 50) x 100 = 2.29 %, which lands in the intrinsic band above 2 %.
Urine Na 5, serum Na 140, urine Cr 100, serum Cr 1.0?
(5 x 1.0) / (140 x 100) x 100 = 0.04 %, deep in the prerenal zone.
Same sodiums with urine Cr 8840 µmol/L and serum Cr 88.4 µmol/L?
Still 0.07 %. The creatinine unit cancels top and bottom, so mg/dL and µmol/L agree when the selector matches.
Questions
What is FENa?
The percentage of filtered sodium that ends up in the urine. Under 1 % means the tubules are holding on to sodium, which fits a prerenal cause.
How do I read the result?
Under 1 % is a prerenal pattern, 1 to 2 % is indeterminate, and over 2 % points to intrinsic injury. The tool prints the band with the number.
Why do the creatinine units cancel?
Creatinine sits on both sides of the fraction, so any consistent unit works. Enter urine and serum in the same unit, and pick µmol/L when your lab reports SI units.
Does diuretic use break the reading?
Yes. Diuretics push sodium into the urine on their own, so a prerenal patient can read over 1 %. The fractional excretion of urea is often used in that setting.
Does chronic kidney disease change things?
Baseline excretion can differ, so read the number against the full history and exam rather than as a stand alone verdict.
Where should the samples come from?
A spot urine and a serum draw taken at the same time. Mixed timing lets sodium and creatinine drift between specimens and skews the ratio.
Sources
- Weill Cornell PICU: Fractional Excretion of Sodium (FENa)
- PMC: Fractional Excretion of Sodium (FENa), An Imperfect Tool for a Flawed Question