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Fractional Excretion of Urea (FEUrea) Calculator

Enter urine and plasma urea and creatinine to get FEUrea.

How it is calculated

FEUrea compares how much urea leaves in the urine with how much creatinine does, using one blood and one urine sample taken at about the same time. The formula is FEUrea (%) = (Uurea × Pcreat) / (Purea × Ucreat) × 100, where U is urine and P is plasma. The units cancel, so any unit works as long as both urea values share one unit and both creatinine values share another (for example urea in mg/dL and creatinine in mg/dL). The tool shows the result as a percentage with two decimals. Plasma urea and urine creatinine must be above zero, the urine urea and plasma creatinine cannot be negative, and a missing field gives no result. The tool reports only the percentage; interpreting it belongs to the treating clinician.

Worked example

Suppose urine urea is 500, plasma urea 80, urine creatinine 60 and plasma creatinine 3 (urea values in one unit, creatinine values in one unit). Numerator: 500 × 3 = 1500. Denominator: 80 × 60 = 4800. Ratio: 1500 / 4800 = 0.3125. Multiply by 100 to get 31.25%. Doubling the urine urea while leaving everything else unchanged would double the result to 62.5%.

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Common mistakes

  • Mixing units inside a pair, such as urine urea in mmol/L but plasma urea in mg/dL. The units only cancel when both urea values (and both creatinine values) use the same unit.
  • Entering blood urea nitrogen (BUN) for one sample and urea for the other. BUN and urea differ by a conversion factor, so use the same quantity in both.
  • Swapping the urine and plasma fields. The creatinine values sit on the opposite side of the fraction from their urea values, so a swap gives a wrong percentage without any error message.
  • Using samples taken hours apart. The urine and blood values describe the same moment, and a plasma creatinine from a different day will distort the ratio.
  • Typing the result as a fraction such as 0.31 into other notes. The tool already multiplies by 100 and reports a percentage.

About this calculator

FEUrea measures what fraction of the urea filtered by the kidneys actually makes it into the urine: FEUrea % = (urine urea × plasma creatinine) ÷ (plasma urea × urine creatinine) × 100. It does the same job as the better-known FENa (fractional excretion of sodium) — distinguishing prerenal azotemia (kidneys are fine, blood flow is low) from intrinsic acute kidney injury like acute tubular necrosis — but urea handling isn't disrupted by diuretics the way sodium handling is, so FEUrea stays reliable in patients already on loop or thiazide diuretics.

Nephrologists and intensive care physicians use it at the bedside when a patient with rising creatinine has recently received diuretics, muddying FENa: a value under 35% suggests a prerenal cause (volume depletion, low cardiac output), while values above 50% point toward intrinsic renal injury.

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