Fractional Excretion of Urea (FEUrea)

The Fractional Excretion of Urea (FEUrea) is a calculated index used to determine whether oliguria or acute kidney injury is caused by pre-renal hypoperfusion (functional, reversible) or intrinsic renal injury such as acute tubular necrosis (ATN). It expresses what proportion of the urea filtered at the glomerulus is ultimately excreted in the urine, rather than reabsorbed by the tubules. In pre-renal states — dehydration, haemorrhage, heart failure, sepsis — the kidneys respond to low perfusion by maximally conserving sodium and water. Urea is passively reabsorbed along with water in the collecting duct under the influence of antidiuretic hormone (ADH), so a disproportionately small fraction of filtered urea is excreted. FEUrea below 35% therefore suggests the tubules are intact and functioning appropriately in the face of underperfusion. In ATN, tubular cells are damaged and cannot reabsorb sodium or concentrate urine effectively. Water (and urea) is excreted in larger proportions, yielding FEUrea above 50%. The critical clinical advantage of FEUrea over Fractional Excretion of Sodium (FENa) is its independence from diuretic use. Furosemide and thiazide diuretics inhibit tubular sodium reabsorption, artificially elevating FENa into the intrinsic range even when the underlying cause of AKI is pre-renal. FEUrea is not meaningfully affected by loop or thiazide diuretics, making it the preferred test in the common clinical scenario of a patient with AKI who has received diuretics.

PrimeCalcPro provides professional-grade tools trusted by businesses and academics.

1. 1Collect simultaneous blood and urine samples: measure plasma urea (Purea), plasma creatinine (PCr), urine urea (Uurea), and urine creatinine (UCr) — all in the same units (mmol/L or mg/dL).
2. 2The formula derives from the concept of filtration fraction: FEUrea = (amount of urea excreted / amount of urea filtered) × 100.
3. 3Since filtered load = plasma concentration × GFR, and GFR can be estimated from creatinine clearance, the creatinine terms cancel algebraically, leaving FEUrea = (Uurea × PCr) / (Purea × UCr) × 100.
4. 4Interpret FEUrea <35%: the kidney is conserving urea — tubular function is intact — consistent with pre-renal AKI. Volume resuscitation is likely the appropriate intervention.
5. 5Interpret FEUrea >50%: a large fraction of filtered urea is being excreted — tubular reabsorption is impaired — consistent with ATN or other intrinsic renal injury.
6. 6The grey zone of 35–50% represents indeterminate results; integrate with clinical context, urine microscopy (muddy brown casts favour ATN), and response to fluid challenge.
7. 7Reassess after treating the underlying cause — FEUrea normalises rapidly in pre-renal AKI with adequate rehydration, but remains elevated in established ATN until tubular recovery occurs (typically days to weeks).

• !Using FENa instead of FEUrea in patients who have received furosemide or thiazides — diuretics invalidate FENa but not FEUrea.
• !Mixing SI and US units in the formula (e.g., plasma creatinine in mg/dL and urine creatinine in mmol/L) — all four values must be in the same units.
• !Applying FEUrea thresholds in chronic kidney disease or stable patients — the index is validated only for acute kidney injury.
• !Ignoring the indeterminate zone (35-50%) and forcing a binary pre-renal vs ATN decision without additional clinical data.
• !Not accounting for the effect of high protein intake or catabolic states on plasma urea — a very high plasma urea from catabolism (not just renal retention) can distort the ratio.
• !Failing to exclude urinary tract obstruction with imaging before using urinary indices to classify AKI.