Fractional Excretion of Urea (FEUrea)

Background

The fractional excretion of urea (FEUrea) is a urinary index used in acute kidney injury (AKI) to distinguish a pre-renal cause (reduced renal perfusion) from intrinsic renal damage such as acute tubular necrosis (ATN). It is calculated from paired urine and plasma measurements as (urine urea × plasma creatinine) ÷ (plasma urea × urine creatinine) × 100, expressed as a percentage. It works on the principle that, in pre-renal states, the tubules avidly reabsorb urea (and sodium), so very little of the filtered urea is excreted. Its main advantage over the fractional excretion of sodium (FENa) is that urea handling is far less affected by diuretics, so it remains usable in patients already taking them.

Interpreting the result

A FEUrea below about 35% suggests a pre-renal picture, where intact tubules are conserving urea in response to hypoperfusion, and the kidney is likely to recover with restoration of perfusion. A value above roughly 50% points towards intrinsic tubular injury (ATN), where damaged tubules can no longer reabsorb urea efficiently. The 35–50% band is indeterminate and should not be over-interpreted. The index supports, but never replaces, the overall clinical assessment of volume status, urine output and the trajectory of renal function.

Worked example

A patient on a loop diuretic presents with AKI: urine urea 250 mmol/L, plasma urea 20 mmol/L, urine creatinine 5000 µmol/L, plasma creatinine 200 µmol/L. FEUrea = (250 × 200) ÷ (20 × 5000) × 100 = 50%. This borderline value, taken with the diuretic use, is more reliable than FENa, which the diuretic would have falsely elevated.

Pearls / pitfalls

• Use the same units for urea and for creatinine across urine and plasma — the ratio cancels, so unit consistency matters more than the specific unit.
• Prefer FEUrea over FENa when the patient is on (or recently took) a loop or thiazide diuretic, which invalidates FENa.
• It is not reliable in chronic kidney disease, with osmotic diuresis, or where urea handling is otherwise disturbed (e.g. high-protein load, gastrointestinal bleeding).
• No urinary index alone diagnoses the cause of AKI — interpret alongside volume status, history, urinalysis and the clinical course.

Evidence & validation

Described by Carvounis and colleagues, who showed FEUrea retained discrimination between pre-renal AKI and ATN in patients on diuretics where FENa failed. It is widely taught as a complementary urinary index, although no single index is diagnostic in isolation and its performance varies across populations.

Frequently asked questions

References

1. Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223–2229.
2. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012.

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