Fractional excretion of urea (FEUrea) estimates the percentage of filtered urea excreted in the urine and is used as an adjunct when differentiating prerenal from intrinsic (tubular) acute kidney injury (AKI), especially in oliguric adults who have recently received diuretics.

The most cited prospective derivation study by Carvounis and colleagues enrolled 102 consecutive adult episodes of acute renal failure referred to a nephrology service (77 prerenal, including 27 on diuretics, and 25 acute tubular necrosis). Using cut points of FEUrea <35% to indicate prerenal azotemia and >50% to indicate acute tubular necrosis, FEUrea showed high diagnostic performance for distinguishing prerenal from tubular causes, including in patients treated with loop or thiazide diuretics.

Later cohorts have shown more modest discrimination. In a tertiary-centre prospective study of adults with AKI, FEUrea <35% had only moderate sensitivity and specificity for transient (prerenal) AKI, and performance was similar in patients with and without diuretics. In a critically ill ICU cohort, FEUrea at the traditional 35% threshold had poor ability to separate transient from persistent AKI, with sensitivity and specificity each around 60% and receiver-operating characteristic curves close to 0.6.

A recent systematic review and meta-analysis pooling hospitalised patients reported overall moderate diagnostic accuracy for FEUrea to distinguish prerenal from intrinsic AKI, with pooled sensitivity and specificity in the mid-70% range and only small advantages over fractional excretion of sodium. The AACC Academy guidance document recommends using FEUrea and other urine biochemistry indices as supportive markers alongside history, examination, haemodynamics, urine microscopy and imaging, rather than as stand-alone diagnostic tests.

FEUrea can be misleading in several situations, including advanced chronic kidney disease, high or low urea generation (for example high protein intake, gastrointestinal bleeding, severe liver disease, catabolic states or malnutrition), osmotic diuresis and intrinsic renal diseases such as glomerulonephritis, contrast nephropathy, rhabdomyolysis or obstructive uropathy. In these settings, a low FEUrea does not reliably imply a purely prerenal process, and interpretation should be cautious, particularly in septic or otherwise critically ill patients.

Fractional Excretion of Urea (FEUrea)