FEUrea (fractional excretion of urea) is a diagnostic index that helps distinguish prerenal azotemia — a volume-responsive drop in kidney function — from intrinsic acute kidney injury, using paired serum and urine urea nitrogen and creatinine values. It is most useful as a diuretic-safe alternative to FENa, since urea handling is far less affected by loop and thiazide diuretics than sodium handling is.
How FEUrea works
FEUrea compares the fraction of filtered urea that ends up in the urine to the fraction of filtered creatinine that does the same, expressed as a percentage: FEUrea = (Urine urea nitrogen x Serum creatinine) / (Serum urea nitrogen x Urine creatinine) x 100. Because creatinine is freely filtered by the glomerulus and undergoes minimal tubular reabsorption, it serves as a stable reference point against which urea's more variable handling can be measured.
When renal perfusion is reduced — as in dehydration, heart failure, or cirrhosis — the kidneys respond by reabsorbing more urea (and sodium) from the tubular fluid, driving FEUrea down. When the kidney tubules themselves are damaged, as in acute tubular necrosis, this reabsorptive capacity is impaired and FEUrea rises.
Why FEUrea instead of FENa
FENa (fractional excretion of sodium) is the classic index for the same prerenal-versus-intrinsic distinction, but it has a well-known limitation: loop diuretics (like furosemide) and thiazides directly increase urinary sodium excretion by blocking tubular sodium reabsorption, independent of the patient's underlying volume status. This can push FENa above its prerenal cutoff even when the true cause is still a reversible, perfusion-related one.
Urea reabsorption in the proximal tubule is largely governed by different transport mechanisms that are much less sensitive to loop and thiazide diuretics, so FEUrea's cutoffs remain more reliable in patients who are actively receiving these medications — a very common scenario in hospitalized patients with acute kidney injury.
Reading the result and its limits
A FEUrea below 35% suggests prerenal azotemia, and a FEUrea above 50% suggests intrinsic acute kidney injury; values between 35% and 50% fall into an indeterminate gray zone that does not clearly separate the two. FEUrea should always be interpreted alongside the full clinical picture — history, physical exam, medication list, and other labs — rather than as a stand-alone diagnostic test. It is less well-validated than FENa in some populations, and neither index is reliable in chronic kidney disease, obstructive (postrenal) causes, or when urine studies are collected outside a steady state. This calculator is for education and clinical decision support only and does not replace the judgment of a treating clinician.