FEUrea Calculator: Acute Kidney Injury Cause Differentiation

Fractional excretion of urea (FEUrea) is a diagnostic test used in nephrology and critical care medicine to determine the underlying cause of acute kidney injury (AKI). When a patient presents with rising serum creatinine, the first clinical question is whether the kidney itself is damaged (intrinsic AKI) or whether the kidneys are responding appropriately to reduced blood flow (prerenal azotemia). Making this distinction correctly is essential because the treatments are fundamentally different: prerenal AKI usually improves with fluid resuscitation and correction of the underlying hemodynamic problem, while intrinsic AKI may require nephrology consultation, avoidance of nephrotoxins, and supportive care. The traditional test for this distinction is the fractional excretion of sodium (FENa), which measures how much filtered sodium is reabsorbed. However, FENa has a significant limitation: it is unreliable in patients who have received diuretics, particularly loop diuretics like furosemide, because diuretics force urinary sodium excretion regardless of the underlying mechanism of AKI. Since many patients with AKI in clinical practice have already received diuretics before the test is ordered, FENa can give false-positive results for intrinsic injury in patients who actually have prerenal AKI. FEUrea was proposed as an alternative because urea reabsorption is not directly affected by loop diuretics, making it more reliable in this setting. The calculation is straightforward. FEUrea is the fraction of filtered urea that is ultimately excreted in the urine, expressed as a percentage. It is calculated as: FEUrea (%) = (Urine Urea / Serum Urea) × (Serum Creatinine / Urine Creatinine) × 100. All four values must be expressed in consistent units — either all in SI units (mmol/L for urea, µmol/L for creatinine) or all in conventional US units (mg/dL) — as long as the same unit system is used for both urine and serum of the same analyte. Interpretation follows established clinical thresholds. A FEUrea below 35% suggests prerenal azotemia: the kidneys are retaining urea efficiently because tubular function is intact and the body is conserving filtered solutes in response to low flow. A FEUrea above 50% suggests intrinsic kidney injury (most commonly acute tubular necrosis, or ATN): tubular dysfunction means that urea is not being reclaimed and a large proportion of filtered urea appears in the urine. Values between 35% and 50% fall in an indeterminate zone and require clinical correlation with the history, physical examination, fluid challenge response, and other investigations. It is important to recognize the limitations of FEUrea alongside its strengths. FEUrea can be falsely elevated in conditions that cause tubular urea losses without true intrinsic injury, such as high-protein intake, catabolic states, or certain medications. It can also be falsely low in early ATN before tubular dysfunction is established, or in patients with underlying chronic kidney disease who have already adapted their tubular handling of urea. Like all biomarkers, FEUrea is most useful when interpreted in the clinical context by a qualified nephrologist or intensivist, not as a standalone diagnostic criterion.