FEUrea Calculator
Calculate fractional excretion of urea to help differentiate the cause of acute kidney injury.
Serum / Blood Values
Urine Values
What Is the Fractional Excretion of Urea (FEUrea)?
The fractional excretion of urea (FEUrea) is a diagnostic calculation used to help distinguish between prerenal acute kidney injury (AKI) and intrinsic renal AKI. It measures the percentage of filtered urea that is excreted in the urine rather than reabsorbed by the kidneys.
In prerenal states, the kidneys conserve urea, leading to a low FEUrea. In intrinsic renal injury, the kidneys lose this concentrating ability, resulting in a higher FEUrea. This makes the FEUrea a more reliable marker than the fractional excretion of sodium (FENa) in certain clinical scenarios, particularly when patients are on diuretics.
How the FEUrea Is Calculated
The FEUrea is derived from four lab values: serum urea nitrogen (SUN), serum creatinine, urine urea nitrogen (UUN), and urine creatinine. The formula expresses the clearance of urea relative to the clearance of creatinine:
FEUrea (%) = (Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine) × 100
This calculation assumes that creatinine is freely filtered and not reabsorbed, providing a reliable reference for glomerular filtration. Urea, by contrast, is partially reabsorbed in the proximal tubule and collecting duct, making its excretion rate sensitive to renal perfusion.
Interpreting FEUrea Results
The FEUrea result is interpreted within a clinical context, but general thresholds are well established:
- FEUrea < 35% suggests a prerenal cause of AKI, such as volume depletion, heart failure, or decreased renal perfusion.
- FEUrea > 50% suggests intrinsic renal injury, such as acute tubular necrosis (ATN).
- FEUrea between 35% and 50% is considered indeterminate and requires further clinical correlation.
The FEUrea is particularly useful when the FENa is unreliable, such as in patients receiving diuretic therapy, where sodium handling is pharmacologically altered.
When to Use FEUrea Over FENa
The FENa has long been the standard for differentiating prerenal AKI from ATN. However, the FEUrea offers advantages in specific situations:
- Diuretic use: Diuretics increase sodium excretion, falsely elevating the FENa in prerenal states. The FEUrea is less affected by diuretics.
- Chronic kidney disease: The FENa may be elevated at baseline in CKD, reducing its diagnostic utility. The FEUrea retains better discriminatory value.
- Early AKI: The FEUrea may change earlier than the FENa in evolving AKI, potentially allowing for earlier recognition of prerenal physiology.
Limitations of the FEUrea
While the FEUrea is a valuable diagnostic tool, it has limitations that must be considered:
- Not a standalone test: The FEUrea must be interpreted alongside clinical history, physical examination, urine microscopy, and other lab values.
- Indeterminate range: Values between 35% and 50% do not clearly differentiate prerenal from intrinsic causes and require additional investigation.
- Confounding factors: High-protein diets, gastrointestinal bleeding, and corticosteroid use can increase urea production and affect the calculation.
- Timing matters: The FEUrea is most useful when measured early in the course of AKI. Delayed measurement may yield less discriminatory results.
Practical Use Cases
The FEUrea calculator is most commonly used in hospital settings where AKI is suspected. Typical scenarios include:
- A patient with acute kidney injury and unclear volume status, where the history is unreliable or conflicting.
- A patient on diuretics who develops AKI, where the FENa may be misleading.
- Serial monitoring of AKI progression, where trends in FEUrea may provide insight into the evolving pathophysiology.
FAQ
What is a normal FEUrea?
There is no single "normal" FEUrea value. In healthy individuals with normal renal function and euvolemia, the FEUrea typically ranges between 50% and 60%. The diagnostic thresholds for AKI differentiation are based on the expected physiological response to prerenal stress versus intrinsic injury.
Can FEUrea be used in patients with CKD?
Yes, the FEUrea is often preferred over the FENa in patients with chronic kidney disease. In CKD, the FENa may be chronically elevated due to impaired sodium handling, reducing its diagnostic accuracy. The FEUrea retains better discriminatory value in this population, though interpretation should account for baseline renal function.
Why is FEUrea more reliable than FENa in patients on diuretics?
Diuretics directly inhibit sodium reabsorption in the renal tubules, causing a pharmacologically induced increase in sodium excretion. This artificially elevates the FENa even when the kidney is experiencing prerenal hypoperfusion. Urea handling is less directly affected by diuretics, making the FEUrea a more accurate reflection of the kidney's response to perfusion status.
What does an FEUrea of 25% mean?
An FEUrea of 25% is below the 35% threshold and suggests a prerenal cause of acute kidney injury. This indicates that the kidneys are appropriately conserving urea in response to decreased perfusion, such as from volume depletion, heart failure, or hypotension. Clinical correlation with volume status and other findings is still necessary.
How quickly does FEUrea change in AKI?
The FEUrea can change relatively quickly in response to alterations in renal perfusion. In prerenal states, the FEUrea may decrease within hours as the kidneys conserve urea. In evolving ATN, the FEUrea may rise as tubular function deteriorates. Serial measurements over 12 to 24 hours can provide useful trend information.