From the Guidelines
A fractional excretion of urea (FEurea) of 40% is likely indicative of acute tubular necrosis (ATN) or other intrinsic renal causes, as it falls above the threshold suggested to separate hepatorenal syndrome (HRS) from non-HRS causes.
Causes of Elevated FEurea
- Intrinsic renal causes: Such as ATN, which is characterized by damage to the renal tubules, leading to impaired reabsorption of urea.
- Prerenal causes with significant renal impairment: Although prerenal causes typically have a low FEurea, severe renal impairment can lead to an elevated FEurea due to decreased urea reabsorption.
Rationale
The FEurea is a useful marker in differentiating between prerenal and intrinsic renal causes of acute kidney injury (AKI) 1. A FEurea of <28.16% has been suggested to indicate HRS, while values above this threshold are more likely to represent ATN or other intrinsic renal causes. However, it is essential to consider the clinical context and other diagnostic markers, such as the fractional excretion of sodium (FENa), to accurately diagnose the underlying cause of AKI.
Diagnostic Considerations
- FENa: A FENa <1% suggests prerenal causes, including HRS, while a FENa >1% is indicative of intrinsic renal causes, such as ATN 1.
- Clinical context: The patient's medical history, physical examination, and laboratory results should be considered when interpreting the FEurea and FENa values.
From the Research
Causes of a Fractional Excretion of Urea (FEurea) of 40%
- A FEurea of 40% can be indicative of transient acute kidney injury (AKI) in critically ill patients, especially if diuretics have been administered 2.
- However, the diagnostic performance of FEurea in evaluating AKI is not consistent across studies, with some finding it to be of little help in distinguishing between transient and persistent AKI 3.
- In patients with sepsis, a high FEurea (>31.5%) in combination with a high fractional excretion of sodium (FENa) (>0.36%) is strongly predictive of transient AKI 4.
- In contrast, a low FEurea (<35%) is often seen in patients with prerenal azotemia, and can be a more sensitive and specific index than FENa in differentiating between AKI due to prerenal azotemia and acute tubular necrosis (ATN) 5.
- The use of diuretics can affect the interpretation of FEurea, with some studies finding that FEurea is less influenced by diuretic therapy than FENa 5, 2.
- Overall, a FEurea of 40% should be interpreted in the context of the patient's clinical presentation and other laboratory results, including FENa and serum creatinine levels 4, 6.