From the Guidelines
To calculate a fractional excretion of urea (FeUrea), you need to use the formula: FeUrea = (Urine Urea × Serum Creatinine) ÷ (Serum Urea × Urine Creatinine) × 100, as this method has been shown to be useful in differentiating between prerenal and intrinsic causes of acute kidney injury, particularly in patients with cirrhosis 1. When calculating FeUrea, it is essential to measure both urine and serum concentrations of urea and creatinine.
- Collect a spot urine sample and a blood sample at approximately the same time.
- Send these samples to the laboratory requesting urea and creatinine measurements for both.
- Once you have all four values, plug them into the formula to calculate the percentage. The FeUrea calculation is particularly useful because urea handling by the kidneys is less affected by diuretics, making it a more reliable indicator of kidney function in patients who have received these medications, as noted in a recent clinical practice update on acute kidney injury in patients with cirrhosis 1. A FeUrea less than 28.16% has been suggested to have a sensitivity of 75% and specificity of 83% in separating hepatorenal syndrome (HRS) from non-HRS, according to a small retrospective study cited in the update 1. This test can help differentiate between prerenal and intrinsic causes of acute kidney injury, which is crucial for guiding treatment decisions and improving patient outcomes.
From the Research
Calculating FeA Urea
To calculate the FeA urea, we need to understand the concept of urea excretion and its relationship with renal function.
- Urea excretion is influenced by various factors, including renal perfusion and ureagenesis 2.
- The study published in The International journal of artificial organs in 1996 found that urinary urea nitrogen excretion rates can distinguish between renal hypoperfusion and hyperureagenesis as causes of hyperuremia 2.
Urea Excretion Rates
- The study measured 24-hour urinary urea nitrogen levels and found significant differences between patients with renal hypoperfusion (group I) and those with hyperureagenesis (group II) 2.
- The 24-hour urinary urea nitrogen levels were 4.8 +/- 2.9 gm in group I and 13.6 +/- 3.2 gm in group II, with a p-value of < 0.001 2.
Calculating FeA Urea
- Although the study does not provide a direct formula for calculating FeA urea, it suggests that timed and random urine urea nitrogen determinations can assist in differentiating prerenal azotemia due to renal hypoperfusion from hyperureagenesis 2.
- Further research is needed to establish a specific formula or method for calculating FeA urea, as the provided study focuses on differentiating causes of prerenal azotemia rather than calculating FeA urea directly 2.