American Board of Internal Medicine (ABIM) Certification Practice Exam

What does a FeUrea value of less than 35% suggest?

1. A renal state

2. A prerenal state

3. A postrenal state

4. A stable state

The correct answer is: A prerenal state

A FeUrea value of less than 35% indicates a prerenal state, suggesting that the patient's renal function is affected by factors before the kidneys, such as decreased renal perfusion. In prerenal azotemia, the kidneys are functioning properly but are not receiving adequate blood flow, which can occur in situations like dehydration, heart failure, or hypovolemia. The rationale behind this interpretation is based on the understanding of the fractional excretion of urea. In conditions of low perfusion, the kidneys conserve urea to maintain serum levels, resulting in lower fractional excretion values. A FeUrea value below 35% reflects this conservation of urea, thus supporting the diagnosis of prerenal azotemia. In contrast, a higher FeUrea value would suggest possible intrinsic renal pathology or postrenal obstruction, as the kidneys would not be functioning properly in these states.