American Board of Internal Medicine (ABIM) Certification Practice Exam

What does the formula for Fractional Excretion of Sodium (FeNa) involve?

1. USodium, UCr, PSodium, and PCr

2. UUrea, UCr, PUrea, and PSodium

3. PSodium, UUrea, and USodium

4. PCr, PSodium, and UUrea

The correct answer is: USodium, UCr, PSodium, and PCr

The correct answer involves understanding the components that make up the Fractional Excretion of Sodium (FeNa) formula. FeNa is calculated to help differentiate between prerenal and intrinsic renal causes of acute kidney injury. The formula is given by: FeNa = (U_Na / P_Na) / (U_Cr / P_Cr) × 100 Where: - U_Na refers to the urine sodium concentration, - P_Na refers to the plasma sodium concentration, - U_Cr refers to the urine creatinine concentration, and - P_Cr refers to the plasma creatinine concentration. By utilizing these measurements, FeNa can determine the percentage of filtered sodium that is excreted in the urine. A lower FeNa (typically <1%) suggests a prerenal cause of acute kidney injury, indicating that the kidneys are conserving sodium due to low blood volume or low perfusion. In contrast, a higher FeNa (typically >2%) points towards intrinsic renal problems. In contrast, the other options do not include the necessary components for calculating FeNa. They either incorporate the wrong substances or omit critical elements required to accurately evaluate sodium handling by the kidneys. By staying aligned with the specific components of the