Which statement accurately describes the use of FENa in differentiating prerenal from intrinsic AKI, considering diuretic effects?

• A. FENa is always accurate and not affected by diuretics
• B. FEUA may be used when diuretics interfere
• C. FENa <10% confirms prerenal AKI
• D. FENa <1% suggests prerenal; >2% suggests ATN; diuretics can confound; FEUA may be used when diuretics interfere

Answer: (D)

The key idea is how the kidney handles sodium under different injury patterns and how that changes what FENa can tell us. In prerenal AKI, the kidneys conserve sodium to maintain volume, so there is very little sodium lost in urine and the fractional excretion of sodium tends to be very low (usually under about 1%). In intrinsic AKI like ATN, tubular damage impairs sodium reabsorption, so more sodium spills into the urine and FENa tends to rise (often above 2%).

But diuretics muddy this picture. Diuretics block sodium reabsorption, especially in the loop of Henle, which pushes FENa up even when the injury is prerenal. That makes FENa unreliable in patients who are actively taking diuretics. To work around this, clinicians turn to a different measure that is less affected by diuretics: the fractional excretion of urea (FEUrea, sometimes referred to as FEUA in shorthand). FEUrea remains more helpful because diuretics don’t alter urea handling in the same way, so a low FEUrea supports prerenal causes and a higher FEUrea supports intrinsic injury.

So the statement aligns with the reality that while FENa thresholds (low for prerenal, high for ATN) are useful, diuretics can confound them, and FEUrea can be used as an alternative when diuretics interfere.