FENa

FENa measures the percentage of sodium filtered by the kidney that is actually excreted in the urine. In prerenal states, the kidneys attempt to maximize volume by reabsorbing sodium. In ATN, tubular damage prevents efficient reabsorption, leading to "salt wasting."

FENa is less reliable in patients with pre-existing CKD, as damaged tubules in a chronic state may have a baseline defect in sodium handling regardless of the acute insult.

Before the FENa, clinicians relied on simple urine sodium concentration. However, urine sodium is highly dependent on urine volume. Dr. Espinel's introduction of "fractional excretion" normalized sodium clearance to the GFR (using creatinine as a proxy), providing a far more accurate reflection of tubular function.