The Fractional Excretion of Sodium (FENa) is a diagnostic tool used in nephrology to evaluate the cause of acute kidney injury (AKI). By assessing the kidney’s handling of sodium, FENa helps differentiate between prerenal azotemia, typically due to reduced renal perfusion, and intrinsic renal damage, such as acute tubular necrosis (ATN). The principle behind FENa lies in the fact that in states of hypoperfusion, the kidneys conserve sodium to maintain intravascular volume, resulting in a low FENa. Conversely, when the tubules are damaged, the ability to reabsorb sodium is impaired, leading to higher sodium excretion and a higher FENa.

Clinical application of FENa is particularly valuable in guiding fluid management, determining the need for further investigations, and avoiding unnecessary interventions. It is important to note limitations: FENa is less reliable in patients on diuretics, chronic kidney disease, or non-oliguric states. In such cases, other measures such as the Fractional Excretion of Urea (FEUrea) may be more accurate. Despite these caveats, FENa remains a simple, cost-effective, and widely used calculation in clinical practice for rapid assessment of renal function dynamics.