Your FENa result helps classify the etiology of acute kidney injury (AKI). A FENa below 1% suggests pre-renal azotemia — the kidneys are intact and appropriately conserving sodium in response to decreased renal perfusion. Common causes include dehydration, hemorrhage, heart failure, and hepatorenal syndrome. The kidney tubules are functioning normally and avidly reabsorbing sodium and water to maintain intravascular volume.

A FENa between 1% and 2% falls in an indeterminate zone that may represent either pre-renal or intrinsic renal disease, and additional clinical context is needed. A FENa above 2% suggests intrinsic renal disease, most commonly acute tubular necrosis (ATN), where damaged tubules can no longer effectively reabsorb sodium. Other causes include interstitial nephritis and acute glomerulonephritis. The FENa result should always be interpreted alongside the clinical presentation, urinalysis findings, and imaging.

Use FENa as part of the initial workup of acute kidney injury when you need to differentiate between pre-renal azotemia and intrinsic renal disease. It is most valuable in oliguric patients with a rising creatinine where the clinical distinction between volume depletion and tubular injury is uncertain. The test requires simultaneous measurement of urine sodium, serum sodium, urine creatinine, and serum creatinine — ideally obtained before any interventions.

FENa is most commonly calculated in the emergency department, inpatient wards, and ICU settings during the evaluation of new-onset AKI. It is a standard component of the nephrology consultation workup and is taught as a foundational concept in renal physiology and clinical medicine.

FENa is unreliable in patients receiving diuretics, which increase urinary sodium excretion and falsely elevate FENa even in pre-renal states. In diuretic-treated patients, the Fractional Excretion of Urea (FEUrea) is preferred because urea handling is less affected by diuretics. FENa is also unreliable in the setting of glycosuria (e.g., DKA), recent contrast dye administration, and chronic kidney disease where tubular adaptation has occurred.

Importantly, some causes of intrinsic renal disease can present with a low FENa (<1%), mimicking pre-renal physiology. These include contrast nephropathy, myoglobinuria (rhabdomyolysis), early ATN (before tubular damage is established), and acute glomerulonephritis. Conversely, pre-renal states superimposed on chronic kidney disease may show higher-than-expected FENa values. The test is most accurate when applied early in the course of AKI and before therapeutic interventions.