Fractional Excretion of Sodium Calculator

FENa answers a question that creatinine alone cannot: in a patient with acute kidney injury, are the kidneys structurally intact and simply under-perfused (prerenal azotemia), or has actual tubular damage occurred (acute tubular necrosis, ATN)? A structurally healthy but under-perfused kidney aggressively reabsorbs filtered sodium to conserve volume, producing concentrated, sodium-poor urine and a low FENa (typically under 1%). Once the tubular epithelium is injured, that reabsorptive capacity is lost, sodium spills into the urine relatively unchecked, and FENa rises above 2%, regardless of how much volume the patient actually has. The test's biggest limitation is diuretic use: loop and thiazide diuretics directly block tubular sodium reabsorption, artificially raising FENa in a volume-depleted patient and mimicking ATN even when the kidney is prerenal. In diuretic-exposed patients, fractional excretion of urea (FEUrea) is used instead, since urea handling is less affected by diuretics, with a cutoff below 35% suggesting prerenal physiology. Because FENa reflects tubular sodium handling and not a fixed disease category, it is interpreted alongside the clinical picture — volume status, medication history, and urine microscopy for granular casts — rather than as a standalone diagnosis.

The physiological rationale for fractional excretion measurements traces to mid-20th-century renal physiology research on how the kidney regulates sodium and water balance under conditions of reduced perfusion, building on the clearance-ratio concepts pioneered by Homer Smith. Applying this specifically to distinguish prerenal azotemia from intrinsic acute renal failure required a clinical dataset linking urine chemistry patterns to biopsy-confirmed or clinically adjudicated causes of AKI. Carlos Espinel formalized the test and established the widely cited 1% and 2% cutoffs in a study published in the mid-1970s, comparing urine and serum sodium and creatinine ratios against clinical diagnoses of prerenal azotemia versus acute tubular necrosis. FENa was rapidly adopted into nephrology teaching and hospital medicine as a simple, low-cost bedside test requiring only a spot urine and serum sample, well before more invasive diagnostic tools were considered. Subsequent literature through the 1990s and 2000s identified its diuretic-related limitations and established fractional excretion of urea as the preferred alternative test in diuretic-exposed patients, refinements that are now standard teaching alongside the original FENa cutoffs in KDIGO and nephrology reference guidelines.