Fractional Excretion of Sodium (FENa) Calculator

About

Fractional Excretion of Sodium (FENa) is a diagnostic index used in the evaluation of acute kidney injury (AKI) to differentiate prerenal azotemia from intrinsic renal failure, particularly acute tubular necrosis (ATN). The concept of FENa was first introduced by Espinel in 1976, who demonstrated that the percentage of filtered sodium excreted in urine could reliably distinguish prerenal from intrinsic causes of AKI. FENa is calculated from simultaneous measurements of sodium and creatinine in both serum and urine using the formula: (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100. In prerenal azotemia, the kidney tubules are intact and respond appropriately to decreased perfusion by avidly reabsorbing sodium, resulting in low fractional sodium excretion (FENa <1%). In contrast, in ATN, the damaged tubular epithelial cells lose their ability to reabsorb sodium, leading to high fractional sodium excretion (FENa >2%). Values between 1% and 2% are considered indeterminate and may be seen in a variety of conditions including prerenal azotemia with diuretic use, early or mild ATN, and certain glomerulonephritides. The test has several important limitations. Diuretics (especially loop and thiazide diuretics) increase sodium excretion and can falsely elevate FENa even in prerenal states, limiting their utility. In patients with chronic kidney disease, the baseline FENa may already be elevated due to compensatory natriuresis in remnant nephrons. In certain intrinsic renal diseases such as acute interstitial nephritis, contrast-induced nephropathy, and rhabdomyolysis, FENa may be less than 1% despite intrinsic damage, leading to misclassification. In these situations, the Fractional Excretion of Urea (FeUrea) is often preferred because urea handling is less affected by diuretics and provides better diagnostic accuracy in the setting of diuretic use. FENa remains a widely taught and utilized tool in nephrology, critical care, and internal medicine, valued for its simplicity and rapid diagnostic refinement in the oliguric AKI patient.

Formula

FENa (%) = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100

FENa = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100. This equation calculates the percentage of filtered sodium that is excreted in the urine. The numerator (Urine Na × Serum Cr) represents the actual sodium excreted, scaled by serum creatinine to normalize for kidney function. The denominator (Serum Na × Urine Cr) represents the filtered sodium load — the amount of sodium that passes through the glomeruli. The ratio of these two products gives the fractional excretion. All units cancel out because sodium (mEq/L) and creatinine (mg/dL) appear in both numerator and denominator, yielding a dimensionless percentage. A critical concept is that FENa reflects tubular function rather than glomerular function. When renal perfusion decreases in prerenal states, the body activates the renin-angiotensin-aldosterone system and sympathetic nervous system, causing the distal tubule and collecting duct to reabsorb more sodium and water. The result is low urine sodium concentration (<20 mEq/L) and low FENa (<1%). In ATN, the proximal tubular cells are damaged and cannot reabsorb sodium normally. The distal tubule receives a higher sodium load and cannot compensate, resulting in high urine sodium (>40 mEq/L) and high FENa (>2%). The units must be consistent — sodium in mEq/L, creatinine in mg/dL. The test requires simultaneous (same time of collection) serum and urine samples to be accurate. Neither sample can be more than a few hours apart, as serum values can change rapidly in AKI. The choice between spot and timed collection: FENa can be calculated from a spot urine sample and simultaneous serum sample, making it convenient and rapidly available. A 24-hour urine collection is not required, though timed collection may provide more accurate results in certain settings. Important mathematical considerations: if the patient has very low urine creatinine (very dilute urine), the denominator becomes small, and FENa may be artifactually elevated. Conversely, very concentrated urine (high urine creatinine) lowers FENa. This is why the ratio of sodium to creatinine clearance is more reliable than urine sodium concentration alone.

Score Interpretation

FENa is a fundamental diagnostic tool in the evaluation of acute kidney injury and is widely taught in nephrology, critical care, and internal medicine training programs. Its primary clinical value lies in rapidly differentiating between prerenal azotemia — a condition of decreased renal perfusion that is typically reversible with fluid resuscitation — and intrinsic renal failure, specifically acute tubular necrosis, which requires fundamentally different management including optimization of hemodynamics, avoidance of nephrotoxins, and supportive care. The clinical impact of correctly differentiating prerenal from intrinsic AKI is substantial. Prerenal AKI responds to volume expansion with isotonic crystalloids, and fluid challenge may rapidly restore renal function. Conversely, administering large volumes of fluid to a patient with ATN may cause volume overload, pulmonary edema, and worsen outcomes. FENa aids in this critical therapeutic decision. The KDIGO 2012 Clinical Practice Guideline for Acute Kidney Injury recommends the use of FENa as part of the diagnostic workup for AKI, though it emphasizes that FENa should be interpreted in the context of the full clinical picture, not in isolation. The guideline highlights specific situations where FENa may be misleading: patients receiving diuretics, those with CKD (where baseline FENa is already elevated), and early stages of certain intrinsic diseases where FENa may be paradoxically low. In these situations, the guideline suggests using FeUrea as an alternative. Specifically, in patients on diuretics, FeUrea <35% suggests prerenal etiology, while FeUrea >50% suggests intrinsic AKI. The diagnostic performance of FENa has been studied extensively. A classic study by Miller et al. reported that FENa <1% had a sensitivity of 78-85% and specificity of 75-85% for prerenal azotemia, with positive predictive value of approximately 80%. The test performs best in oliguric AKI and when measured prior to diuretic administration. FENa is less reliable in non-oliguric AKI, where values may be higher even in prerenal states. In modern clinical practice, FENa is used alongside urinary biomarkers such as neutrophil gelatinase-associated lipocalin and kidney injury molecule-1, which provide earlier detection of tubular injury before serum creatinine rises. However, FENa remains a valuable, low-cost, widely available first-line test that can be calculated from routine laboratory values without specialized equipment.

Reference Ranges