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The Fractional Excretion of Sodium (FENa) Calculator determines the percentage of filtered sodium that is excreted in the urine. FENa is one of the most important biochemical tests for differentiating pre-renal azotemia from acute tubular necrosis (ATN) in patients presenting with acute kidney injury. This test capitalizes on the kidney's ability to avidly reabsorb sodium during states of reduced perfusion, a response that is lost when the tubules are damaged.
FENa is calculated as: FENa (%) = (Urine Na x Plasma Cr) / (Plasma Na x Urine Cr) x 100. This formula effectively compares sodium clearance to creatinine clearance, providing a normalized measure of sodium handling independent of urine flow rate and concentration. A FENa below 1% indicates that the kidney is reabsorbing more than 99% of filtered sodium, consistent with an intact tubular response to reduced perfusion (pre-renal azotemia). A FENa above 2% indicates impaired tubular sodium reabsorption, suggesting tubular damage (ATN).
The physiological basis for FENa interpretation lies in the kidney's response to hypoperfusion. When effective circulating volume is reduced, the renin-angiotensin-aldosterone system (RAAS) is activated, antidiuretic hormone (ADH) is released, and sympathetic nervous system activity increases. These neurohormonal responses drive the proximal and distal tubules to maximally reabsorb sodium and water, resulting in low urine sodium concentration and low FENa. In ATN, the damaged tubular epithelium cannot effectively reabsorb sodium regardless of hormonal stimulation, resulting in sodium wasting and elevated FENa.
Clinical applications of FENa extend beyond simple pre-renal versus ATN differentiation. FENa below 1% is also seen in early urinary tract obstruction, acute glomerulonephritis, contrast-induced nephropathy in the first 24 hours, rhabdomyolysis-induced AKI, and some cases of sepsis-associated AKI where renal perfusion is compromised but tubular function is preserved. FENa above 2% can occur in patients receiving diuretics (which pharmacologically increase sodium excretion), underlying CKD with impaired concentrating ability, and salt-wasting nephropathies.
The most significant limitation of FENa is its unreliability in patients receiving diuretics, which artificially elevate urinary sodium excretion and can produce FENa above 1% even in pre-renal states. In these patients, fractional excretion of urea (FEUrea) is preferred because urea handling is less affected by diuretics. Other limitations include the requirement for simultaneous blood and urine samples, interference by IV normal saline infusion (which increases urinary sodium), and the indeterminate zone between 1% and 2% where either diagnosis is possible.
Timing of the urine sample is also important. FENa should be obtained before starting IV fluids or diuretics for optimal diagnostic accuracy. A spot urine sample obtained simultaneously with the blood draw is sufficient; a 24-hour urine collection is not required. The test provides the most useful information during the early evaluation of AKI when the distinction between reversible pre-renal azotemia and established ATN has the greatest impact on management decisions.
FENa is calculated as (Urine Na x Plasma Creatinine) / (Plasma Na x Urine Creatinine) x 100. This ratio normalizes sodium excretion to creatinine clearance, making it independent of urine volume. A value below 1% indicates avid sodium reabsorption (pre-renal), while above 2% indicates impaired tubular sodium reabsorption (intrinsic renal disease, typically ATN).
FENa below 1% suggests pre-renal azotemia (volume depletion, heart failure, hepatorenal syndrome). FENa above 2% suggests intrinsic renal disease, particularly ATN. FENa 1-2% is indeterminate. Important exceptions: diuretic use invalidates FENa (use FEUrea instead), and some intrinsic causes (contrast nephropathy, early ATN, rhabdomyolysis) may have FENa below 1%.
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FENa of 0.12% indicates avid sodium retention consistent with pre-renal cause. Volume resuscitation is the initial treatment.
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FENa of 4.35% indicates impaired tubular sodium reabsorption, consistent with ATN. Supportive care and avoidance of nephrotoxins are key.
FENa measures the percentage of sodium filtered by the glomerulus that ends up being excreted in the urine. It reflects the kidney's tubular handling of sodium. Normal FENa is approximately 1%. Values below 1% indicate enhanced sodium reabsorption, while values above 2% indicate impaired reabsorption.
FENa is unreliable when the patient has recently received diuretics (which increase urinary sodium), during IV normal saline infusion, in patients with CKD (baseline impaired sodium reabsorption), in glycosuria (osmotic diuresis), and in adrenal insufficiency (aldosterone deficiency impairs sodium reabsorption).
FENa measures fractional excretion of sodium, while FEUrea measures fractional excretion of urea. FEUrea is preferred when diuretics have been given because urea handling is less affected by diuretics. FEUrea below 35% suggests pre-renal azotemia, while above 50% suggests intrinsic renal disease.
No. FENa is calculated from a simultaneous spot urine and blood sample. The mathematical normalization to creatinine clearance eliminates the need for timed urine collection. A 24-hour urine is unnecessary and would only delay the result.
Yes. Some causes of intrinsic AKI can have FENa below 1% early in their course, including contrast-induced nephropathy (first 24-48 hours), rhabdomyolysis-induced AKI, pigment nephropathy, early sepsis-associated AKI, and acute glomerulonephritis. Clinical context and serial measurements help resolve ambiguity.
In pre-renal states, reduced renal perfusion activates RAAS, ADH, and sympathetic nervous system, which stimulate maximal sodium and water reabsorption in the proximal and distal tubules. The kidney retains more than 99% of filtered sodium, resulting in FENa well below 1%.
Patients with CKD have baseline impaired tubular function and may have FENa above 1% even in pre-renal states. In CKD patients, FENa thresholds are less reliable, and clinical judgment, trending of creatinine, and response to fluid challenge become more important for diagnosis.
Hepatorenal syndrome (HRS) characteristically has very low FENa (often below 0.2%) because renal hypoperfusion from splanchnic vasodilation drives maximal sodium reabsorption. Low FENa in a cirrhotic patient with rising creatinine should raise suspicion for HRS.
FENa below 1% supports a trial of volume resuscitation as initial therapy. If creatinine improves with fluids, pre-renal etiology is confirmed. FENa above 2% suggests established tubular damage where aggressive fluid resuscitation alone is unlikely to restore function, shifting focus to supportive care and avoidance of further insults.
The FENa formula works with any consistent units for sodium (mEq/L) and creatinine (mg/dL or micromol/L, as long as both plasma and urine creatinine use the same units). The units cancel in the ratio, producing a dimensionless percentage result.
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