Fractional Excretion of Sodium (FENa) Calculator
Calculate fractional excretion of sodium (FENa) to differentiate prerenal azotemia from ATN. Enter serum and urine sodium and creatinine for instant clinical interpretation.
About This Calculator
The Fractional Excretion of Sodium (FENa) Calculator helps healthcare professionals differentiate between prerenal azotemia and intrinsic renal failure, particularly acute tubular necrosis (ATN). This nephrology tool uses serum and urine sodium and creatinine values to calculate the percentage of filtered sodium excreted in the urine. It is most valuable in assessing patients with acute kidney injury (AKI) when prerenal versus intrinsic causes need to be distinguished.
The FENa formula calculates: FENa = (Urine Na × Serum Creatinine) / (Serum Na × Urine Creatinine) × 100. The urinary flow rate cancels out in the derivation, making a spot urine sample sufficient for calculation. A FENa below 1% suggests sodium retention by the kidneys (prerenal physiology), while a value above 2% indicates sodium wasting (intrinsic tubular damage). Values between 1-2% are indeterminate and require clinical correlation with history, physical examination, urine microscopy, and response to fluid resuscitation.
Important limitations: FENa is unreliable in patients taking diuretics, those with chronic kidney disease, urinary tract obstruction, acute glomerulonephritis, or contrast-induced nephropathy. In patients on diuretics, consider using FEUrea (fractional excretion of urea) instead. The test is also less reliable in neonates and children due to immature tubular function.
Regional considerations: In India and other countries where µmol/L is commonly used for creatinine reporting, the same formula applies as the units cancel out in the ratio. The FENa interpretation (below 1% prerenal, above 2% ATN) is universally accepted in nephrology practice across IN, US, and UK. Reference ranges for serum sodium (135-145 mmol/L) and creatinine (0.6-1.2 mg/dL or 53-106 µmol/L) are consistent globally.
Frequently Asked Questions
What does FENa measure?
FENa (fractional excretion of sodium) measures the percentage of sodium filtered by the kidneys that is excreted in the urine. It helps differentiate prerenal azotemia (caused by decreased blood flow to the kidneys) from intrinsic renal failure such as acute tubular necrosis (ATN).
What do FENa results mean?
FENa below 1% indicates prerenal azotemia where the kidneys are conserving sodium due to decreased perfusion. FENa above 2% suggests intrinsic renal failure, most commonly acute tubular necrosis. Values between 1-2% are indeterminate and require clinical correlation.
When should FENa not be used?
FENa should not be used in patients taking diuretics, as these drugs alter renal sodium handling and invalidate the result. FENa is also unreliable in chronic kidney disease, urinary tract obstruction, and acute glomerular disease. In patients on diuretics, fractional excretion of urea (FEUrea) is preferred.
What formula is used for FENa?
FENa is calculated as: (Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine) × 100. The units cancel out, so serum and urine creatinine can be in any consistent unit (mg/dL or µmol/L), and sodium in any consistent unit (mmol/L).
Can FENa differentiate hepatorenal syndrome?
No. FENa below 1% cannot differentiate hepatorenal syndrome (HRS) from other causes of prerenal disease. Both conditions result in low FENa due to effective volume depletion. Clinical context, history of liver disease, and response to volume expansion help distinguish HRS.
Is FENa accurate in children and neonates?
FENa must be interpreted cautiously in pediatric patients because immature renal tubules cannot reabsorb sodium maximally. In term neonates, FENa below 3% represents volume depletion. In critically ill preterm neonates, a FENa as high as 4% may represent maximal sodium conservation.
What are alternatives to FENa?
Fractional excretion of urea (FEUrea) is the preferred alternative in patients on diuretics. Other alternatives include fractional excretion of uric acid and lithium, urine osmolality, and the BUN-to-creatinine ratio. Each has specific indications and limitations.