The fractional excretion of sodium, or FENA, is a calculated value used in the assessment of acute renal failure. It is an assessment of how much sodium is being excreted in the urine in comparison to how much total sodium the kidneys are “seeing” in the plasma. It is essentially a measure of how well the kidney tubules are able to perform their resorptive functions. If the FENA, is low, it indicates both that the kidneys are functionally capable of reabsorbing sodium, and also that there is a physiologic stimulus to conserve sodium (i.e. hypovolemia). If the FENA, is high, it indicates an intrinsic problem with the kidneys themselves, such as ATN, because they are unable to reabsorb sodium.
The FENA can calculated by dividing the plasma to urine creatinine ratio by the ratio of the plasma to urine sodium.
FENA = 100 ((Urinary sodium X Plasma creatinine)/(plasma sodium X urinary creatinine))
As above, a FENA of < 1% correlates with a hypovolemic, or “pre-renal” state, and one in which the kidney function will increase via volume resuscitation. Values above 2% correlate with intrinsic damage to the kidneys, such as acute tubular necrosis. The above rule is less accurate in the setting of diuretic use, FENA values near 1, or in patients with advanced stable chronic renal failure.
R W Steiner Interpreting the fractional excretion of sodium. Am. J. Med.: 1984, 77(4);699-702 [PubMed:6486145]