The fractional excretion of sodium (FENa) is a measure of the percentage of sodium excreted in the urine versus the sodium reabsorbed by the kidney. It is measured in terms of plasma and urine sodium, rather than by the interpretation of urinary sodium concentration alone, as urinary sodium concentrations can vary with water resorption. Therefore the urinary and plasma concentrations of sodium must be compared to get an accurate picture of renal clearance. FENa can be calculated by multiplying the plasma sodium concentration by the glomerular filtration rate. Renal function, in Nephrology, is an indication of the state of the Kidney and its role in Renal physiology. It may also be calculated by multiplying the urine sodium concentration by the urinary flow rate. This translates into the formula:
(Sodiumurinary×Flow rateurinary)÷(Sodiumplasma×(Creatinineurinary×Flow rateurinary÷Creatinineplasma)×100
Sodium (mmol/l) Creatinine (mg/dl)
The flow rates cancel out in the above equation, simplifying to the standard equation:
(Sodiumurinary×Creatinineplasma)÷(Sodiumplasma×Creatinineurinary)×100
This can also be rearranged in a more intuitive ratio of fractions:
(Sodiumurinary÷Creatinineurinary)÷(Sodiumplasma÷Creatinineplasma)×100
Interpretation
FENa is an accurate screening test for differentiating prerenal failure versus acute tubular necrosis. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Creatinine is a break-down product of Creatine phosphate in Muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass Creatinine is a break-down product of Creatine phosphate in Muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Creatinine is a break-down product of Creatine phosphate in Muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Creatinine is a break-down product of Creatine phosphate in Muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass Acute tubular necrosis or ( ATN) is a medical condition involving the death of tubular cells that form the Tubule that transports Urine to the Ureters A value below 1 percent suggests prerenal disease, as the physiologic response to a decrease in renal perfusion is an increase in sodium reabsorption to control hypovolemia. Values above 2 percent usually indicate acute tubular necrosis: either excess sodium is lost due to tubular damage, or the damaged glomeruli result in hypervolemia resulting in the normal response of sodium wasting. Values between 1 and 2 may be seen in either disorder. In renal tract obstruction, values may be either higher or lower than 1%. [1]
Alternatives
Fractional excretion of other substances can be measured to determine renal clearance including urea, uric acid, and lithium. These can be used in patients undergoing diuretic therapy, where the urinary sodium concentrations may be higher despite possible prerenal pathology. [2]
References
- ^ Steiner R (1984). "Interpreting the fractional excretion of sodium". Am J Med 77 (4): 699–702. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 6486145.
- ^ Steinhäuslin F, Burnier M, Magnin J, Munafo A, Buclin T, Diezi J, Biollaz J (1994). "Fractional excretion of trace lithium and uric acid in acute renal failure". J Am Soc Nephrol 4 (7): 1429–37. PMID 8161725.
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