Fractional sodium excretion
Encyclopedia
The fractional excretion of sodium (FENa) is the percentage of the sodium filtered by the kidney which is excreted in the urine. 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 reabsorption. Therefore the urinary and plasma concentrations of sodium must be compared to get an accurate picture of renal clearance. In clinical use, the fractional excretion of sodium can be calculated as part of the evaluation of acute renal failure
in order to determine if hypovolemia or decreased effective circulating plasma volume is a contributor to the renal failure. "Measurement of the urine sodium concentration provides information on the integrity of tubular reabsorptive function. Low urine sodium concentration thus indicates not only intact reabsorptive function but also the presence of a stimulus to conserve sodium "
First, the actual amount of sodium excreted is calculated by multiplying the urine sodium concentration by the urinary flow rate. This is the numerator in the equation. The denominator is the total amount of sodium filtered by the kidneys. This is calculated by multiplying the plasma sodium concentration by the glomerular filtration rate calculated using creatinine filtration. This formula is represented mathematically as:
(Sodium
urinary × Flow rateurinary) / (Sodium
plasma × Creatinine
urinary × Flow rateurinary ÷ Creatinine
plasma) × 100
Sodium (mmol/l)
Creatinine (mg/dl)
The flow rates cancel out in the above equation, simplifying to the standard equation:
For ease of recall, one can just remember the fractional excretion of sodium is the clearance of sodium divided by the glomerular filtration rate (i.e. the "fraction" excreted).
and oliguria
. Low fractional excretion indicate sodium retention by the kidney, suggesting pathophysiology extrinsic to the urinary system such as volume depletion or decrease effective circulating volume (e.g. low output heart failure). Higher values can suggest sodium wasting due to acute tubular necrosis
or other causes of intrinsic renal failure. The FENa may be affected or invalidated by diuretic
use, since many diuretics act by altering the kidney's handling of sodium.
Acute renal failure
Acute kidney injury , previously called acute renal failure , is a rapid loss of kidney function. Its causes are numerous and include low blood volume from any cause, exposure to substances harmful to the kidney, and obstruction of the urinary tract...
in order to determine if hypovolemia or decreased effective circulating plasma volume is a contributor to the renal failure. "Measurement of the urine sodium concentration provides information on the integrity of tubular reabsorptive function. Low urine sodium concentration thus indicates not only intact reabsorptive function but also the presence of a stimulus to conserve sodium "
Calculation
FENa is calculated in two parts—figuring out how much sodium is excreted in the urine, and then finding its ratio to the total amount of sodium that passed through (aka "filtered by") the kidney.First, the actual amount of sodium excreted is calculated by multiplying the urine sodium concentration by the urinary flow rate. This is the numerator in the equation. The denominator is the total amount of sodium filtered by the kidneys. This is calculated by multiplying the plasma sodium concentration by the glomerular filtration rate calculated using creatinine filtration. This formula is represented mathematically as:
(Sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
urinary × Flow rateurinary) / (Sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
plasma × Creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
urinary × Flow rateurinary ÷ Creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
plasma) × 100
Sodium (mmol/l)
Creatinine (mg/dl)
The flow rates cancel out in the above equation, simplifying to the standard equation:
For ease of recall, one can just remember the fractional excretion of sodium is the clearance of sodium divided by the glomerular filtration rate (i.e. the "fraction" excreted).
Interpretation
FENa can be useful in the evaluation of acute renal failureAcute renal failure
Acute kidney injury , previously called acute renal failure , is a rapid loss of kidney function. Its causes are numerous and include low blood volume from any cause, exposure to substances harmful to the kidney, and obstruction of the urinary tract...
and oliguria
Oliguria
Oliguria is the low output of urine, It is clinically classified as an output below 300-500ml/day. The decreased output of urine may be a sign of dehydration, renal failure, hypovolemic shock, HHNS Hyperosmolar Hyperglycemic Nonketotic Syndrome, multiple organ dysfunction syndrome, urinary...
. Low fractional excretion indicate sodium retention by the kidney, suggesting pathophysiology extrinsic to the urinary system such as volume depletion or decrease effective circulating volume (e.g. low output heart failure). Higher values can suggest sodium wasting due to acute tubular necrosis
Acute tubular necrosis
Acute tubular necrosis or is a medical condition involving the death of tubular cells that form the tubule that transports urine to the ureters while reabsorbing 99% of the water . Tubular cells continually replace themselves and if the cause of ATN is removed then recovery is likely...
or other causes of intrinsic renal failure. The FENa may be affected or invalidated by diuretic
Diuretic
A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :...
use, since many diuretics act by altering the kidney's handling of sodium.
| Value | Category | Description |
|---|---|---|
| below 1% | prerenal disease | the physiologic response to a decrease in renal perfusion is an increase in sodium reabsorption to control hypovolemia. |
| above 2% or 3% | acute tubular necrosis Acute tubular necrosis Acute tubular necrosis or is a medical condition involving the death of tubular cells that form the tubule that transports urine to the ureters while reabsorbing 99% of the water . Tubular cells continually replace themselves and if the cause of ATN is removed then recovery is likely... or other kidney damage |
either excess sodium is lost due to tubular damage, or the damaged glomeruli result in hypervolemia resulting in the normal response of sodium wasting. |
| intermediate | either disorder | In renal tract obstruction, values may be either higher or lower than 1%. The value is lower in early disease, but with renal damage from the obstruction, the value becomes higher. |
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.External links
- FENa Calculator
- Online FENa calculator by Cornell UniversityCornell UniversityCornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...
- FENa calculator available online & for iPhone, BlackBerry and Android by QxMD Medical.