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Physicians can monitor the health of the kidneys by comparing the sodium filtered and absorbed to that excreted in urine. Fractional sodium excretion is the calculation of this measurement often used when a kidney disease is suspected. This number can also show what kind of acute renal failure a person has, particularly if the problem is with the blood flow to the kidney or with the organ itself. Sodium in the urine as well as plasma concentrations is typically used in the measurement. These values, along with urinary flow rate, creatinine levels, and the sodium is filtered by the kidney, are often used in calculating the fractional excretion of sodium (FENA).
Fractional sodium excretion is usually determined by a blood test and taking a urine sample. Blood chemistry can be analyzed to detect the salt and creatinine levels, and during the test, the only discomfort is typically the sensation of a needle. The test is usually performed on patients with illnesses that affect kidney function. It can detect if there is a lack of blood flow to the kidneys, in which the FENA will typically be measured at 1% or less. A value of more than 3% often indicates that there is damage to the kidneys.
Illnesses of the circulatory system are typically called prerenal diseases when fractional sodium excretion is measured. If there is kidney damage or failure, acute tubular necrosis can be the cause. An intermediate value can indicate either type of condition, or a blockage in or near the kidneys. Mathematical calculations are done after the blood and urine test results are analyzed. Sometimes the fractional excretion of other substances in urine are measured, such as urea, uric acid, or lithium.
There is typically not much preparation before a fractional sodium excretion test. Doctor’s often suggest avoiding diuretics because they can alter the absorption of sodium in the kidneys, as well as other medications that can do so. The testing is generally low risk, while complications from blood tests can include bleeding, fainting, or infection in some cases.
Conditions that can be detected from fractional sodium excretion include cirrhosis of the liver or congestive heart failure. Kidney conditions such as acute glomerulonephritis, renal failure, or rhabdomyolysis can be associated with a low FENA count. The number can be high if a person vomited or is being treated with a nasogastric tube for suctioning, as biological processes can prevent the re-absorption of sodium.