Uric acid is a naturally occurring waste product in the body, formed primarily from the breakdown of purines. These purines are chemical compounds found in various foods and are also produced by the body’s cells. The kidneys play a central role in managing uric acid levels by filtering it from the blood. Fractional Excretion of Uric Acid (FEUA) is a specialized test that provides insight into how efficiently the kidneys process and remove uric acid. This measurement helps healthcare providers understand the kidney’s specific handling of this substance.
Uric Acid and Kidney Function
Uric acid originates from the metabolism of purines, which are components of DNA and RNA found in both the foods consumed and the body’s own cells. When purines break down, they are converted into hypoxanthine, then xanthine, and finally uric acid through enzymatic reactions, mainly in the liver. Approximately 20% of uric acid comes from dietary purines, while the remaining 80% is produced internally.
The kidneys are responsible for filtering uric acid from the blood and excreting it into the urine. This process involves several steps within the kidney’s nephrons, the tiny filtering units. Uric acid is freely filtered from the blood in the glomeruli. After filtration, a significant portion, about 90%, of the filtered uric acid is reabsorbed back into the bloodstream in the proximal tubules. A smaller amount is then secreted into the tubular lumen. The remaining uric acid is ultimately excreted in the urine, with about 75% of the body’s uric acid being eliminated this way. Imbalances in uric acid levels can contribute to various health issues.
Measuring Fractional Excretion of Uric Acid
Fractional Excretion of Uric Acid (FEUA) quantifies the percentage of filtered uric acid that is ultimately excreted in the urine. This measurement offers a more comprehensive assessment of kidney function regarding uric acid, accounting for the complex processes of filtration, reabsorption, and secretion, compared to simply measuring blood or urine uric acid levels alone.
To perform the FEUA test, both blood and urine samples are required. A 24-hour urine collection is requested to gather all urine produced over a full day. This collection involves discarding the first morning urine sample and then collecting all subsequent urine for the next 24 hours in a special container, which should be kept refrigerated. A blood sample is also drawn to measure serum uric acid and creatinine levels. Creatinine, a waste product from muscle breakdown, is used as a reference to normalize the uric acid measurements, helping to evaluate how well the kidneys are filtering. The FEUA is calculated using a formula that compares the concentration of uric acid and creatinine in both the urine and the blood.
Understanding FEUA Results
Different FEUA values provide specific insights into how the kidneys are handling uric acid. A normal FEUA falls within a range of 4% to 11%, indicating effective regulation of uric acid levels by the kidneys. This range suggests a balanced interplay between uric acid production and excretion.
Low FEUA
A low FEUA, below 4% to 7%, suggests that the kidneys are reabsorbing too much uric acid, leading to underexcretion and elevated uric acid levels in the blood (hyperuricemia). Underexcretion is the predominant mechanism for hyperuricemia in approximately 90% of individuals with primary gout. Conditions associated with low FEUA include underexcretor gout, where uric acid crystals accumulate in joints causing inflammation. Dehydration can also lead to a low FEUA, as the body attempts to conserve fluid, which can increase uric acid reabsorption. Certain genetic factors that affect uric acid transporters in the kidney, such as URAT1 and GLUT9, can also contribute to reduced uric acid excretion.
High FEUA
A high FEUA, above 10% to 11% or even 20% in some cases, indicates that the kidneys are excreting too much uric acid or that there is an overproduction of uric acid in the body. Overproduction of uric acid can stem from increased synthesis of purine precursors or conditions involving high cell turnover, such as certain cancers like leukemia, myeloproliferative diseases, or psoriasis. In some instances, overproducer gout is characterized by the body producing excessive amounts of uric acid.
A high FEUA can also point to kidney disorders that impair the kidney’s ability to reabsorb filtered substances. For example, Fanconi syndrome is a rare kidney tubule disorder where the proximal tubules fail to adequately reabsorb various small molecules, including uric acid, leading to their excessive excretion in urine. This results in lower blood uric acid levels and a high FEUA. Medications can also influence FEUA results; for instance, some uricosuric agents, which are drugs designed to increase uric acid excretion, can lead to a higher FEUA by inhibiting uric acid reabsorption in the renal tubules. Additionally, loop diuretics can affect fractional excretion values, including FEUA.
FEUA measurements help healthcare providers differentiate between the various causes of abnormal uric acid levels, which is important for guiding appropriate treatment decisions. For instance, knowing whether hyperuricemia is due to under-excretion or overproduction allows for targeted interventions, such as dietary adjustments, specific medications to lower uric acid levels, or managing underlying conditions.