Uric Acid
Last updated: October 13, 2014
Synonyms: Urate, monosodium urate.
Description: Uric acid is the end product of purine metabolism and is excreted in the urine. It is primarily used in the diagnosis of gout. Uric acid is not soluble at a pH <7.4.
Method: Uric acid may be measured singly or as part of an automated chemistry panel. These automated enzymatic methods depend on the generation of peroxide during oxidation of urate by uricase.
Normal Values: In men, serum uric acid levels increase during childhood and reach adult levels after puberty. In women, urate levels remain constant until after menopause, when they increase (as does the incidence of gout). Gender differences are owing to estrogen, which exerts a uricosuric effect. Although normal values vary between laboratories, serum values in men range from 4.0 to 8.6 mg/dL and in women from 3.0 to 5.9 mg/dL. Urinary uric acid levels are normally <750 mg/24 hours. Urinary levels >750 mg/24 hours in gout (or >1,100 mg/24 hours in asymptomatic hyperuricemia) indicate that the patient is a urate overproducer and may need allopurinol therapy.
Abnormal in: Serum uric acid values may be abnormally high or low (Table 17). Although hyperuricemia may indicate gout, levels do not correlate with the severity of disease. Hyperuricemia levels >9 mg/dL are associated with increased risk of gout and nephrolithiasis. Nonetheless, treatment of asymptomatic hyperuricemia may not be necessary until values are >13 mg/dL in men or 10 mg/dL in women. Although nearly all patients with gout demonstrate hyperuricemia at some time during their illness, as many as 40% of patients having an acute gouty attack have normal serum uric acid levels.
| Table 17: Abnormalities of Serum Uric Acid | ||||
| Increased Values | Decreased Values | |||
|---|---|---|---|---|
| Renal Failure | Drugs | |||
| Gout | Adrenocorticotropic hormone | |||
| Asymptomatic hyperuricemia | Uricosuric drugs (sulfinpyrazone, probenecid, high-dose salicylates) | |||
| Increased purine turnover | Allopurinol | |||
| Lymphoproliferative disorders | Wilson disease | |||
| Myeloproliferative disorders | Fanconi syndrome | |||
| Chemotherapy or radiotherapy | ||||
| Hemolytic anemia | ||||
| Toxemia of pregnancy | ||||
| Psoriasis | ||||
| Drugs | ||||
| Diuretics (except p-spironolactone) | ||||
| Low-dose salicylates | ||||
| Ethanol | ||||
| Diet: purine-rich foods (meat, legumes) | ||||
| Metabolic acidosis | ||||
| Lead poisoning | ||||
| Hypoparathyroidism | ||||
| Primary hyperparathyroidism | ||||
| Hypothyroidism | ||||
| Sarcoidosis | ||||
Indications: Serum uric acid assays are most useful in monitoring the response to treatment in gout, renal failure, and neoplasia or during chemotherapy; 24-hour urinary uric acid determinations may be valuable in assessing the risk of nephrolithiasis or in making therapeutic decisions in gout (e.g., whether to treat with probenecid or allopurinol).
Cost: $10–25.
Comments: Urinary uric acid determinations are most reliable when the patient is on a low-purine diet and not taking uricosuric drugs. Serum uric acid levels are labile and vary from day to day.
BIBLIOGRAPHY
Wallach J. Core blood analytes: alterations by diseases. In: Interpretation of diagnostic tests, 6th ed. Boston: Little, Brown, 1996:37–38.
