Uric acid is the ultimate catabolite of purine metabolism in humans and higher primates. It is a weak organic acid that under physiologic conditions exists mainly as a monosodium salt. At a pH less than 5.75, as may occur in the urine, the predominant form is nonionized uric acid. The solubility of monosodium urate is about 18 times greater than uric acid in aqueous solutions. This solubility differential provides the therapeutic rationale for alkalinization of the urine pH to greater than 6.0 in patients forming uric acid stones.
The upper limit of plasma uric acid may be defined by a statistical range in a normal population. Epidemiologic studies in the United States have generally accepted 7.0 mg/dl as the upper limit in adult men and 6.0 mg/dl in women.
The physiochemical definition of hyperuricemia may be considered 7.0 mg/dl measured by the specific uricase method. This represents the solubility limit of urate in plasma at 37°C. Levels beyond 7.0 result in supersaturated solutions that are prone to crystal formation.
Uric acid levels are influenced by age and sex. Prior to puberty, the average serum uric acid is 3.6 mg/dl for males and females. Following puberty, values rise to adult levels with women typically 1 mg/dl less than men. This lower level in women apparently reflects estrogen-related enhancement of renal urate clearance and disappears at the menopause. Many additional factors, including exercise, diet, drugs, and state of hydration, may result in transient fluctuations of uric acid levels.
Copyright © 1990, Butterworth Publishers, a division of Reed Publishing.