Xanthine and Hypoxanthine, 24-Hour Urine
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Xanthine and hypoxanthine are the direct precursors of uric acid, the end product of purine metabolism. Two inborn errors of metabolism are characterized by elevated excretion of xanthine and hypoxanthine. Patients with isolated xanthine dehydrogenase (XDH, xanthine oxidase) deficiency may remain asymptomatic, but nephrolithiasis, due to the insolubility of xanthine, may occur at any age. Some patients also develop a myopathy with crystalline xanthine deposits in muscle. Combined deficiency of XDH and the related enzyme sulfite oxidase (SO) is also characterized by nephrolithiasis, but more prominently by the symptoms of SO deficiency (isolated SO deficiency also occurs) including neonatal seizures, myoclonus, lens dislocation, and severe mental retardation. This form of xanthinuria is caused by molybdenum cofactor deficiency, which is required for the activity of both oxidases. Elevations of xanthine and hypoxanthine and abnormally low levels of uric acid are found in both disorders, while in patients with XDH/SO deficiency sulfites and sulfur-containing metabolites (S-sulfocysteine, thiosulfate, taurine) also accumulate.
Analysis of xanthine and hypoxanthine alone, allows the diagnosis of xanthinuria, is helpful for the evaluation of low serum and/or urine uric acid concentrations, and for the evaluation of allopurinol (a xanthine oxidase inhibitor) treatment in hyperuricemic disorders (eg, Lesch-Nyhan syndrome).
Diagnosis and confirmation of xanthinuria
Evaluation of low serum or urine uric acids
Evaluation of allopurinol treatment in hyperuricemic disorders (eg, Lesch-Nyhan syndrome)
Abnormal concentrations of xanthine and hypoxanthine will be reported along with an interpretation. The interpretation of an abnormal metabolite pattern includes an overview of the results and of their significance, a correlation to available clinical information, possible differential diagnoses, recommendations for additional biochemical testing and confirmatory studies (enzyme assay, molecular analysis), name and phone number of contacts who may provide these studies at the Mayo Clinic or elsewhere, and a number for 1 of the laboratory directors in case the referring physician has additional questions.
Increased urinary xanthine and hypoxanthine with low urinary uric acid are characteristic of xanthine oxidase deficiency.
Increased urinary excretion of xanthine, hypoxanthine, and uric acid are indicative of hyperuricemia disorders treated with allopurinol.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
No significant cautionary statements
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
20-100 mcmol/24 hours
20-60 mcmol/24 hours
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Raivio KO, Saksela M, Lapatto R: Xanthine oxidoreductases-Role in human pathophysiology and in hereditary xanthinuria. In The Metabolic and Molecular Bases of Inherited Disease. Eighth edition. Edited by CR Scriver, AL Beaudet, WS Sly, et al. New York, McGraw-Hill Book Company, 2001, pp 2639-2652