Plasma and blood xanthine and hypoxanthine levels were assayed using a sensitive and specific method involving gas chromatography-mass spectrometry, associated with an optimized sample preparation procedure. Physiological variation was studied in 224 subjects with no purine metabolism disorders. An age dependency for both compounds was found, comparable with that known for uric acid. The mean plasma levels for the 224 subjects were 0.65 +/- 0.24 microM for xanthine and 1.65 +/- 0.78 microM for hypoxanthine. Corresponding mean blood levels were 0.59 +/- 0.21 microM for xanthine and 1.72 +/- 0.74 microM for hypoxanthine. Plasma and blood levels were significantly different, by ca. 10%. Rapid in vitro release of hypoxanthine from erythrocytes and continuation of intraerythrocytal metabolism lead to overestimation exceeding 10% within half an hour after sample blood collection. Hence samples must be deproteinized promptly. Blood can therefore be conveniently used for oxypurine assay instead of plasma when prompt spinning of samples is difficult to manage, as is usually encountered in clinical practice.
