Xanthurenic acid excretion in urine after oral intake of 5 grams L-tryptophan by healthy volunteers: standardisation of the reference values.

The excretion of xanthurenic acid in urine after oral intake of 5 grams of L-tryptophan was studied and correlated with test and test-person parameters. The xanthurenic acid was determined by a colorimetric method, which gives reliable and reproducible results. Two groups, of 11 healthy volunteers each, participated. The first group started the test at 08.00 hours and the second group at 22.00 hours. Both groups repeated the same test, within one weed of the first test, at 22.00 hours. The xanthurenic acid excretion in the 24-hour urine correlated significantly (p = 0.04) with urine volume, but did not correlated with age, sex, height, body-weight, urine pH and urinary output of creatinine or 17-hydroxysteroids. The intraindividual variability of the test is not significant. The test should be started in the evening and the urine should be collected over 24 hours. The new reference values for the xar thurenic acid excretion under these conditions are 68.8 +/- 19.0 mumols/24 h.