Second, xanthinuria is typed using urinary metabolomics: N1-methyl-2-pyridone-5-carboxamide (2PY) and N1-methyl-4-pyridone-5-carboxamide (4PY) are the final products excreted in urine in the nicotinamide catabolism and these products are results of the oxidation of N1-methylnicotinamide by aldehyde oxidase (AO)), Finally, the results are confirmed by molecular genetics. This evidence concerns the gene AOX1 and xanthinuria.