Maple syrup urine disease: newborn screening fails to discriminate between classic and variant forms.

tems calibrators for metanephrines. The increase in sources of calibrators did not cause a decrease in the accuracy of the results during external quality control program in the 2004– 2006 period. The observed differences in published reference values for urinary metanephrines are not mainly because of analytical reasons but most probably are due to differences in the characteristics of reference groups. The present availability of consistent calibrators should prompt efforts to define and validate reference values based on a large pool of observations, which then could be used in multiple laboratories.