Every infant born in the US is now screened for up to 42 rare
genetic disorders called "
inborn errors of metabolism". The screening method is based on tandem mass spectrometry and quantifies acylcarnitines as a screen for organic acidemias and also measures
amino acids. All states also perform enzymatic testing for
carbohydrate disorders such as
galactosemia. Because the results can be non-specific, follow-up testing of positive results is required using a more definitive method. The present report describes the "
urease" method of sample preparation for inborn error screening. Crystalline
urease enzyme is used to remove
urea from body fluids which permits most other water-soluble metabolites to be dehydrated and derivatized for gas chromatography in a single procedure.
Dehydration by evaporation in a
nitrogen stream is facilitated by adding
acetonitrile and
methylene chloride. Then, trimethylsilylation takes place in the presence of a unique catalyst, triethylammonium
trifluoroacetate. Automated injection and chromatography is followed by macro-driven custom quantification of 192 metabolites and semi-quantification of every major component using specialized libraries of mass spectra of TMS derivatized
biological compounds. The analysis may be performed on the widely-used Chemstation platform using the macros and libraries available from the author. In our laboratory, over 16,000 patient samples have been analyzed using the method with a diagnostic yield of about 17%--that is, 17% of the samples results reveal findings that should be acted upon by the ordering physician. Included in these are over 180 confirmed inborn errors, of which about 38% could not have been diagnosed using previous methods.