Mass screening of newborn infants for inherited metabolic disorders began in the early 1960s with the development of the Guthrie bacterial inhibition assay for the detection of phenylketonuria (PKU).117 This simple assay utilizing dried filter paper blood specimens collected by heel prick from newborn infants has resulted in mass screening for PKU throughout most of the world. The field of newborn screening, however, has not remained limited to search for patients with PKU. It has evolved into an exciting area of preventive medicine which has seen the introduction of additional bacterial inhibition assays for other inborn errors of metabolism,118-120 automation of the screening laboratories,121 and multistate regionalization of screening programs.122 This field has also seen the introduction of other specimens for mass screening including the use of cord blood, follow-up blood specimens,122 and urine specimens collected on filter paper.123,124 In order to obtain an optimum number of specimens, the multitest laboratory might be a centralized laboratory serving a single large state or a regional laboratory serving a newborn population of several smaller states.125
APPROACHES TO UREA CYCLE SCREENING
In the development of any screening test, there are a number of different parameters that may be examined. An investigator may choose to develop a simple screening test which measures the presence or absence of an enzyme or he may choose one which measures the accumulation of a substrate or secondary metabolic product. In the case of urea cycle disorders, there are a number of important markers that can be used as the basis for developing simple screening tests.