ABSTRACT
The infant before birth can be thought of as undergoing continuous dialysis through the placenta. Thus, most disorders of small molecule metabolism are of very little consequence until the time of birth. Intermediates in metabolism, which accumulate after birth, are removed before birth through the mother’s blood stream. Most products needed for normal development that are reduced by a metabolic error e.g. tyrosine in infants with phenylketonuria (PKU) are provided through the placenta by the mother. There is a period of time after birth when intermediates in metabolism, which are toxic to the infant, have not yet accumulated to the point where
they may cause irreversible damage but elevated enough to ensure their detection and identification as abnormal. This time interval is the period when newborn screening by substrate level is possible. Newborn screening by enzymatic assay (e.g., in galactosemia) and for low or high hormone levels (hypothyroidism or congenital adrenal hyperplasia) must also be accomplished during this predamage window of opportunity. Table 1 gives representative examples and frequencies of disorders screened by newborn screening programs.
