In this chapter, we will talk about both screening and testing. What is the difference? A screen is population based and is administered to most members of a group, such as newborn babies or pregnant women. A test is an assay requested by a doctor on an individual basis with the patient's consent, usually in response to some medical information or risk factors indicating the need for the test. Thus we talk in terms of newborn screening, which looks for infants in which a metabolic defect of genetic origin can be detected without actually testing the genetic material itself.
A genetic test may look at the patient's DNA or may be a biochemical test for an enzymatic function. Tests of genetic material mostly either test for mutations or look at chromosomes under a microscope. A biochemical test will evaluate whether a gene product is correctly performing its function, either by a direct test of the gene product (such as a test of enzymatic activity) or through assaying for levels of metabolites, such as sugars or amino acids, to determine whether the body is maintaining the correct levels of molecules handled by the biochemical pathway. Why do a biochemical screen
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