The classic example of population screening is testing new-born babies for phenylketonuria (PKU). Individuals with PKU fail to make a protein, a certain enzyme, and develop mental retardation. The absence of the enzyme results in both an accumulation of phenylalanine, which causes the mental retardation, and a deficiency of tyrosine in the body, as shown in Figure 1.
Because both phenylalanine and tyrosine are constituents of a normal diet, in what way could mental retardation be prevented in babies born with PKU?
By restricting the intake of phenylalanine in the diet.
PKU was the first disease for which treatment in the form of dietary restriction was successfully used. Most children are relatively free of symptoms when given such treatment, hence screening of new-born babies. The technique, first used in 1960, is not strictly a genetic test. No fancy DNA technology is involved, just a needle prick on the baby's heel to produce a blood sample (Figure 2). A cheap biochemical test then identifies children who have more phenylalanine than usual in their blood. Only some of these have PKU, and more tests are then needed to identify them more definitely. Babies who do have PKU are put on a phenylalanine-restricted diet, but this is harmful to those without the disorder.
Not completely straightforward, then. As with all tests there is a possibility of misdiagnosis (of false positive results) from the heel-prick test. For those correctly diagnosed, the special diet is both expensive and unappetizing – not an ideal combination for child rearing. And a woman with PKU who bears children herself must also adopt a special diet when she is pregnant. On the whole, though, it works. Although PKU is fairly rare (around 1 in 15 000 live births), it used to account for roughly one per cent of profound mental retardation. Screening has practically eliminated this. And as the test is so cheap, and long-term care of the disabled children so costly, universal screening is cost-effective. In the UK, where almost all new-born babies are tested, the money saved is estimated at four times the cost. In 1995, it cost around £800 000 to screen 100 000 babies, but caring for the ones who would have been damaged by excess phenyalanine would have cost over £3 million. The cost saving, of course, goes along with sparing those parents the distress of raising a child with brain damage, but policy-makers like to have both kinds of benefit.