Decisionanalytic Model Of Genetic Screening

Consider genetic testing in utero (Campbell, 1992). Once the mother becomes aware she is pregnant, typically a range and sequence of tests are made available to her. Some are invasive (maternal blood serum tests, chorionic villus sampling, amniocentesis), and some are not (ultrasound scanning), and there are varying degrees of risk attached to each as well as varying degrees of expense. Assume for simplicity that expense constrains the range of tests available, but that the tests available are free to the mother and not rationed. So the main determinants affecting the choice the mother makes in respect of any given test are the risk attached to the test (e.g., the probability of miscarriage), the desire for the information, and the utility of that information. The latter two determinants are linked but separate, since a mother may not desire information because she will not act on it (if she is against all abortions in principle, for instance), or she may desire it even though she will not act on it (for the sake of preparedness or peace of mind).

According to a simple theory of rationality, this can be described using a decision tree. Consider amniocentesis, and (in this section only) assume for simplicity that we are only interested in Down's syndrome. With a certain probability p, the test will cause miscarriage; so with probability 1-p the foetus will be unharmed and information obtained. Using epidemiological data we can determine the probability this information will indicate that the child has Down's; call this probability q. So with probability 1-q the information will tell us that the child does not have Down's.

Here the mother is faced with two decisions: to test or not to test, and to abort or not to abort, conditional on a positive test for Down's. What should she do? This depends on the utilities she assigns (or we ascribe to her in the light of her behaviour) to the different outcomes. It may be that this is her (actual or perceived) last chance to conceive—perhaps this pregnancy arises out of IVF treatment. So, she may judge that the risk of miscarriage caused by amniocentesis is not worth taking. The negative utility of a miscarriage is in far excess of the utility of any information she may get if she does not suffer a miscarriage. Or it may be that she prefers to be childless rather than have a child with Down's (for whatever reason), so much so that the risk of the test is worth taking.4

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