Screening for carriers of cystic fibrosis

BMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6942.1451 (Published 04 June 1994) Cite this as: BMJ 1994;308:1451
  1. S Raeburn

    At the core of genetic management is the principle that once people have been informed about all the options they can then choose. Clinical geneticists facilitate decision making by confirming the diagnosis, providing information non-directively, and arranging counselling that takes account of emotional aspects. Decision making is not always logical, and geneticists (and other doctors) should know this. People's feelings are as important as the facts; decisions should emerge from both.

    Two papers in this week's journal concern screening for carriers of cystic fibrosis, although both use quite different models. Can such studies provide an approach to screening that is applicable to other autosomal recessive conditions? If so, and if health authorities were to introduce screening for carriers of the commoner autosomal recessive diseases, which should be chosen? The feelings of health professionals and of those who allocate resources may contribute (alongside the facts) to such strategic decisions.

    Livingstone and colleagues from Edinburgh describe “couple screening” in pregnancy to identify carriers of cystic fibrosis (p 1459).1Following suggestions by Wald et al2,3 they reported back to those at high risk of having a child with cystic fibrosis. In this first large scale trial of “couple screening” three quarters of nearly 8000 eligible couples agreed to take part - that is, both partners accepted the offer of testing for the cystic fibrosis carrier state. If both were found to be carriers the couple was informed that the result was “high risk”; in all other situations, even when one of the partners was a carrier, the test was reported as “couple negative.” Four high risk couples were identified, all of whom chose to have prenatal diagnostic testing. Altogether 238 women were heterozygotes. Only 89 women asked for their carrier status, and two were carriers. Thus 238 carriers were identified, of whom only six were told of their status.

    In contrast, Super and colleagues studied “cascade testing” in families of people known to have cystic fibrosis (p 1462).4 So far they have tested 1122 relatives from 607 families with members with cystic fibrosis and the partners of the 441 carriers identified. They report that an average of 16 relatives per family would accept testing and enable other relatives to be contacted. Confirming this figure is difficult because the authors included other groups in the cascade, such as those opportunistically screened. Holloway and Brock have calculated that less than a quarter of couples where one partner is a carrier of cystic fibrosis is identifiable by cascade testing of relatives of patients with cystic fibrosis.5 This puts Super et al's method in context and emphasises that screening in the healthy population is the only way to identify most carriers.

    These studies must also be contrasted with other approaches to carrier screening. The Edinburgh team have described “two step” pregnancy screening for carriers of cystic fibrosis, testing the woman first and offering tests to those men whose partner was a carrier.6Of eligible women, 83% took up the offer. Studies of pregnancy screening in primary care7,8 or in family planning clinics,7or by the family doctor when pregnancy is first diagnosed9 have shown that if an offer was made directly by an interested health professional the take up rate exceeded 70%. Catch 22 seems to be that if there is an offer before pregnancy it may have to be proactive (possibly directive!) whilst offers during pregnancy mean that high risk situations are identified at a difficult time. Thus, the situations in which there is a high uptake may not allow people to be fully informed or high risk couples to have the choice of all options. The response of older school students to questions about carrier screening is relevant here10;86% were in favour, provided that no one assumed that identified carriers would choose prenatal tests.

    Do we now have sufficient data on which to judge the pros and cons of genetic screening for important autosomal recessive conditions in Britain? Perhaps several complementary approaches are relevant, and no single choice will be right for a mixed population. In the community, an offer of screening, which includes cascade testing on the relatives of identified carriers, might be an appropriate start, combined with an offer of couple testing in pregnancy. It has been calculated that if 10% of young adults chose to be tested (and to allow their relatives to be informed if the result was positive) up to 30% of all carriers could be identified after testing around 11% of the population.11The increased community knowledge achieved by such initiatives would then offset the disadvantages of couple screening in pregnancy that most carriers cannot be counselled because they are not informed of the result. Such a policy would address positively several issues raised by the recent report on the bioethics of genetic screening.12

    Carrier screening might be introduced more wisely if it started with those who actively seek it, not those who accept it passively. If modern genetic counselling, with freedom of informed choice, is to be distinct from eugenics it must respond to the wishes of individual couples rather than to screening programmes aimed at the termination of affected pregnancies. The advent of molecular genetics provides more options suited to a wide range of people. Can we decide now to launch an ethical genetic screening programme for autosomal recessive conditions such as cystic fibrosis? If not, is the barrier logical or emotional - a feeling that if we allow choice for all, then some people will not follow the path that we would have wished.


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