Serum screening for Down's syndrome: experiences of obstetricians in England and Wales

Introduction

Screening for fetal abnormality has become a major part of antenatal care, and there has been much discussion of the pros and cons of various tests - notably, serum screening for Down's syndrome. A survey in 1991 found that the use of such screening in the United Kingdom was expanding rapidly but in a piecemeal and sometimes inappropriate fashion.1 This survey was based on returns from directors of public health and asked about provision in 1991 and expected provision in 1992. Debates in the medical press suggest that many of those at the front of service provision are unhappy about the test, but there is no systematic information about the attitudes and experiences of obstetricians on this subject.

I carried out a survey of the attitude of consultant obstetricians towards prenatal screening and diagnosis and towards termination of pregnancy. I report here on the questions relating to serum screening for Down's syndrome. Other aspects of the survey have been reported elsewhere2 and are being prepared for publication.

Results

Discussion

Serum screening for Down's syndrome is being offered on some basis by virtually all obstetricians in England and Wales, with about one half offering the test routinely to all women in their care. My questionnaire did not ask which biochemical test was being used or the basis for defining a subgroup to whom to offer the test. Comments suggest problems in both of these areas. Wald et al found that several districts were offering serum screening only to older women as an alternative to amniocentesis,1 and this was also evident in my survey. For some obstetricians this was a non-clinical decision: “The triple test is offered to women of 35 and over, but this is because of other factors within the administration.” For others it was reasoned policy: “We have taken a policy decision as a unit not to offer a screening service for Down's to the whole population, only to those who request and those aged 35-37 who prefer not to opt directly for amnio. This is because of the 60% detect rate <37 years and 80% >37 years which we feel misses too many cases.”

There were also difficulties integrating serum screening for Down's syndrome with scanning: “False positive Down's anxiety would be less if we could take on first trimester scans.” Another respondent said: “Scan restrictions (financial constraint) mean routine dating and anomaly scan are combined at 18 weeks, so serum screening is not undertaken until after this.”

Other comments highlighted the complexities associated with the decision to offer screening; some obstetricians thought that they had a negligible input to the decision, others felt overwhelmed by the choices about which markers to measure and to which women to offer the test. A number commented on the need for a national policy.

Serum screening for Down's syndrome was seen as consistently more problematic than serum screening for neural tube defects. Why should this be? Firstly, the test is replacing two existing tests: serum screening for neural tube defects and age as a screening test for Down's syndrome. The test procedure is the same as for neural tube defects so women may mistakenly think that they understand.4 The relation between age and Down's syndrome has been accepted by women and midwives but is not recognised as screening, and thus most older women (those whose babies do not have Down's syndrome) were not being seen as “false positives,” just as the young women who gave birth to Down's babies were not seen as “false negatives.” The introduction of a new screening method has challenged old ideas by drawing attention to the fact that any pregnant woman could be carrying a fetus with Down's syndrome.

Difficulties in understanding the test arise partly because details (what is measured and what cut off points are used) vary from place to place. A more fundamental difficulty, however, is a failure to distinguish between screening and diagnosis. The test is seen as poor because it has false positive and false negative results; one obstetrician commented that if the pick up rate was only going to be 57% rather than 66% “why not toss a coin?.” In fact the triple test performs relatively well on these criteria with a positive predictive value of 1 in 68, which is about twice that of maternal age.

Nearly half of those offering serum screening said that they did not have adequate facilities to counsel the women to whom the test is offered. This is an issue raised by the recent report from the Nuffield Council on Bioethics Genetic Screening: Ethical Issues.5 There are clearly questions about who should counsel, where and when, and what counselling actually means. Should information about the test be given by the hospital or the primary care team? This is an issue in need of resolution given the current trend towards returning antental care to the community. Women may turn to their general practitioners and community midwives for information and support but find that knowledge is lacking.4

There are different requirements for counselling before and after tests. Pretest counselling should give women the information on which to decide whether or not to be screened.6 This may include consideration about how they might react to certain scenarios - for example, a positive result of screening or amniocentesis - but does not involve many of the emotional and therapeutic ingredients which are often associated with the word “counselling” in other contexts and which may be needed after positive test results. Clarification of terminology and of the relative roles of obstetricians and the primary care team may help to ensure that women receive the counselling that they require, which is not currently the norm.4, 7

This survey was funded by a fellowship awarded by the Institute for Social Studies in Medical Care.