Figures on screening for Down's syndrome are inaccurate

BMJ 1996; 312 doi: http://dx.doi.org/10.1136/bmj.312.7027.379a (Published 10 February 1996) Cite this as: BMJ 1996;312:379
  1. Anne Kennard,
  2. Eva Alberman,
  3. Mike Gill
  1. Lecturer Professor Wolfson Institute of Preventive Medicine, Medical College of St Bartholomew's Hospital, London EC1M 6BQ
  2. Director of public health Brent and Harrow Health Authority, Harrow, Middlesex HA1 3EX

    EDITOR,—Philip Steer's comments about serum screening for Down's syndrome1 should not go unchallenged. Firstly, it is inappropriate to cite a 48% detection rate. For a 5% false positive rate the estimated detection rate is 59% when dates are used to estimate gestational age and 65% if an estimate based on an ultrasound scan is used.2 The Barts Down's syndrome demonstration project, in which 48% of cases were detected, was not designed to estimate the detection rate and was not large enough to do so.

    Secondly, a serum screening test carried out on a community basis costs much less than the pounds sterling80 cited. There are existing NHS programmes which are provided at a quarter of this sum.

    Thirdly, contrary to Steer's statement, we know that screening has had a considerable effect on reducing the birth prevalence of Down's syndrome. In the absence of screening and at a time of increasing maternal age, the birth prevalence of Down's syndrome would have increased from 1.41/1000 in 1989 to 1.47/1000 in 1993,3 assuming a natural fetal loss rate of 27% among the women who had terminations. In fact it has decreased from 1.11/1000 in 1989 to 0.92/1000 in 1993. This is equivalent to an increase in the percentage of affected births avoided through screening from about 21% to 37%.

    Finally, it is acknowledged that the provision of adequate information before the screening test and appropriate counselling afterwards is important and is not always carried out satisfactorily. This is not the fault of the screening test but is a fault in the way the screening test is delivered to women. It underlines the fact that the test itself is but one component of the screening procedure.


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