Prenatal screening for Down's syndrome
Authors: Z. Alfirevic
Journal: BMJ (2009)

Cite this as: BMJ 2009;338:b140 doi: 10.1136/bmj.bl40 Screening for Down's syndrome is the most controversial aspect of antenatal care offered to pregnant women, and it is a highly emotionally charged subject. In a recent editorial, two senior executives of Down Syndrome Education International claimed that current public health policies leading to an estimated annual loss of 400 normal fetuses to prevent the birth of 600 babies with Down's syndrome were shocking.' Both the estimated numbers and the tone of the statement were disputed by-among others-members of the editorial board of the journal published by Down Syndrome Education International.2 Two BMJpapers provide additional information for the debate. In the first, Ekelund and colleagues described real life data from Denmark after the introduction of the first trimester combined screening programme in 2005.1 In the second, Gekas and colleagues calculated the cost effectiveness of 19 different screening algorithms using data generated by computer simulations.4 Can prospective parents and clinicians make any sense of all this information? The language currently used to describe the evaluation of various screening policies is incomprehensible to most couples deciding whether or not the woman should be screened. Traditionally, a screening programme is deemed superior if the detec tion rate (sensitivity) is high and the false positive is rate is low. But these figures can be manipulated by using different  offs for a screen positive result. A cut off of one in 300 may yield a high detection rate but will result in many unnecessary invasive procedures. Increasing a cut off to one in 100 means fewer false positive results, but also lower detection rates. In other words, somebody needs to decide what is more important to avoid-the unsuspected birth of a baby with Down's syndrome or the loss of a healthy baby as a consequence of an inva sive test (amniocentesis or chorionic villus sampling) triggered by a false positive test. Herein lies a problem. Can an individual make this judgment or is it the domain of policy makers in public health? The UK National Screening Committee has set the Down's detection rate benchmark at 75%, with a false positive rate of less than 3%. It claims that four tests meet these criteria-the quadruple second trimester test, the serum first and second trimester integrated test, the full integrated test, and the first trimester combined test (table).5 From April 2010 the NHS target is even tougher-detection rate of more than 90% with a false positive rate of less than 2%. The Danish paper found that the first trimester combined test had a false positive rate of 3.3% (95% confidence interval 3.1% to 3.4%).3 Could the NHS can do better? Should we implement the integrated test (table) and force women to wait for the results until 16 weeks of gestation, or should we trust the Canadian data and implement contingency screening in the hope that everyday practice mirrors their modelled data?4 In reality, prospective parents demand a personalised approach, entirely in keeping with the NHS's promise of responsive, convenient, and personalised services.6 For antenatal Down's syndrome screening, equal atten tion has to be given to women who choose not to be screened, those who think that any risk is unacceptable, and those who want a formal risk assessment to decide what to do. At any stage of the decision making process women are entitled to change their minds-specialists in this field see this on a daily basis. Women who decline Down's syndrome screening often demand