Down's syndrome screening: a controversial test, with more controversy to come!

By 1998, most health authorities offered antenatal screening for Down's syn drome, usually by biochemical methods. To date, the development of this for m of screening has not been coordinated by a national body and, consequentl y, there are wide variations in practice between localities. Fortunately, m any of these variations have not led to any noticeable inequality of health provision, but the wide variation in risk cut offs used by different centr es does. Other variations merely lead to potentially unnecessary expenditur e; whereas it is believed that adding extra tests to the screening procedur e is beneficial (such as double test to triple test), statistical evaluatio n of the confidence intervals for the detection rates quoted indicates that there is no evidence that the extra test provides an increase in detection . The cervical screening programme has progressively improved, partly throu gh the auspices of a national framework. A similar national approach would benefit Down's screening and is only now being considered: the national scr eening committee (NSC) is currently drafting recommendations. To ensure opt imum screening performance, the NSC should specify the risk thresholds appl ied, the screening protocols to be used-that is, an opt-in programme with a minimum (possibly even a maximum) of two biochemical analytes or a nuchal fold evaluation-and perhaps should even recommend national population param eters to be used for risk calculation. It might even be advisable for stati stical work to be carried out to determine whether local derivation of medi ans is truly necessary. Furthermore, defined options for older women could be specified-for example, should all older patients have the option to proc eed directly to amniocentesis if they wish or should National Health Servic e amniocentesis only be available for those with a "high risk" screening re sult. The difficulties that will face the NSC in deciding which screening p olicy to adopt are also considered; specifically, the lack of evidence to s uggest that triple testing is superior to double testing, and the lack of e vidence to prove the superiority of one analyte over another. This inadequa cy of evidence is not from want of trying, but is caused by the problems of collecting enough data to provide statistical significance. Finally, there is one important difference between cervical and Down's syndrome screening that has a major impact on the advice given by any "expert"; namely, paten ts. Many aspects of Down's screening are subject to patents and, therefore, there is more potential for apparently uncontroversial decisions to reboun d with future retrospective patent infringement claims. Thus, it would be s ensible to insist that any member of a national body deciding upon Down's s creening policy must fully disclose all potential conflicts of interest, bo th personal and family, before they are allowed to sit on the committee. Fu rthermore, ifa national policy is decided upon, worldwide patent searches s hould be carried out to determine whether there are any possible unforeseen legal consequences of any recommendation.