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Editor,—The paper describing screening for cystic fibrosis (CF) by Cunningham and Marshall1 has a number of important flaws. Any screening programme is targeted at a defined population. The title refers to a “population in one region” without being geographically precise, but the study is of children born in two hospitals. If the total number of screened pregnancies in the hospitals had been stated then the incidence of CF conceptions in this group could have been calculated.
The paper quotes without referencing a CF “gene frequency”, but the authors clearly mean CF carrier frequency of nearly 1 in 20. This gives an incidence of 1 in 1600 live births (not 1 in 2000). The only published long term population data estimating CF incidence in Scotland are the UK data, which at the time of this study was fractionally under 1 in 2500 live births.2 Thus, the authors suppose that the CF incidence is 50% higher than this. This has important consequences, which the authors have ignored in the interpretation of their results.
The births to Edinburgh residents is not helpful because this was not the population screened. Given the number of cases found per year prescreening (approximately five), and assuming the UK incidence, there should have been about 12 500 live births per year. The births in Edinburgh were about 5000 per year, suggesting about two CF cases per year. This discrepancy warrants comment by the authors.
The prescreening data raise more issues that they solve. The direct reduction in CF live births is given by the number of abortions. This can only be converted to incidence if the exact population studied is known. Historical comparisons, with small numbers of cases, risks misinterpreting random variations and can magnify the effects of bias.
While it is helpful to know what peoples’ reproductive intentions are, the overall effect of neonatal screening on reproductive behaviour can only be determined by long term population studies of disease incidence and gene frequency. As the members of a population become better informed about the nature and scope of antenatal screening they will adopt more sophisticated strategies to exploit its potential for their own benefit. It is difficult to anticipate the net effect of these strategies.
Organisations such as the UK National Screening Committee are more likely to accept the case for CF screening if scrupulously accurate data, properly analysed and clearly interpreted, are available. Anything less leads to confusion and can only damage the case.
Drs Cunningham and Marshall reply:
Information with regard to carrier frequency in this region has recently been reported.1-1 The 1 in 20 frequency quoted was a general introductory statement that we did not attach to this region and did not use in any calculation.
The numbers of screened pregnancies1-2 in this area have previously been reported by Professor Brock. Our paper attempted to mirror the time period and population used in Professor Brock’s report, to allow the reader to collate both pieces of information. Edinburgh Sick Children’s Hospital CF centre serves the population of south east Scotland. Even when considering this whole region, the effect of antenatal screening stated in our paper remains true. In the seven years before screening there were 39 children born with CF (mean 5.57) and in the seven years after screening was initiated (in Edinburgh only) there were 20 children with CF born in the south east of Scotland (mean 2.85) (including two children born in 1996 and four in 1997 not included in our original paper).
We accept that the data of births to Edinburgh residents (used to intimate stable demographics) may have been misinterpreted, as they do not represent the total number of births in the city (which is more typically 8500 per annum).
Potential discrepancies in our data as a result of small numbers of births were highlighted as the most likely explanation for differences in observed and expected numbers of CF births, other possible causes were discussed in recognition of this. As no other area in the world has a routine antenatal screening programme for CF we were unable to amalgamate data. Our aim was to highlight that we now have approximately 20 fewer children with CF under 6 years old in our clinic than we might have expected, information that we consider will be further able to inform the debate on antenatal and postnatal screening for CF.