Are Down’s syndrome tests an unnecessary risk?

Women could be being given unnecessary tests which put their pregnancy at risk, reported The Daily Telegraph . The newspaper said, “Screening tests for Down's syndrome could lead women to undergo needless invasive tests and risk miscarrying healthy babies.”

Screening tests are offered to all pregnant women to determine their risk of having a baby with Down’s syndrome or another genetic abnormality. In the UK, screening includes blood tests and a nuchal thickness measurement. If the screening test result put a woman at high risk of having a child with Down’s syndrome, she is offered an invasive test, such as an amniocentesis. Only high-risk women are offered amniocentesis, as there is a 1% chance that the procedure will cause a miscarriage.

The_ Daily Mail_ reported that, “3000 healthy babies could be dying in the womb each year” due to the pointless use of invasive tests.

This story comes from a lecture delivered in 2006 by a renowned expert in the field of clinical ultrasound. His opinions are supported by references and include a calculation based on an assumption about the accuracy of ultrasound when used as a single screening test, and the risk from amniocentesis. The additional use of blood tests, which takes place in UK pregnancy screening, is not considered and the headline figure may be inaccurate.

Where did the story come from?

Dr Hylton Meire delivered the Donald, MacVicar and Brown lecture in 2006, and a transcript was published recently. Dr Meire was employed by the Medical Research Council as a full-time consultant in clinical ultrasound from 1975 to 1982. From 1982 until his retirement in 2000 he was a consultant radiologist with responsibility for the Radiology Ultrasound Service at King's College Hospital and he was simultaneously director of ultrasound at the Portland Hospital for Women and Children, London. This opinion piece was published in Ultrasound , the peer-reviewed journal of the British Medical Ultrasound Society.

What kind of scientific study was this?

This is a transcript of an invited lecture. The lecturer reviews a number of primary and secondary research articles, providing some scientific critique and personal analysis of the results regarding the ‘20-week anomaly scan’ and ‘nuchal thickness measurements’.

What were the results of the study?

People normally have 23 pairs of chromosomes, however Down’s syndrome (or trisomy 21) is a genetic disorder caused by the presence of all or part of an extra 21st chromosome.  The risk of carrying an affected baby increases with the mother’s age, though most Down’s syndrome babies are born to mothers under 35, reflecting the fact that this group of women have more babies. This lecture discusses the 20-week anomaly scan and also the nuchal translucency (or thickness) test which is a separate ultrasound test typically performed between 11 and 13 weeks.

What interpretations did the researchers draw from these results?

The lecturer concludes that, “Rapidly progressive improvements in the quality of ultrasound scans have led to widespread claims about the utility of the technique as a routine screening procedure for all obstetric patients. The large scientific studies that might ‘prove’ this utility have still not been performed and those studies published to date have generally failed to support the value of population screening.”

What does the NHS Knowledge Service make of this study?

The lecturer identifies several topics for debate with regard to screening, and has identified the common problem for screening programmes of ‘false positives’. These are the number (or proportion) of women who have been incorrectly reported as positive for Down’s syndrome on a test, despite having a healthy baby. All tests will have a small rate of false positives, but in a population of healthy women and babies this rate must be as low as possible. The detection rate - the number or proportion of people correctly identified with the condition - is another feature of a test that needs to be balanced against the false positive rate. This is important, as detecting a high proportion of affected babies with Down’s syndrome is also of value to the mothers who would choose termination of an affected baby if this was offered. 

The addition of blood tests (‘triple or quad screen’) to ultrasound screening can increase the detection rate at the same time as reducing the false positive rate.  This combined blood and ultrasound testing at different stages in pregnancy is currently the recommended method of screening in the UK, and will reduce the false positive rate below that quoted in this report. The risk of miscarriage with invasive techniques may also be overestimated. Both these estimates will influence the calculation of “needless miscarriages” reported, which may therefore be inaccurate.

Dr Muir Gray adds...

It is difficult to assess how much of Dr Meire’s lecture is based on the current situation, as we do not have information about the time period to which his data refer. However, Dr Meire identifies the reason for setting up the National Down’s syndrome Screening Programme, which has two primary objectives:

• Ensure that the number of false positives is minimised; and
• Ensure that women who are offered the test are aware of its risks and implications.