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The importance of accuracy in Down’s syndrome testing

Posted in Diagnostic testing, Screening by Matt at WelcometoIllinois on October 28, 2008

The Independent has a really interesting story today about the comparative accuracy of the various tests available for Down’s syndrome in the UK. The report’s author, Annalisa Barbieri, asks why the most accurate test is only availble at two NHS hospitals even though the estimated cost of adding the test is “about £25”.

Barbieri’s argument is not based on a desire to improve detection rates in order to increase terminations but to give parents an accurate picture of the various risks they face.

Having been through the process of trying to weigh up various conflicting statistics in order to decide whether to continue with the pregnacy of our child I have to agree that, if you assume that there is zero chance of getting rid of testing for Down’s syndrome, then the most important issue has to be ensuring that the tests are as accurate as possible.

Based on the combined nuchal fold and blood test we were told there was a one in 76 (1.3%) chance that our son had Down’s syndrome, while the combined test had a 95% accuracy rate. So it was 3.8 times more likely that the result was wrong than there was that he had Down’s syndrome. How is someone supposed to make a decision based on statistics like those? 

This is why I am in favour of new non-invasive DNA testing currently being developed, although that’s another matter.

As Barbieri points out “The bottom line is that if women have screening tests with a low rate of accuracy, the reality is that they are more likely to opt for invasive tests that put their babies at risk.”

Reducing the number of amnios and CVS tests should also help reduce the number of healthy babies that are miscarried thanks to invasive testing (400 for every 660 babies with Down’s syndrome identified and terminated according to recent research).

From the Independent story:

“Let’s take the tests in turn. The first was the double test. This took maternal age into account as well as looking for two blood markers: alpha fetoprotein (AFP) and human chorionic gonadotrophic (hCG). AFP tends to be decreased in Down foetuses, hCG increased. It has a 59 per cent accuracy rate: that is, it picks up 59 per cent of babies affected. The triple test added another marker to the blood test, uE3, which tends to be decreased in affected babies. Its detection rate is 63 per cent. The quadruple test added a further marker, inhibin A, which tends to be increased in foetuses with trisomy-21; the quad test is picks up 72 per cent of affected babies.

After this came the combined test, which added to the blood tests by combining data from a nuchal fold scan (which must be done between 11 and 13.6 weeks). Research had found that the thicker the nuchal fold the more likelihood of a baby having Down syndrome. (New markers now also look at other markers such as the presence of the nasal bone – the sooner this is visible, the less likelihood of the syndrome – and the tricuspid valve flow through the heart.) The combined test is said to be 76 per cent accurate. The nuchal test (NT) on its own is only 69 per cent accurate, but relies heavily on the skill of the sonographer.

Then came what should be the industry standard: the integrated test. This combines maternal age, NT result, one blood test at about 12 weeks that looks for PAPP-A (pregnancy associated plasma protein A) and another blood test at 15 weeks that looks at further biochemistry: AFP, hCG, uE3 and inhibin-A. At worst, it has an 89 per cent detection rate, at best 98 per cent (it is more accurate the older the mother is). Put personally, at 37 I had a risk of 1:200 vs the less than 1:50,000 the integrated test gave me. At 42, I had a risk of 1:55 vs 1:1,900.”

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2 Responses

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  1. […] testing techniques prompt wider debate I wrote yesterday about the importance of accuracy in ante-natal testing for chromosome abnormalities such as […]

  2. […] the focus on the unnecessary deaths of healthy foetuses is understandable, I would also suggest that the inaccuracy of the testing process also results in the unnecessary deaths of a number of […]


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