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How sensitive are the screening tests for Down's syndrome?
About 60 per cent sensitive. That means about 60 per cent of all affected fetuses will be detected and 40 per cent will be left undetected by this standard screening test. It is clearly unsatisfactory. However, better screening methods with higher sensitivity are already available (see below) but not yet universally adopted. Cost and in some cases lack of facilities are some of the factors.
For those babies that slip through the screening blood test, is there any other detecting test?
There is still a possibility that an affected fetus can be detected using the detailed anatomy ultrasound scan, usually performed at 18 to 20 weeks of gestation. As mentioned before, Down's babies tend to have a number of physical abnormalities and organ malformations, which may be detectable by the scan. If a number of those are detected, in spite of a "negative" screening test, the suspicion may be aroused to justify suggesting a definitive diagnostic test. As it happens, not all Down's babies have these abnormalities and therefore a substantial proportion still slip through and are born when not expected.
What are the definitive diagnostic tests?
For all chromosomal disorders, including Down's syndrome, a confirmatory diagnostic test is based on obtaining fetal (or placental) cells which are processed to enable the mapping of the chromosomal make-up of that baby. Once this is achieved, a chromosomal defect, be it an extra chromosome (as in Down's) or deletion of part or a whole chromosome , will be established.
Non-invasive tests may be available in the not too distant future
How are the fetal or placental cells obtained?
There are three main ways of getting the cells. The most widely used is amniocentesis, where fluid is obtained from around the fetus and fetal cells isolated from this and then cultured. The second method is chorionic villus sampling (CVS), where a tiny piece of placenta is obtained; and the third is cordocentesis, where fetal blood is obtained from the umbilical cord.
How do the three tests compare?
As can be appreciated, all three are invasive. Amniocentesis is considered to be the least invasive because it avoids both the baby and the placenta. The potential for serious consequences is also considerably less with amniocentesis. The flip side is that it tends to take between two to three weeks to get the results. This is a long time for most parents, because of the anxiety associated with it. In addition, since most times the amniocentesis is carried out on the basis of screening test results, it is therefore done at around 17 - 18 weeks. This means the results are obtained at about twenty weeks; rather late if results mean some major intervention.
CVS can be done quite early in pregnancy (at 11 to 12 weeks) or later. The problem is that the miscarriage risk as a consequence of the procedure is about twice that of amniocentesis. However, results are available in two to four days, far quicker than with amniocentesis.
Cordocentesis can be performed only beyond eighteen weeks. The miscarriage rate is even higher than CVS. There is usually little justification to resort to cordocentesis for the sake of chromosomal analysis.