| Until now, prenatal diagnosis of Downs syndrome | | | | Eight of the mothers had their blood samples taken in |
| (Trisomy 21) for the unborn baby has required an | | | | the first trimester (13 weeks or below). |
| invasive test. This has taken the form of an | | | | The breakthrough was that they managed to |
| amniocentesis where fluid is taken from around the | | | | successfully identify and separate the small amount |
| fetus in the womb after 14 weeks of gestation. The | | | | of fetal DNA that cross into the maternal circulation. |
| alternative, available about 3 weeks earlier is CVS | | | | This was analyzed via what are known as SNP (single |
| where a tiny piece of the afterbirth (placenta) is | | | | nucleotide polymorphisms) tests to identify where |
| extracted. | | | | there was an extra chromosome on set No. 21 |
| Screening Tests | | | | (Trisomy 21). |
| The screening tests which are non-invasive have | | | | Their tests in this study correctly identified 58 of the |
| varying degrees of detection rates from a lowly | | | | 60 chromosomal make up. Among the 58 correctly |
| 30% for maternal age alone to a clearly impressive | | | | analyzed, two had Trisomy 21 (Downs). The two |
| 85% for a combination of maternal age, fetal nuchal | | | | that were not correctly analyzed included one who |
| translucency (scan) and maternal serum biochemistry | | | | had Downs but labeled as normal (false negative) and |
| at 11-14 weeks. This is called the Integrated Test. | | | | the other one was incorrectly identified to have |
| Research has also shown that fetuses affected by | | | | Downs syndrome (false positive). |
| Downs syndrome have a missing nasal bone on an | | | | What does this imply? |
| ultrasound scan performed at 11-14 weeks. This test | | | | This is potentially quite a major advance. Its strength |
| alone has been shown to identify around 70% of | | | | lies in the fact that it does away with an invasive |
| Downs syndrome babies. Crucially however, it did not | | | | procedure therefore eliminating in a stroke the |
| appear to give any advantage over the longer | | | | spectre of miscarriage that hangs over the currently |
| established nuchal translucency scan performed at | | | | used procedures. This risk is indeed small but quite |
| the same gestation and therefore the uptake has | | | | real and when it occurs, is devastating. |
| been poor. | | | | However, it is important to stress that this is only an |
| One of the most frustrating weaknesses of all | | | | early stage of this potential breakthrough. Here are |
| screening tests is the fact that they also falsely | | | | the facts: |
| detect unaffected babies, occasionally leading to | | | | The study had quite a small sample (60) |
| unnecessary invasive diagnostic tests and always | | | | and it is therefore crucial that it is replicated in larger |
| causing significant stress to prospective parents. The | | | | studies. |
| false positive rate of screening tests is around 5%. | | | | There is still a lot of room for refinement. |
| Moreover, even the most sensitive screening test will | | | | In this study, whilst the earliest sample was taken at |
| still miss some babies that are affected and for all | | | | 8 weeks gestation, the first case of Downs was |
| parents this means a little seed of doubt until the | | | | identified at just over 17 weeks. For this to be truly |
| baby arrives. | | | | useful, it will have to have the ability to establish the |
| Diagnostic Tests | | | | diagnosis early, ideally in the first trimester. Also the |
| The definitive diagnostic tests mentioned | | | | interval between blood sample collection and getting |
| (amniocentesis and CVS) suffer from one Achilles | | | | the results need to be reasonably short, lasting no |
| heel: They are invasive and therefore carry the | | | | more than 2 or 3 weeks. |
| potential risk of provoking miscarriage. This can | | | | The current diagnostic tests have quite a |
| happen regardless of whether the baby is affected | | | | high 99% accuracy rate. This test will need to match |
| by Downs syndrome or not. Because of this, there | | | | that and ideally surpass it. |
| has been a lot of research activity to find a risk-free | | | | Science research thrives on this kind of challenge and |
| reliable diagnostic prenatal test for Downs Syndrome. | | | | there is a lot of excitement in the medical research |
| This is why the recent announcement of a | | | | world about this potential breakthrough. |
| development of such a test reported in the medical | | | | In fact, this research work by Ravgen Inc. in the |
| journal The Lancet (Feb. 3, 2007) has caused such a | | | | United States is not the only current development in |
| stir. | | | | this area. Researchers in Hong Kong are also |
| Maternal Blood Test | | | | reportedly on the verge of a similar breakthrough but |
| Dr Ravinder Dhallan of Ravgen Inc. has reported | | | | employing a different method which utilises placental |
| results of a study to diagnose Downs syndrome for | | | | DNA in maternal circulation. |
| the unborn baby using Maternal blood and therefore | | | | These methods might usher in a new era of safe |
| practically eliminating the potential risk of miscarriage | | | | and effective prenatal diagnostic tests. If the hurdles |
| associated with the available invasive tests. What Dr | | | | are overcome, the rough estimate is that the tests |
| Dhallan and colleagues did in this study was to collect | | | | may be available for use in about three years. |
| blood from 60 mothers whose median age was 34. | | | | |