Tuesday July 31, 2007
Scan is not so new
I REFER to your article, New scan in town (StarTwo, July 10), and would like to share the following points to assist pregnant women in their decision on the appropriate tests for their pregnancy.
Firstly, the nuchal scan is not a new scan in town as it has been offered by several medical centres for over a year now.
The Nuchal Translucency (NT) is an ultrasound feature and the blood test involves measuring the chemicals, free beta HcG and PAPP-A. The two combined is called the First Trimester Screening (FTS) test. The FTS, it must be emphasised, is a screening test and hence it is not diagnostic, which means if the FTS test showed an abnormal result, one has still to undergo an amniocentesis to confirm the diagnosis.
The NT scan has its limitations as it is dependent on the accuracy of the gestational age of the foetus, the abdominal size of the pregnant woman, the training of the operator, the type of machines used and the strict adherence to guidelines on measurements of the NT. In comparison, there are fewer limitations on the amniocentesis procedure.
Blood is also used in the second trimester screening which measures four chemicals – MSAFP, total or beta-hCG, unconjugated estradiol, and serum inhibin A. This is called the “quad” screening and offers a comparable detection rate to the FTS, which has a 79% detection rate of Down syndrome, with a 5% false positive rate. In comparison, amniocentesis detects 100% of Down syndrome with a 0% false positive rate.
While the FTS is a screening test for Down Syndrome, and in some instances, for Trisomy 13 and 18, amniocentesis detects all known chromosomal defects.
The Integrated Test incorporating the First and Second Trimester blood markers has a 88% detection rate which is higher than the FTS, and it does not incorporate NT.
The nasal bone is more difficult to detect and seems to have a less prominent role in identifying the foetus at risk of Down syndrome due to its lower detection rate.
A recent trial (FASTER) has demonstrated that the miscarriage rate from amniocentesis is only 0.15% and not 0.5% as believed, thus making it a safe option.
There are several prenatal tests available and there is not one that fits all, including the NT. The tests have to be individualised according to the patient’s risk profile.
There is no doubt that NT and FTS have an important role in screening for Down syndrome but pregnant women should be given accurate information on detection rates and false positive rates of any Down syndrome screening test, and further diagnostic tests that may be offered. They should also be offered both screening and diagnostic tests as options, and the woman’s right to accept or decline the test should be made clear.
Dr Gunasegaran
Kuala Lumpur
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