The Combined Blood and Ultrasound Test for Down Syndrome
What is the combined test?
This test has recently become available in Victoria. It is a
screening test which combines the results of two proteins in
the mothers blood with ultrasound to determine your risk of
having a baby with Down syndrome and other chromosomal abnormalities.
The accuracy of detecting Down Syndrome is approximately 85-90%.
The advantage of this test over others is that a result is obtained
early in pregnancy, around 11-14 weeks.
How is the test performed?
Blood is taken from your arm at approximately 10 weeks gestation.
This should be arranged through your nearest approved pathology
collection service as advised by your doctor. The blood is
then dispatched to the Murdoch institute at the Royal Children's
Hospital, in Parkville. Two blood proteins (PAPP-A and beta-HCG)
are measured. Approximately 2 weeks later when the pregnancy
is around 12 weeks an ultrasound scan is performed. At this
examination the fetal development, size, and nuchal translucency
are documented (refer Nuchal Translucency leaflet). The nuchal
translucency result is faxed to the Murdoch Institute and
the results of the blood test and ultrasound are combined
to give one result.
How should I interpret the results?
Your results will be given to you as a risk estimate; i.e.
the chance of your baby having Down syndrome. For example
if you are 28 years old your chance of having a baby with
Down syndrome is about 1 in 600. But if you ultrasound result
is favourable ie. a thin nuchal translucency, and biochemistry
results favourable i.e. low levels of both proteins, this
risk will be reduced to say 1 in 3500 and therefore would
be reassuring. The combined will result will not however provide
you with a definite 'yes' or 'no' answer as to whether your
baby has Down Syndrome.
Do all pregnant women have this combined
test?
No. The decision to have the blood test, the ultrasound, or
both is entirely up to you. It is, however, worth considering
a number of issues for example:
- If testing showed that I was carrying a Down Syndrome
baby what would I do?
- The test does not identify ALL pregnancies with Down Syndrome.
- At what risk figure should I proceed with CVS or amniocentesis.
Most patients choose to proceed with CVS or amniocentesis when
the combined ultrasound and biochemistry risk reaches 1 in 300.
But it is your choice as to what level you wish to rest with.
These difficult issues may be discussed further with your doctor
or ultrasound specialist.
Should I have CVS or amniocentesis
instead of the combined test?
You may ask yourself should I not just proceed to the definitive
test with CVS or amniocentesis and avoid having 2 tests. There
are 2 principle reasons why you may choose not to proceed
directly to needle testing. Firstly CVS and amniocentesis
both carry a significant risk of miscarriage (CVS 1% and amniocentesis
0.5%) and secondly 95% of patients who have the combined test
will be given a favourable result, lowering their age related
risk.
How accurate is the Combined Test?
By combining the results of the early ultrasound and the early
blood tests (ideally at 10 weeks) the test can identify approximately
9 out of 10 pregnancies in which the baby has Down syndrome.
This is more accurate than each test alone. It is however
important to realize that this test is only for chromosomal
abnormalities such as Down syndrome and does not ensure that
the baby is free of other possible birth defects.
If the ultrasound shows a thick nuchal translucency and
the blood levels are high, your age related risk at 28yrs
may be increased from say 1:600 to say 1 in 60. If this situation
arises then you have the choice of a definitive test such
as chorionic villus sampling or amniocentesis.
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