What is screening?
Screening is the systematic application of a test or inquiry, to identify individuals at sufficient risk of a specific disorder to benefit from further investigation or direct preventive action, among persons who have not sought medical attention on account of symptoms of that disorder.
Antenatal screening for Down's syndrome and open neural tube defects identifies women with a high risk of an affected pregnancy so that they can be offered a diagnostic test, and if a diagnosis of Down's syndrome or an open neural tube defect is made have the option of a termination of pregnancy.
The screening tests use your age and the levels of various markers (see
calculation of the risk of Down's syndrome for information on the markers) to calculate your individual chance (risk) of your baby having Down's syndrome. Using this risk you will be classified as screen-positive (high risk) or screen-negative (low risk). The risk is estimated using a computer program: details are at www.lmsalpha.com.
What is a risk?
A risk is the chance of an event occurring. For example,
a risk of Down's syndrome of 1 in 100 means
that if 100 women have this test result,
we would expect that 1 of these women would
have a baby with Down's syndrome and that
99 would not. This is the same as a 1% chance
that the baby has Down's syndrome and a
99% chance that the baby does not.
What do screen-positive and screen-negative results mean?
Screen-positive
A screen-positive result means that you
are in a higher risk group for having a
baby with Down's syndrome or a neural tube
defect. If your result is in this group,
you will be offered a diagnostic test.
The result is called screen-positive if
i) the risk of Down's syndrome in your pregnancy is above a specified risk cut-off (1 in 150 for the Combined and Integrated tests or 1 in 200 for the Quadruple). About 1 in 60 women screened using the Integrated test will be in this group. About 1 in 40 using the Combined test and about 1 in 20 using the Quadruple test.
or
ii) the AFP level is at least two and a half times
higher than the normal (median) level. About
1 in every 100 women screened will be in this
group.
iii) you have had a previous pregnancy affected with either Down's syndrome or an open neural tube defect. For more information please click here.
Most women with screen-positive results do not have a pregnancy with Down's syndrome or an open neural tube defect. For example of 6 women with screen-positive results from the Integrated test, only one would actually have a pregnancy with Down's syndrome, and of 20 women with screen-positive results from the Quadruple test, only one would have a pregnancy with Down's syndrome
Screen-negative
If
the risk of Down's syndrome, based on your
age and the levels of the markers is lower
than a specified cut-off (1 in 150 for the
Combined and Integrated tests or 1 in 200 for the Quadruple test) and the AFP level is not
high, then the result is called 'screen-negative'
and a diagnostic test would not
usually be offered.
Although a screen-negative result means that you are not at high risk of having a baby with Down's syndrome or an open neural tube defect, a screen-negative result does not rule out the possibility of a pregnancy with either of these abnormalities.
Why do women with screen-negative results occasionally have babies with Down's syndrome or an open neural tube defect?
It is unusual for a women to have a baby with either of these abnormalities, and it is even more unusual for a woman with a screen-negative result, but it does sometimes happen. This is because the screening test cannot completely distinguish affected from unaffected pregnancies. However small the risk is, we cannot rule out the possibility of the baby having Down's syndrome or a neural tube defect.
If I do not have an affected pregnancy, how could I have a screen-positive result?
The screening result is based on your age and the levels of the markers used in the test. You are therefore more likely to have a screen-positive result if you are older or if your marker levels are falling in a pattern typical of a Down's syndrome pregnancy (see
calculation of the risk of Down's syndrome for information on the markers). However, since the markers also naturally vary between women, there is usually no apparent reason for women having either high or low levels and so most women with screen-positive results will not have an affected pregnancy.
A screen positive result only indicates who is in the higher risk group so that we know who should be offered a diagnostic test.
Why do you take my age into account?
Any woman could have a baby with Down's syndrome, whatever her age, but the likelihood of this happening does increase as a woman gets older and so we use age as one of the factors when working out your risk of a pregnancy with Down's syndrome. It also means that an older woman is more likely to have a result in the higher risk group (screen-positive) and so be offered a
diagnostic test.
