The result is called screen-positive if:
A screen-positive result means that you are in a higher risk group for having a baby with Down’s syndrome or an open neural tube defect. If your result is in this group, you will be offered a diagnostic test.
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, which is why we use age as one of the factors when calculating your risk. This means that an older woman is more likely to have a result in the higher risk and so be offered a diagnostic test.
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.
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. 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.
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 test and Integrated test 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.
It is unusual for a woman 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 an open neural tube defect.
The Antenatal Screening Service provides five screening tests for Down’s syndrome. The following table compares the performance of each test:
|Test||Cut-off (at term)||Detection rate (DR) %||False Positive Rate (FPR) %||Odds of being affected given a positive result (OAPR)|
|Integrated test||1 in 150||90||1.3||1:5|
|Serum Integrated test||1 in 150||82||2.7||1:12|
|Combined test||1 in 150||84||2.2||1:9|
|Quadruple test||1 in 150||80||3.5||1:15|
|Quadruple plus NT test||1 in 150||80||1.6||1:6|
The DR is the proportion of affected individuals with screen-positive results – the proportion of women with pregnancies affected by Down's syndrome who receive a screen-positive result.
The FPR is the proportion of unaffected individuals with screen-positive results – the proportion of women whose pregnancies are not affected by Down's syndrome who receive a screen-positive result.
The OAPR is the ratio of affected screen-positive pregnancies to unaffected screen-positive pregnancies. An OAPR of 1:5 means that among women with a screen-positive result, 1 will have an affected pregnancy for every 5 that do not.