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Wolfson Institute of Preventive Medicine - Barts and The London

Screening test results

The information on your page can help you to understand your test results. It also provides a comparison of the performance of the main screening tests.

Understanding your results

The results of your antenatal screening test will be classified as either screen-positive (high risk) or screen-negative (low risk).  These are the statistical terms used in screening.  Nowadays it is common to use the term ‘high chance’ to describe a screen-positive result and ‘low chance’ to describe a screen-negative result.  We will use both terms  

Screening tests use your age and the levels of various markers to calculate your individual risk or chance of your baby having Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18) or Patau syndrome (Trisomy 13).

Screen-positive (high risk or chance)

The result is called screen-positive (high risk or chance) if:

  • The risk or chance of Down syndrome, Edwards syndrome or Patau syndrome in your pregnancy is above a specified cut-off (1 in 150 for the Combined test and the Quadruple test).
    About 1 in 40 women screened using the Combined test will be in this group. About 1 in 20 using the Quadruple test.

If your test includes an assessment for your risk or chance of an open neural tube defect -

  • 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.
  • You may also be classified as screen-positive (high risk or chance) if you have had a previous pregnancy with any of the above conditions

A screen-positive result (high risk or chance) for having a baby with Down syndrome, Edwards syndrome, Patau syndrome will mean you are offered a diagnostic test such as chorionic villus sampling (CVS) or amniocentesis 

A screen-positive result (high risk or chance) for having a baby with an open neural tube defect will mean you are offered a detailed ultrasound scan at around 18 weeks of pregnancy.

Any woman could have a baby with Down syndrome, Edwards syndrome or Patau 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 or chance. This means that an older woman is more likely to have a result in the high risk or chance group and so be offered a diagnostic test.

Most women with screen-positive (high risk or chance) results do not have a pregnancy with Down syndrome, Edwards syndrome or Patau syndrome. For example, of 10 women with screen-positive (high risk or chance) results from the Combined test, only one would actually have a pregnancy with Down syndrome.

If I do not have an affected pregnancy, how could I have a screen-positive (high risk or chance) 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 (high risk or chance) result if you are older or if your marker levels are falling in a pattern typical of a pregnancy with Down syndrome, Edwards syndrome or Patau syndrome. 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 (high risk or chance) results will not have an affected pregnancy.

A screen positive result (high risk or chance) only indicates who should be offered a CVS or amniocentesis.

Screen-negative (low risk or chance)

If the risk or chance of Down syndrome, Edwards syndrome or Patau 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  the Quadruple test) and the AFP level is less than two and a half times higher than the normal (median) level, then the result is called 'screen-negative' (low risk or chance) and a diagnostic test would not usually be offered.

Although a screen-negative (low risk or chance) result means that you are not at a high risk or chance of having a baby with Down syndrome, Edwards syndrome, Patau syndrome or an open neural tube defect, a screen-negative (low risk or chance) result does not rule out the possibility of a pregnancy with any of these abnormalities.

Why do women with screen-negative (low risk or chance) results occasionally have babies with Down syndrome, Edwards syndrome, Patau syndrome or an open neural tube defect?

It is unusual for a woman to have a baby with any of these abnormalities, and it is even more unusual for a woman with a screen-negative (low risk or chance) result, but it does sometimes happen. This is because the screening test cannot completely distinguish affected from unaffected pregnancies. However small the risk or chance is, we cannot rule out the possibility of the baby having Down syndrome, Edwards syndrome, Patau syndrome or an open neural tube defect.

Comparison of screening performance

The Antenatal Screening Service provides three screening tests for Down syndrome, Edwards syndrome and Patau 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)
Combined test 1 in 150 84 2.2 1:9
Quadruple test 1 in 150 80 3.5 1:15
Reflex DNA test 1 in 150 92 0.02 20:1

Detection Rate 

The Detection Rate (DR) is the proportion of affected pregnancies with screen-positive (high risk or chance) results – the proportion of women with pregnancies affected by Down syndrome, Edwards syndrome or Patau syndrome who receive a screen-positive result.

False Positive Rate

The False-positive rate (FPR) is the proportion of unaffected pregnancies with screen-positive (high risk or chance) results – the proportion of women whose pregnancies are not affected by Down syndrome, Edwards syndrome or Patau syndrome who receive a screen-positive result.

Odds of being affected given a positive result

The Odds of being affected given a positive result (OAPR) is the ratio of affected screen-positive (high risk or chance) pregnancies to unaffected screen-positive (high risk or chance) pregnancies. An OAPR of 1:5 means that among women with a screen-positive (high risk or chance) result, 1 will have an affected pregnancy for every 5 that do not.