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The combined test

The Combined test only uses first trimester markers, and is performed at around 11 weeks of pregnancy (but any time between 10 and 13 weeks is acceptable) when a blood sample is obtained and an ultrasound examination is carried out.

This page explains:

What does the Combined test involve?

A sample of your blood is taken between 10 and 13 weeks of pregnancy. At the same time an ultrasound scan is performed. This test is suitable for women who do not want to wait until 15 weeks to receive their test result and understand it is a less effective test than the Integrated test.

The two blood markers are:

  1. pregnancy associated plasma protein-A (PAPP-A)
  2. free ß-human chorionic gonadotrophin (free ß-hCG)
    and the ultrasound marker is:
  3. nuchal translucency (NT).

In pregnancies with Down's syndrome, PAPP-A tends to be low, NT and free ß-hCG levels tend to be raised.

The values of these three markers are used together with your age to estimate the risk of having a pregnancy with Down's syndrome.

Can any other abnormalities be identified?

Yes, the Combined test also identifies pregnancies at a high risk of Edwards' syndrome (trisomy 18) . The risk of Edwards' syndrome can be identified using NT, PAPP-A and free ß-hCG.

The Combined test does not screen for open neural tube defects.

What is defined as a screen-positive result?

Down's syndrome

If the risk of having a term pregnancy affected with Down's syndrome is 1 in 150 or higher the result will be screen-positive and you will be offered a CVS or amniocentesis.

Edwards' syndrome

If the risk of having a term pregnancy affected with Edwards' syndrome is 1 in 100 or higher you will be offered another ultrasound examination and CVS or amniocentesis.

Performance of the Combined test using a 1 in 150 at term cut-off

Down's syndrome

Detection Rate (DR)

84% of women carrying Down's syndrome pregnancies will receive a screen-positive result. (The remaining 16% of women carrying Down's syndrome pregnancies will receive a screen-negative result).

False Positive Rate (FPR)

2.2% of women who are not carrying Down's syndrome pregnancies will receive a screen-positive result. (97.8% of women who are not carrying Down's syndrome pregnancies will receive a screen-negative result).

Odds of being affected given a positive result (OAPR)

1:9 – Among women in the screen-positive group, 1 woman will have a pregnancy with Down's syndrome for every 9 who do not.

Edwards' syndrome

Detection Rate (DR)

About 85%

False Positive Rate

0.2%

Age specific performance for Down's syndrome

Maternal age group (years) Probability of a screen-positive result Proportion of Down’s syndrome pregnancies detected (%)
Under 25 1 in 120 71
25-29 1 in 100 72
30-34 1 in 50 78
35-39 1 in 20 86
40-44 1 in 7 93
45 and over 1 in 4 95
All 1 in 40 84

(mid-trimester estimates of performance, test performed at 11 weeks GA)

Arranging a Combined test

The Combined test can be performed at the London Ultrasound Centre. For further information please contact the London Ultrasound Centre on +44 (0)20 7935 4450.

Cost

The cost of the Combined test at the London Ultrasound Centre is £192.

Information leaflets

The Antenatal Screening Service has produced two information leaflets for the Combined test.

The Questions and Answers leaflets are designed for women considering the test and contain the basic information about the screening test and the results.

The Information for Health Professionals leaflets are aimed at staff and contain more detailed information about the tests. Many women considering the tests also find this information useful.

Download the leaflets in PDF format:

See all of our Antenatal Screening Service information leaflets.

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