Which test?
Our Antenatal Screening Service provides five screening tests for Down’s syndrome. These are the Integrated test, the Serum Integrated test, the Combined test, the Quadruple test, and the Quadruple plus NT test.
There are several different markers that are used in screening for Down’s syndrome, some are useful only in the first trimester of pregnancy (10-13 weeks) and some are useful only in the second trimester of pregnancy (14-22 weeks). The five screening tests use different combinations of these markers. For details of the markers see calculation of the risk of Down’s syndrome.
The Integrated test
The Integrated test is the most effective method of screening for Down's syndrome as it looks at all of the screening markers from both the first and second trimesters of pregnancy. The test is performed in two stages. The first stage is ideally carried out at 11 weeks of pregnancy (but any time between 10 and 13 weeks is acceptable), when a blood sample is obtained and an ultrasound examination carried out. The second stage involves a second blood sample which is ideally taken at 15 or 16 weeks, but between 14 and 22 weeks is possible. The integrated test includes screening for open neural tube defects (spina bifida and anencephaly). Using a 1 in 150 at term cut-off the Integrated test has a detection rate of 90% for a 1.3% false positive rate (see below).
If the fetal nasal bone (NB) is used in the first stage, using a 1 in 150 at term cut-off the Integrated test has a detection rate of 94% for a 1.0% false positive rate (see below). For more detailed information see the Integrated test.
The Serum Integrated test
The serum integrated test is an option for women who are unable to have a nuchal translucency measurement performed. The test is performed in two stages. The first stage is ideally carried out at 11 weeks of pregnancy (but any time between 10 and 13 weeks is acceptable), when a blood sample is obtained. The second stage involves a second blood sample which is ideally taken at 15 or 16 weeks, but between 14 and 22 weeks is possible. The Serum Integrated test includes screening for open neural tube defects (spina bifida and anencephaly). Using a 1 in 150 at term cut-off the Serum Integrated test has a detection rate of 82% for a 2.7% false positive rate (see below). For more detailed information see the Serum Integrated test.
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. The Combined test is not as
effective as the integrated test, but is suitable
for women who are prepared to accept a less
effective test in order to have an earlier
result. Using a 1 in 150 at term cut-off the
Combined test has a detection rate of 84%
for a 2.2% false positive rate (see below).
For more detailed information see the Combined
test.
The Quadruple test
The Quadruple test only uses second trimester markers, and is performed at around 15 or 16 weeks of pregnancy (but between 14 and 22 weeks is possible) when a blood sample is obtained. The test is suitable for women who book after 13 weeks of pregnancy and are too late for the Integrated test. Using a 1 in 150 at term cut-off the Quadruple test has a detection rate of 80% for a 3.5% false positive rate (see below). For more detailed information see the
Quadruple test.
The Quadruple plus NT test
The Quadruple plus NT test is an option for women who have had a nuchal translucency (NT) performed but wish to utilise the second trimester serum markers to improve screening performance. The NT measurement is incorporated into the Quadruple test interpretation. The Quadruple plus NT test includes screening for open neural tube defects (spina bifida and anencephaly). Using a 1 in 150 at term cut-off the Quadruple plus NT test has a detection rate of 89% for a 1.6% false positive rate (see below). For more detailed information see the Quadruple plus NT test.
Comparison of screening performance
|
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 |
|
|
|
|
|
Quadruple test
|
1 in 150
|
80 |
3.5
|
1:15 |
Quadruple plus NT test
|
1
in 150
|
89 |
1.6 |
1:6
|
Detection Rate (DR) :
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.
False Positive Rate (FPR) :
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.
Odds of being affected given a positive result (OAPR) :
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.
