Combined pregnancy test (assessment tests)

Content

Introduction

The combined test is a non-invasive test that provides an estimate of the risk (screening test) that the fetus is affected by some diseases present before birth (prenatal) (Video).

It is performed in the first trimester of pregnancy and allows you to estimate the risk of the fetus having a chromosomal abnormality such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) or Patau syndrome (trisomy 13).

In order not to confuse the different tests that use substances, called markers, measurable in maternal blood in the first or second trimester (double, triple, quadruple test and integrated test) it is useful to know that, sometimes, the combined test is also called bi test or duotest.

Any examination aimed at ascertaining the risk or the presence of anomalies before birth must include, as a first step, correct information to the couple that allows them to make free choices, including that of not performing any prenatal tests.

The information should increase not only knowledge about the practical aspects of the test but also awareness about the reliability and significance of the test result.

For the combined test, it is important that parents are aware that the test response does not equal the certainty of the presence of a chromosomal disease but only expresses the probability that it may be present.

The test makes it possible to identify fetuses at risk of these three pathologies as early as the 11th week of pregnancy with a more accurate estimate than that which can be obtained by taking into consideration only the maternal age.

The risk of chromosomal abnormalities, such as Down syndrome, increases, in fact, with maternal age and significantly increases after the age of 35. For this reason, since 1998 the National Health Service has offered amniocentesis and CVS free of charge to women aged over 35 years of age who wish to perform prenatal diagnosis.

However, these invasive procedures carry an additional risk of miscarriage while the combination test can be used without complications, for either the fetus or the mother, by all pregnant women, regardless of their age.

The test allows to identify an increased risk of chromosomal abnormalities to be confirmed by means of CVS or amniocentesis, in cases where the test is positive.

The Ministry of Health has included the combined test among the services that all Regions since 2017 offer free and exempt tickets to pregnant women who wish to perform it. This is in line with the ISS national guidelines on physiological pregnancy, which recommend limiting the free offer of invasive investigations (villocentesis and amniocentesis) based only on the maternal age criterion and to prefer tests that are based on the individual risk.

The exam consists of:

  • a blood draw from the mother to dose two substances specific to pregnancy
  • an ultrasound examination to measure the thickness of a space between the skin and the spine behind the nape of the fetus (nuchal translucency)

By combining the maternal age with the results of blood tests and ultrasound, through predefined algorithms, it is possible to estimate the risk that the fetus is affected by Down syndrome, trisomy 18 or trisomy 13.

The combined test cannot assess the risk of neural tube closure defects, such as spina bifida.

The test

The combined test it is performed in the first trimester of pregnancy (between the 11th and 14th week) to estimate early the risk that the fetus is affected by chromosomal abnormalities, primarily Down syndrome, (trisomy 21), which is the most frequent, in addition Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13), much rarer but more severe.

Antenatal counseling must be an integral part of the screening to define the indications and characteristics of the test, provide information on the risks and benefits, discuss the acceptability of the examination by the couple and inform them of the available assistance options as well as the path for the woman / couple in case the test result was positive.

The examination involves a maternal blood sample and an ultrasound examination which do not require any particular preparation for their execution.

The blood sample taken from a vein in the woman's arm is sent to a testing laboratory to test for two pregnancy-specific substances, human chorionic beta-gonadotropin (β-HCG) and pregnancy-associated plasma protein A.

During the ultrasound, the operator measures the thickness of the space, between the skin and the spine, present behind the nape of the fetus (nuchal translucency) and visible in the ultrasound images as a clear because liquid space.

Once the examination is completed, the woman can immediately go back to carrying out her usual activities.

Results

The data that emerge from ultrasound and blood analysis are processed by a specific program that relates them to maternal age.

The result of the combined test is classified as high risk, or positive, if the estimated probability is equal to or greater than a certain threshold which can vary in different laboratories (generally it is between 1: 250 and 1: 385, value corresponding to the estimated risk for a 35-37 year old woman).

Only the reading of a healthcare professional allows the correct interpretation of the result of this examination.

The combined test correctly identifies about 85% of women who have a fetus with Down syndrome. About 5% of women receive a so-called result instead false positive, that is to say that it indicates the presence of Down syndrome when, on the other hand, the syndrome is not present.

It should always be considered that the result of the combined test only indicates the risk of a specific chromosomal syndrome. Therefore, a negative result does not guarantee that the fetus will not have it, just as a positive result does not guarantee that the fetus will be affected.

In the event of a positive test, the interpretation of the result is left to genetic counseling and to any subsequent investigations by means of a villocentesis or amniocentesis.

The test, in fact, does not replace the villocentesis or the "amniocentesis but allows to evaluate the risk even in young women and to limit the use of invasive techniques, reserving them only for selected cases, thus reducing the number of spontaneous abortions that complicate about 0. 5-1% of amniocentesis and CVS.

Bibliography

SNLG-ISS National Guidelines System. Physiological pregnancy guidelines (2011 update)

Mayo Clinic. First trimester screening (English)

NHS Choices. Screening for Down "s, Edwards" and Patau "s syndromes (English)

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