TRIPLE SCREENING IN PREGNANCY: HOW TO INTERPRET THE VALUES

After explaining what is and when the Triple Screening test is performed in pregnancy, we will now talk more in detail about how to interpret triple screening values.

Many pregnant women receive the test results that have been performed, and of course, they do not understand the data and values that are expressed, which causes great confusion.

We will try in this post to explain in a simple way each one of the parameters that you will find in the test results and clarify when they are normal values and when they can indicate a risk of chromosomal alterations in the fetus.

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We must clarify before something very important. Triple screening does not represent a diagnosis, but a statistical test based on biochemical markers present in the maternal blood that establishes a risk index.

It is not conclusive evidence, it only gives clues with a reliability of between 65 and 90 percent and can yield both false positives (that the result indicates an alteration when there is none) and false negatives (that there is an alteration and is not detected).

How to interpret triple screening: markers and units of measure

The risk assessment of chromosomopathy in the first trimester is obtained by combining three markers:

PAPP-A (protein produced by the fetus) and beta-HCG free (Human chorionic gonadotropin, produced by the placenta). Both are present in the mother's blood
Nuchal translucency (TN) of the fetus, measured by ultrasound.

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In the second quarter, the triple screening assessment is obtained by combining:

AFP (alphafetoprotein), produced by the fetus.
HCG free Y free estriol (estrogen produced by the fetus and placenta).
In some cases a quadruple test is performed, adding a fourth parameter: inhibin A, produced by the placenta.

Although each laboratory presents them in a different way, in the analytical results you will find the information of the three parameters analyzed in triple screening:

Demographic data: mother's age, weight, race, number of fetuses, if she is a smoker or has gestational diabetes, and the date of sample extraction.
Biochemical data: the values of the biochemical markers analyzed in the blood.
Ultrasound datas: where the date on which the ultrasound was performed, gestational age, measurement of nuchal translucency (TN) and fetal length (CRL)

Units of measure (MoM):

You will see that the values are expressed in MoM (multiples of the median). As the levels can be expressed in different units and vary according to the gestation time, in order to compare the results and make them independent of the gestation time the values are corrected and quantified in MoM.

Normal values are those between 0.5 and 2.5 MoM. The normal value of the different markers is 1 MoM, so that the more distant the 1 MoM is, the result of the marker is worse.

Abnormal triple screening values

Low values of PAPP-A and altered values of free Beta hCG or hCG may indicate a chromosomal alteration.
High values of AFP (alpha-fetoprotein) are suspected of fetal malformation (spina bifida, anencephaly, etc.) while in trisomies 21 and 18 it may have low values.
Low values of uE3 (unconjugated estriol) may indicate a possible alteration of chromosomes 21, 13 or 18.
High values of Inhibin A may be suspected of a trisomy 21 or 13.
The pathological values of nuchal translucency (no nuchal fold) usually range between 1.8 - 2 MoM or a measurement greater than 3 mm (regardless of MoM).

Reference values (MoM marker):

A marker is suspicious when the values, expressed in MoM, are:

TN (nuchal translucency) greater than 1.8-2
PAPP-A (pregnancy-associated plasma protein A) less than 0.4
Beta free hCG or hCG (human gonadotropin hormone) less than 0.4 or greater than 2.5
AFP (alphafetoprotein) less than 0.4 or greater than 2.5
uE3 less than 0.5
Inhibin A greater than 2.5

Triple screening cut points

The triple screening test determines a cut-off point for the risk of chromosomal abnormalities.

For Trisomy 21 (Down syndrome) the cut-off point is 1/270 (or 1: 270), although some laboratories place it at 250, that is, a value below 270 (for example, 1/125) is considered high risk, while a value greater than 270 (for example 1/1250) is considered low risk.

To calculate the risk of trisomy 21 the risk index by the maternal age is combined (according to the table you see below) with the biochemical risk index, which results in a combined risk index that is the one taken into account. As we said before, below the cut-off point (1/270) it is considered high risk and above it is considered low risk.

(Source: Hook, E.G., Lindsjo, A. Down syndrome in Live Births by Mother's Age in Years).

For Trisomy 18 (Edwards syndrome) The cut-off point is 1/100 (or 1: 100), that is, a value below 100 is considered high risk, while a value above 100 is considered low risk.

When it is recommended to continue with the studies

If the markers of this test are altered and a high risk of trisomy 21 or trisomy 18The doctor will propose to the parents to continue performing more tests, such as amniocentesis, corial biopsy or non-invasive prenatal tests.

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Finally, I leave this software for the calculation of prenatal risk of trisomies 21 and 18-13 in which you can enter the results of the laboratory analysis and, based on a clinical use system, calculate the risk index of each.

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