Two stage Perinatal screening

Two stage Perinatal screening

Introduction

 Due to the relatively high prevalence of Down syndrome, various methods have been developed to diagnose prenatal abnormalities; However, for various reasons (such as the risk of miscarriage and the cost of testing), it is not possible for all pregnant women to use these methods.

 For this reason, in recent years, a variety of screening methods have been developed to identify pregnant women whose fetuses are at high risk for Down syndrome. If the screening test is positive for this group of pregnant women, diagnostic tests are performed. Amniocentesis and CVS (obtaining embryonic cells and examining their chromosomes) are among these tests.

The difference between diagnostic and screening methods is that in a diagnostic test, a positive result means a definite existence of a specific disease or condition, but in a screening test, the purpose of the test is to assess the risk (probability) of the disease or a specific condition. So in screening tests, a positive result only shows an increase in risk. At the same time, screening methods, unlike diagnostic methods, are safe and low cost.

Down syndrome is directly related to the age of the pregnant woman, so the risk of giving birth to a baby with Down syndrome by the age of 30 is almost constant at about 1 in 1,000; But then it increases exponentially, reaching 1 in 250 at age 35 and 1 in 75 at age 40. However, 80% of Down syndrome cases occur in women under the age of 35 (because the number of pregnancies at this age is much higher).

Types of Down Syndrome Screening Tests

Down syndrome screening tests are divided into three main groups:

1) Tests performed in the first trimester of pregnancy (from the beginning of week 11 to the end of week 13).

2) Tests performed in the second trimester of pregnancy (from the beginning of week 14 to the end of week 22).

3) Integrated tests, some of which are performed in the first trimester (weeks 11 to 13) and the other part in the second trimester (weeks 14 to 22).

Integrated screening tests
These tests are more sensitive than any of the first and second trimester screening tests alone. Integrated screening tests are used to screen for Down syndrome, trisomy 18 (triplet chromosome number 18), and neural tube defects, and are divided into two groups:
Integrated or Integrated Protocols, which are divided into two different types:

1- Seum Integrated Protocol: which includes PAPP-A test in the first trimester and quad marker in the second trimester (85% diagnostic power).

2 - Full Integrated Protocol: which includes PAPP-A testing and NT sono in the first quarter and Quad Marker in the second quarter (detection power 92 to 93%).


Both of these protocols must be requested from the patient from the beginning, and the patient visits the laboratory in two stages. Until the end of the second stage of screening, no response is given to the patient. This means that even if we have a very abnormal sono in the full protocol, we have to wait until the end of the second stage so that we can release the answer and give it to the patient.

In each of the two stages of these tests (the first and second trimesters of pregnancy), a small amount of blood is taken from the pregnant woman and the level of biochemical blood markers is measured (these compounds are produced by the placenta and the fetus and enter the mother's bloodstream). When the fetus is at risk for Down syndrome, trisomy 18, or neural tube defects,

the amount of these biomarkers changes and becomes abnormal. The results of a particular fetal ultrasound called an NT ultrasound, which is done in the first trimester, are combined with the results of blood tests to get a more accurate result (some ultrasound screening protocols, such as serum integrated, do not require this ultrasound).

It is worth noting that in combination screening protocols, both stages of the test are performed for all mothers and the results are obtained only after the second stage of the test, in other words, the pregnant woman has to wait until the second stage of the test to get results.

Sequential screening tests
These tests are also done in two stages, but the preliminary answer that can be obtained in the first stage can be provided, however, the complete and final answer can be provided only after the second stage.
This protocol can be done in two ways:
1- Stepwise sequential step-by-step or step-by-step protocol
2- Contingent sequential protocol

1- Stepwise sequential protocol:

From the beginning, the patient is asked for a screening test in the first trimester, and based on the results of this step, by selecting an almost higher cut-off (eg 1:50 or 1: 100), the patient is a candidate for diagnostic tests (which is the recommended diagnostic test). CVS testing is performed for weeks by sampling of placental villi) and other patients (all patients who have a lower risk of this cut-off, ie the denominator of the fraction is greater than 1:50 or 1: 100). For the second stage screening test, they are referred (emphasizing that he should go to the same laboratory that performed the first stage to make changes to the test median differences). Laboratory dependent does not lead to an increase in laboratory error.

The detection power in this protocol reaches 95%.



2- Contingent sequential protocol

First, a first-trimester screening test is requested for the patient, and based on the response of this stage, patients are divided into three groups:

High-risk: By selecting an almost higher cut-off (for example, 1:50 or 1: 100), the patient is a candidate for diagnostic tests.

Intermediate Risk: Risks between 1:51 and 1: 1100 (different sources use different numbers) are referred to the same laboratory that performed the first step to perform the second stage screening test.

Low risk: The rest of the patients (all patients with a risk of less than 1: 1100) are referred only for AFP testing to rule out nerve rope and abdominal wall disorders at week 15 and an accurate scan anomaly at weeks 18 to 20.

For this protocol, the diagnostic power is reported to be between 88 and 94% according to various sources. This different diagnostic power depends on the extent of the boundary line or cut-off, which we define for the high-risk range and the intermediate risk.

In consecutive protocols, the first step can be performed from the beginning of the eleventh week (11W + 0D) to the end of the thirteenth week (13W + 6D).
The second step can be done between the beginning of week 15 (15W + 0D) and the beginning of the seventeenth week (17W + 0D).
From the combination of the results of the first and second stages, the final and complete result is obtained, which shows the risk of fetal Down syndrome, trisomy 18 and neural tube disorders.

 One of the advantages of consecutive tests over combined tests is that the initial results are obtained in the first stage of the experiment.

Unidentifiable cases
The risk of prenatal disorders in each pregnancy is 3 to 5%, but some of these disorders cannot be identified by screening methods. Examples include autism (and homosexuality or self-loathing), rare mental retardation, some genetic diseases, and some physical disorders.

Understand the concept of results
Low risk in screening tests: This means that the risk of fetal Down syndrome, trisomy 18, or neural tube defects is low, although a percentage of these disorders are not always detected by screening tests.
• Stepwise sequential and Full Integrated screening tests are the most sensitive protocols in this regard, with 95% and 92% of Down syndrome, 90% of trisomy 18 and 80% of tube disorders, respectively. They recognize the nerve.
• High risk in screening tests: means the risk of Down syndrome, trisomy 18, or neural tube defects, but high risk does not mean that the fetus has definitely been diagnosed with the disease, but that the diagnosis means diagnostic procedures. Such as amniocentesis, CVS, or multidimensional ultrasounds at the physician's discretion should be performed to obtain conclusive results.
• Screening test results are positive for about 3 to 5 percent of mothers with healthy fetuses (false positive).
• Most mothers who have a positive screening test have healthy fetuses.
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