Essential Basics of Pregnancy Screening

Essential Basics of Pregnancy Screening



Generalities and concepts

 The most commonly used words in screening tests are:



Risk: In each screening, people are divided into three general groups based on the final answer: High Risk, Intermediate Risk, and Low Risk.



Sensitivity or Detection Rate: The percentage of people diagnosed with a screening test (ie, the patient's response is in the high-risk group).



Specificity: The percentage of healthy people diagnosed by a screening test (ie, the patient's response is in the low-risk or intermediate risk group).



False Positive Rate (FPR): The percentage of healthy cases identified in the above test as a high-risk group.



False Negative Rate (FNR): The percentage of cases that are identified in the above test as a low-risk group.



Positive Predictive Value (PPV): Percentage of cases diagnosed in the above test as a high-risk group and also confirmed by diagnostic tests.



Negative Predictive Value (NPV): The percentage of healthy cases that have been identified as low-risk in the above test and are actually healthy.



Positive Predictive Value (PPV): Percentage of cases diagnosed in the above test as a high-risk group and also confirmed by diagnostic tests.



Odds of being Affected given a Positive Results = OAPR: This is actually the same as PPV, but is expressed as a percentage instead of a percentage. They are diagnosed as a high-risk group and are also confirmed by diagnostic tests.



Median: A median is a number that divides a statistical population or a probabilistic distribution into two equal parts. In other words, the middle of the middle number is in a series of numbers, which divides the series of numbers into two equal parts.



MoM (Multiple of Median): In fact, the ratio of the concentration of each substance in the patient's blood to the median is the same in the normal population with the same fetal age. In other words, the MoM index shows us that this marker (or substance) in the mother's blood is several times the normal population.



The Corrected MoM: MoMs Index obtained from the patient's test results are raw MoMs and should be modified by considering parameters such as weight, smoking, diabetes, IVF pregnancy, twin pregnancy, etc. to correct corrected MoMs [1]. Achieved and then used in statistical calculations.



Another very important point about MoMs is that getting a MoM for a patient should be done using the medins of the same community, and if the medins of other communities are used, the correct risk for Down syndrome will not be calculated.



Clinical variables affecting the risk and level of markers



Maternal age: The probability of having a fetus with chromosomal abnormalities increases with increasing maternal age. On the other hand, the risk of intrauterine death in fetuses with chromosomal defects is higher, so the risk of having fetuses with chromosomal defects decreases with increasing gestational age (spontaneous death rate of fetuses with trisomy 21 between 12 and 40 weeks is about 30% and between 16 weeks And 40 is about 20%; in trisomy 13 and 18 this rate is about 80% between 12 and 40). Therefore, accurate calculation of maternal age is vital.



Fetal age: Accurate calculation of fetal age is very important because the level of markers changes with increasing fetal age. Ultrasound can reduce false positives by up to 2% by increasing the accuracy of fetal age estimation.



Maternal weight: There is an inverse relationship between maternal weight and serum marker levels due to the dilution effect of physiological increase in blood volume. Weight loss will be followed by fatigue and constant tiredness. If the patient's weight is high (blood volume also depends on the amount of weight and the weight increases, the blood volume actually increases), because the test elements are diluted in more plasma, their levels are falsely low. More than the actual value is obtained, and all MoMs must be corrected by a factor of more than 1. If the patient's weight is low, the relevant coefficient will be less than 1.



Racial origin [3]: In blacks, serum levels of alpha-fetoprotein are about 15% higher, hCG 18%, and PAPP-A 35% higher, and Inhibin A 8% lower than whites. In South Asian women, AFP levels are 6% lower, hCG 6%, PAPP-A 17%, and uE3 7% higher than white women. In general, the level of markers in the first trimester is higher in Asian women. Race-related reforms increase the ability to detect screening tests. NT changes are also seen in different races, however they are of little value to correct the risk.



Insulin-dependent diabetes (IDDM): In women with insulin-dependent diabetes, some second-trimester markers tend to decrease (for example, AFP decreases by about 10% and uE3 by about 5%). In diabetic women, Free βhCG levels are 5% lower and PAPP-A levels are 5% higher. NT does not differ significantly in diabetic women.

Pregnancy Using Pregnancy Techniques: If the egg is donated, the age of the egg donor should be used as the mother's age. In IVF, hCG levels increase in the second trimester and uE3 levels decrease. But at the AFP and Inhibin A levels, there is no significant difference in IVF and normal pregnancy. High levels of hCG are probably due to high doses of progesterone in these patients, and if not corrected, the false positives of this group will be twice as high as normal pregnancies. In the first trimester, lower levels of PAPP-A are seen, while NT and Free βhCG do not change much.

 

Previous history of trisomy: In a pregnant woman who has a history of having a fetus or child with trisomy, the risk of developing trisomy will be higher than expected for her age. Therefore, this factor has an effect on the initial risk and has its effects on risk determination.

 

Specific risk calculation: To calculate the specific risk of a pregnant woman for trisomy, the patient's initial risk [4] (according to maternal age at birth and previous history of trisomy) should be considered first, then this initial risk in the probability ratio [5] (Percentage of abnormal embryos to normal with this finding) Each marker is multiplied and finally a specific risk is obtained for each trisomy.] 8-5 [

 

 

[3]. ethnic origin

[4]. prior risk

[5]. likelihood ratio