Alpha-test or triple-test or prenatal 2nd-trimester screening test is performed on pregnant women in the 2nd trimester of pregnancy (between 15th and 19th weeks) to identify possible malformations or serious chromosomal abnormalities. These screening tests may indicate the possibility of fetal abnormalities such as trisomy 21 (Down syndrome) and trisomy 18. They may also indicate the possibility of neural tube abnormalities (myelomeningocele and spina bifida).
The incidence of these abnormalities is directly related to the age of the mother. These are screening tests and are not diagnostic tests. If the results of the screening test are positive, then more direct and definitive examinations can be done, such as chorionic villus sampling (CVS) at the beginning of pregnancy or amniocentesis in mid-pregnancy. Most pregnant women over 35 years of age usually receive chorionic villus sampling or amniocentesis without prior prenatal screening.
Three markers are measured in the triple test: beta chorionic gonadotropin (β-hCG), alpha-fetoprotein (αFP), and unconjugated (free) estriol. ΑFP is produced in the yolk sac and embryonic liver. Unconjugated estriol and β-hCG are produced by the placenta.
Screening the mother with the triple test offers a 50% to 80% probability of detecting trisomy 21 compared to measuring only αFP that has a 30% probability. What is now recommended is the quadruple test which is usually combined with fetal nuchal translucency (FNT). These tests are done more accurately during the second trimester of pregnancy and more specifically between the 14th and 22nd weeks (ideally between the 16th and 18th week) of pregnancy. The use of ultrasound to determine the exact age of pregnancy improves the sensitivity and specificity of all maternal prenatal screening tests.
At trisomy 21, maternal second-trimester serum levels of αFP and unconjugated (free) estriol are approximately 25% lower than normal, and serum hCG levels are approximately two times higher than normal. Test results are expressed in multiples of the mean (MoM). The values of αFP and estriol (E3) during pregnancy with trisomy 21 are lower than those with normal pregnancy, which means that values below the mean are below 1 MoM. The value of hCG for trisomy 21 is above 1 MoM.
MoM values, fetal age, and maternal weight are used to calculate the potential risk for fetal chromosomal abnormalities.
Important Note
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. The correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings, and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
At Diagnostiki Athinon we answer any questions you may have about the test you perform in our laboratory and we contact your doctor to get the best possible medical care.
