Triple Marker Screen Test Health Article

Definition

The triple marker screen test (also called the maternal serum screening test or multiple marker test), is a blood test that is performed usually between the 14th and 18th week of pregnancy. This screening test measures the levels of three substances, alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol (uE3) in the maternal blood. Each level is then divided by the median concentration of that substance for the given week of pregnancy to generate a multiple of the median value (MOM). These values, along with other maternal characteristics, such as maternal age, are analyzed by a computer program to indicate the probability that the fetus has Down syndrome. Down syndrome is a condition that includes mental retardation, skeletal abnormalities such as upslanted eyes and cleft palate, and organ abnormalities such as heart disease and intestinal obstruction. Approximately 80-95% of cases are caused by a nondisjunction of chromosome 21 in the developing gamete resulting in the presence of an additional chromosome 21.

Purpose

Triple marker testing is a screening test that is used to identify the risk that a pregnant woman will give birth to an infant with Down syndrome. The test will also detect pregnancies at increased risk for Edward syndrome (trisomy 18) and Turner syndrome (monosomy X) and developmental defects associated with increased leakage of alpha fetoprotein from the fetus. The criterion used to define cutoff concentrations of the three markers is a risk for Down syndrome of one in 190. This is equal to the risk of miscarriage from amniocentesis. Women who screen "positive" (risk of 1:190 or higher) are recommended for amniocentesis. This procedure provides cells from the fetus that are cultured and analyzed to determine the number or chromosomes within each cell and detect structural chromosome abnormalities. This is the definitive method for diagnosing Down syndrome and other genetic conditions caused by an abnormal number of chromosomes (aneuploidy).

Precautions

It is very important that the correct gestational age be determined by last menstrual period dating and recorded for the risk calculation. Errors in determining the age of the fetus lead to errors when interpreting the test results. Since an AFP test is only a screening tool, an abnormal test result is not necessarily indicative of a birth defect. Accurate gestational dating lowers the false-positive and false-negative rates associated with this screening test.

The nurse or phlebotomist collecting the blood sample for these tests should observe universal precautions for the prevention of transmission of bloodborne pathogens.

Description

Prior to 1964, when the association between low levels of AFP and an increased risk for Down syndrome was reported risk assessment for chromosomal diseases was based upon maternal age. At age 35, the risk of carrying a Down syndrome pregnancy is approximately one in 270, and this was deemed sufficient to warrant amniocentesis. However, three of four Down syndrome pregnancies occur in women under 35 years old. When AFP testing was used along with maternal age, the rate of detection of Down syndrome increased to about 45%, but this level of sensitivity did not justify the screening of younger women because of the risk of miscarriage. The inclusion of uE3 and hCG testing has improved the detection rate to approximately 65-80% of cases for all age groups.

KEY TERMS

Acetylcholinesterase—A chemical found only inside neural tissue. Its presence in the amniotic fluid indicates an opening in the neural tube.

Amniotic fluid—Fluid within the uterine sac in which the fetus lives until birth.

Anencephaly—A severe and usually fatal brain abnormality caused by failure of the neural tube to close at its cranial end.

Embryo—The stage of human development prior to the second month of pregnancy.

Fetus—The stage in human development from the second month of pregnancy until birth.

Karyotyping—Chromosome analysis.

Neural tube—Tube that becomes the brain and spinal cord.

Oligohydramnios—Low amniotic fluid level.