Triple Screen

Definition

Triple screen is a blood test offered to pregnant women during the 16th-18th week of pregnancy to help identify if the unborn baby is at risk for birth defects.

Purpose

Researchers have found that women carrying a fetus with Down syndrome tend to have three substances in their blood in a distinctive abnormal pattern. Both alpha-fetoprotein (AFP), a protein produced by the fetus, and unconjugated estriol (uE3), a hormone manufactured by both the fetus and the placenta, are lower than normal, while human chorionic gonadotropin (hCG), a hormone produced in the placenta, is higher. The level of each substance 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 characteristics of the mother, such as her 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.

Guidelines developed from this research help healthcare workers to determine whether the risk for any one pregnancy is clearly different from that for the general population of the same age. For example, the average 25-year-old woman has one chance in a 1,000 of bearing a Down syndrome baby. After her blood is analyzed for the three hormones, her risk can be refigured. It might be considerably lower—or higher—than average.

The triple screen was first aimed toward detecting fetuses at high risk for Down syndrome, an abnormality that comes about when, at fertilization, the embryo receives three copies of chromosome 21 instead of the usual two. For unknown reasons, extra amounts of genetic material are usually associated with severe developmental defects. Although Down syndrome is the most common such disorder, it is by no means the only one. Chromosomes 13 or 18 can also turn up as a triple set (or trisomy). An abnormal pattern of the three hormones sometimes signals one of these trisomies or another, more rare chromosomal disorder.

Precautions

It is important that the correct age of the unborn baby 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 dating of the fetus lowers the false-positive and false-negative rates associated with this screening test.

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, a more accurate but invasive test that involves using a needle to draw out a sample of the embryonic fluid that surrounds the fetus. 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. The triple screen test costs $90-250 and is covered by most insurance plans.

Preparation

There is no specific physical preparation for this test. Fasting is not required.

Aftercare

After the blood sample is drawn, pressure should be applied to the puncture site until the bleeding stops to reduce bruising. A bandage or taped ball of cotton may be applied to the site. A warm pack may be applied to the site to relieve discomfort.

Risks

The complications associated with drawing blood are minimal, but may include bleeding from the puncture site, feeling faint or lightheaded after the blood is drawn, or blood accumulating under the puncture site (hematoma).

Normal results

Results are expressed as multiples of the median value used by the laboratory. Normal ranges expressed in concentration (for example, ng/mL) are dependent upon the age of the fetus, but MOMs are age-adjusted and do not change. These values are used to calculate risk. If the multiple of the median value is below 2.0 MOM or 2.5 MOM (depending on the laboratory), the fetus is considered to be at a lower risk for a genetic defect.

Abnormal results

If the multiple of the median value is above 2.0 MOM or 2.5 MOM (depending on the laboratory), the fetus is considered to be at a higher risk for a genetic defect. The MOM value for amniotic fluid is then used to calculate the exact probability the fetus is affected (1:100, for example). With respect to Down syndrome and trisomy 18, the MOM values are also used in the calculation of probability. The woman is considered to be "high risk" or "screen positive" for Down syndrome if the risk is greater than the standard risk for women who are 35 years old or older (one in 270). For trisomy 18, the cut-off is one in 150. In one study the triple marker screen test had a detection rate for Down syndrome of 67% and a false positive rate of 5%.

Resources

BOOKS

Health Education Board for Scotland. A Parent's Guide to the Screening Test for Spina Bifida and Down Syndrome: A Blood Test You Can Chose to Have During Your Pregnancy Edinburgh, United Kingdom: Health Education Board for Scotland, 2002.

PERIODICALS

Graves, J. Christopher, et al. "Maternal Serum Triple Analyte Screening in Pregnancy." American Family Physician (March 1, 2002): 915-921.

Pouliot, Janine S. "Triple Screen." Better Homes and Gardens (October 2003): 256.

ORGANIZATION

National Down Syndrome Society. 666 Broadway, New York, NY 10012. (212) 460-9330 or (800) 221-4602. info@ndss.org. http://www.ndss.org.

KEY TERMS

Amniocentesis— A test performed to determine the health, sex, or genetic constitution of a fetus by taking a sample of amniotic fluid through a needle inserted into the womb of the mother.

Amniotic fluid— The fluid that surrounds a fetus while it is developing. It is the fluid that flows out in the "breaking of the water" before a baby is born.

Down syndrome— A genetic disorder characterized by a broad skull, blunt facial features, short stature, and learning difficulties. It is caused by the presence of an extra copy of chromosome 21.

Placenta— An organ that develops inside the uterus of pregnant women to supply food and oxygen to the fetus through the umbilical cord.