The Quad Screen is a second trimester screen that assesses the risk of a baby being born with Down syndrome, trisomy 18, and open neural tube defects (ONTDs). The risk for Smith-Lemli-Optiz syndrome (SLOS) is reported if the patient is high risk. This screen is performed between 15 weeks and 22 weeks, 6 days gestation.
In this test, the levels of four markers in the maternal serum are serum analytes:
- Alpha-fetoprotein (AFP)
- Total beta-human chorionic gonadotropin (total ß-hCG)
- Unconjugated estriol (uE3)
In pregnancies affected by Down syndrome, the levels of AFP and uE3 tend to be lower than normal while the levels of inhibin-A and total ß-hCG are elevated.
In pregnancies affected by trisomy 18, the levels of AFP, uE3, inhibin-A, and ß-hCG tend to be lower than normal.
In pregnancies affected by ONTDs, such as open spina bifida or anencephaly, the level of AFP tends to be elevated.
What Does a Negative Result Mean?
The result is considered screen negative if the risks for Down syndrome, trisomy 18 and ONTDs are lower than the cut-off used. Typically, a cut-off of one in 270 is used for Down syndrome, a cut-off of one in 100 is used for trisomy 18, and a cut-off of one in 104 is used for ONTDs. A patient with a screen negative result is not considered at high risk to have a baby with either Down syndrome or trisomy 18. However, the chance to have a pregnancy with Down syndrome or trisomy 18 is not completely eliminated.
What Does a Positive Result Mean?
A screen positive result for Down syndrome by the Quad Screen means that the patient has a risk to have a baby with Down syndrome of greater than or equal to one in 270. About one in 20 women (5%) who take this test receive a screen positive result. A screen positive result for trisomy 18 means that the patient’s risk of having a baby with trisomy 18 is greater than or equal to one in 100. A screen positive result for ONTDs means that the patient’s risk of having a baby with an ONTD is greater than or equal to one in 104.
A high-risk result for Smith-Lemli-Optiz syndrome (SLOS) will be reported if the patient has a risk of greater than or equal to one in 50. No risk assessment will be provided for this condition if the patient is low risk.
What is the Detection Rate for Down Syndrome, Trisomy 18, and ONTDs?
Among singleton pregnancies, the Quad Screen will detect about 83% of those affected with Down syndrome, 77% affected with trisomy 18, and 85% affected with ONTDs.
What Other Tests may be Offered if a Quad Screen is Positive?
Depending on the results of the Quad Screen, additional testing may be offered including a detailed anatomy ultrasound and prenatal diagnosis (amniocentesis). Genetic counseling is also recommended.