The quad screen is a second trimester blood test that measures the levels of four substances in your blood to tell you if your baby is at increased risk for Down syndrome and a few other conditions. If the results are "positive" you can have an amnio to tell you for sure about your baby's condition.
What is the quad screen?
The quad screen (sometimes referred to as the quadruple screen, AFP4, or multiple marker) is a blood test that can tell you whether your baby is at an increased risk for certain conditions. It's a screening test, which means it can tell you how likely it is that your baby has a condition, but you still need a diagnostic test (amniocentesis) to confirm the results.
A quad screen looks for:
• Down syndrome (trisomy 21, the most common chromosomal disorder)
• Trisomy 18 (Edwards syndrome), a more severe and less common chromosomal disorder
• Neural tube defects, such as spina bifida
• Abdominal wall defects
Certain results may also indicate that you're at a somewhat greater risk for complications such as preeclampsia, premature birth,intrauterine growth restriction, or miscarriage. Knowing this can enable you and your practitioner to be on the lookout for signs of trouble.
When is the quad screen done?
The quad screen is done in the second trimester, between 15 and 22 weeks of pregnancy (16 to 18 weeks is considered best).
Ideally, the test should be performed in conjunction with first-trimester screening tests. This combination is known as integrated or sequential screening. But if you miss the opportunity to start your screening in the first trimester, the quad screen alone can still give you some good information.
How is the quad screen done?
You'll have a blood sample taken and sent to a lab for analysis. Results are usually available in about a week.
The test measures the levels of four substances in your blood:
- Alpha-fetoprotein (AFP), a protein made by the baby
- Human chorionic gonadotropin (hCG), a hormone made by the placenta
- Unconjugated estriol (uE3), a hormone produced by the placenta and the baby
- Inhibin A, a hormone produced by the placenta
High AFP can mean several things. A baby produces AFP throughout gestation, and a certain amount of it should cross the placenta into the mother's bloodstream at each stage.
If there's more AFP than expected, it may mean that you're carrying more than one baby or that your baby is older than your practitioner thought. But in some cases, it's a sign of an abnormal opening in the baby's spine (spina bifida), head, or abdominal wall that's allowing AFP to leak out. In rare cases, it can also signify a problem with the baby's kidneys. And in some cases, it doesn't mean any of those things.
Low AFP, low estriol, high hCG, and high inhibin A are associated with a higher risk for Down syndrome. Low levels of the first three means your baby has a higher than normal risk of having trisomy 18 (Edwards syndrome).
How are the results computed?
To determine your baby's chances of having Down syndrome or trisomy 18, your levels of the four substances are plugged into a formula along with your baby's gestational age and your age.
Your age is factored in because although anyone can have a baby with a chromosomal abnormality, the risk increases as you age. For example, your likelihood of carrying a baby with Down syndrome ranges from about 1 in 1,200 at age 25 to about 1 in 100 at age 40.
To determine your baby's chances of having a neural tube defect, your level of AFP is plugged into a formula along with your age, your baby's gestational age, and other factors.
If you're having the quad screen as part of an integrated or sequential screening, the results will be combined with those from the tests you took in your first trimester for a more comprehensive risk assessment and a higher detection rate.
How do I interpret the results of the quad screen?
Talk with your practitioner or genetic counselor about how to interpret the results, which can be confusing. Many centers will report individual lab results as well as the final calculated result using a special formula.
You'll be given one risk assessment for Down syndrome, one for trisomy 18, and another for neural tube defects. Each one will be in the form of a ratio that expresses your baby's chances of having that problem.
For example, you may be told that your baby's chances of having Down syndrome are 1 in 100 or 1 in 1,200. A risk of 1 in 100 means that for every 100 women with this result, one baby will have Down syndrome and 99 will not. And a risk of 1 in 1,200 means that for every 1,200 women with this result, one baby will have Down syndrome and 1,199 will not. The higher the second number, the lower the risk.
Normal or abnormal results
You may also be told that your results are "normal" or "abnormal" for a particular condition, depending on whether the ratio is below or above a certain cutoff. For example, some genetic screening tests use a cutoff of 1 in 250. So, a result of 1 in 1,200 would be considered normal because the risk that there's a problem is lower than 1 in 250, and 1 in 100 would be considered abnormal because that risk is higher than 1 in 250.
Remember that a normal screening result (also known as "screen negative") isn't a guarantee that your baby has normal chromosomes, but it does suggest that a problem is unlikely. And an abnormal screening result ("screen positive") doesn't mean that your baby has a chromosomal problem – just that he's more likely than the average baby to have one. In fact, most screen-positive babies turn out not to have a problem. The same holds true for an abnormal AFP result and neural tube defects.
How accurate is the quad screen?
The quad screen detects at least 70 percent of babies with trisomy 18 and about 85 percent of those with neural tube defects. For Down syndrome, the quad screen detects about 81 percent of affected babies.
This doesn't mean that a screen-positive baby has an 81 percent chance of having Down syndrome. It just means that 81 percent of babies who have Down syndrome will have screening results that are suspicious enough to recommend additional testing.
False negative results
The quad screen doesn't pick up all cases of Down syndrome, trisomy 18, or neural tube defects, so it may identify your baby as being at low risk for these problems when she really is affected. (While the test will pick up 81 percent of Down syndrome cases, for example, that means there's a 19 percent chance the test will miss the Down syndrome in a baby that has it.) This is called a false negative result, a falsely reassuring result that might lead you to decide against diagnostic testing that would have revealed a problem.
False positive results
On the other hand, this test has a 5 percent false positive rate. That is, it may identify your baby as being at high risk for one of these three problems when she is fine. This may cause you to undergo further testing and to worry about your baby's well-being unnecessarily.
What if the results indicate that my baby is at high risk for a problem?
A genetic counselor or maternal-fetal medicine specialist can help you understand your results and your options. You may want to discuss the pros and cons of undergoing amniocentesis to find out whether your baby does have a problem.
But you'll most likely be offered a detailed (sometimes called level II or targeted) ultrasound first. This test can give you some more immediate information.
If you have high AFP, a level II ultrasound will enable the doctor to check your baby's spine and other parts for defects. It will also confirm your baby's age and whether you're carrying twins. (If the ultrasound shows that your baby is younger or older than your practitioner thought or that you're carrying more than one baby, your results will be recalculated.)
If your baby is found to have spina bifida or another birth defect and you decide to continue the pregnancy, your medical team will be able to monitor your baby's condition during your pregnancy and prepare to do surgery once he's born.
If your baby is screen-positive for Down syndrome or trisomy 18, the ultrasound can enable your doctor to check for several "markers" that may suggest Down syndrome and other chromosomal disorders.
These markers include:
• A bright spot in the muscles of the heart (echogenic intracardiac focus)
• A kidney problem (pyelectasis)
• A thickened nuchal fold (a clear area in the tissue at the back of the baby's neck)
• An abnormally short thighbone or arm bone
• A bright-appearing (hyperechogenic) bowel
If one or more of these markers are found, the likelihood of your baby having a genetic abnormality is increased. A baby with a major structural abnormality, such as a defect in the heart or abdominal wall, is also more likely to have a chromosomal defect.
If everything looks normal on the detailed ultrasound, it's still not a guarantee that everything is okay, but it does mean that your baby has a lower risk of chromosomal defects than your screening results indicate. This information may help you decide whether to go ahead with amniocentesis,