How accurate is it?
A screening test shows the chance that a baby has a certain birth defect. The accuracy of a screening test is based on how often the test correctly finds a birth defect.
- The triple and quad tests correctly find neural tube defects, such as spina bifida, in 80 out of 100 fetuses who have it and find anencephaly in about 90 out of 100 fetuses.1 The tests miss spina bifida in 20 out of 100 fetuses who have it and miss anencephaly in 10 out of 100 fetuses.
- The triple test correctly finds Down syndrome in 69 out of 100 fetuses who have it. It misses the condition in 31 out of 100 fetuses.2
- The quad test correctly finds Down syndrome in 81 out of 100 fetuses who have it. It misses Down syndrome in 19 out of 100 fetuses.2
With the triple or quad test, there is a chance of getting a false-positive test result. This means that the test could show a problem when the baby doesn't have the problem. A false positive may be more likely with the triple screening than the quad screening.
A false-positive result can cause stress and lead to unnecessary testing (such as an amniocentesis). Many women who have a positive screening test result are actually carrying a healthy baby.
Sometimes negative test results can be wrong too. They may show that the baby is fine when he or she does have a birth defect. (This is a false-negative test result.)
Your doctor will use your age and your baby's age to interpret the test results. If your test results are abnormal, your doctor may use a fetal ultrasound to make sure of your baby's age.
What do the results mean?
A "positive" result means that there is a higher-than-average chance your baby has a birth defect. If the result is "negative," or normal, it means that your baby probably doesn't have a birth defect. But it doesn't guarantee that you will have a normal pregnancy or baby.
Your doctor may tell you the result of your test as a set of numbers. Doctors often use a certain number as a cutoff for a positive result. For example, your doctor may say the cutoff is 1 out of 200. This means that if your result is 1 out of 200 or 1 out of a number less than 200 (such as 1 out of 100), you have a positive result and your baby has a higher chance of a birth defect. If your result is 1 out of 300, this means that you have a negative result and your baby has a lower chance of a birth defect.