Combined First Screen
The Combined First Screen is a first trimester screening test that uses maternal blood and ultrasound markers to assess the risk of Down syndrome and trisomy 18 in a pregnancy. The screen is performed between 11 weeks and 13 weeks, 6 days gestation.
Combined First Screen Options
Combined First Screen
- Ultrasound markers including the nuchal translucency (NT) measurement, nasal bone development, and gestational age (crown-rump length measurement).
- Serum analytes measured include pregnancy-associated plasma protein-A (PAPP-A) and total beta-human chorionic gonadotropin (total ß-hCG).
Combined First Screen Plus
- This screen is the same as the basic Combined First Screen except it also measures the serum inhibin-A level for inclusion in the screening algorithm.
In pregnancies with Down syndrome, the levels of PAPP-A tend to be lower than normal while levels of total B-hCG and inhibin-A tend to be elevated. In pregnancies with trisomy 18, the levels of PAPP-A and total ß-hCG tend to be lower than normal.
What Does a Negative Result Mean?
The result is considered screen negative if the risks for Down syndrome and trisomy 18 are lower than the cut-off used. Typically, a cut-off of one in 200 is used for Down syndrome, and one in 46 is used for trisomy 18. A patient with a screen negative result is not considered 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.
Because first trimester serum screening does not assess the risk of open neural tube defects (ONTDs), it is recommended that women who undergo the Combined First Screen should consider maternal serum alpha-fetoprotein (AFP) screening at about 16 weeks of pregnancy.
What Does a Positive Result Mean?
A screen positive result for Down syndrome by the Combined First Screen means that the patient has a risk to have a baby with Down syndrome of greater than or equal to one in 200. About one in 20 women (5%) who take the Combined First Screen receive a screen positive result. A screen positive result for trisomy 18 by the Combined First Screen means that the patient’s risk of having a baby with trisomy 18 is greater than or equal to one in 46.
What is the Detection Rate for Down Syndrome and Trisomy 18?
The Combined First Screen will detect about 85% of singleton pregnancies affected with Down syndrome, and 90% of singleton pregnancies affected with trisomy 18.
What Tests May be Offered if a Combined First Screen is Positive?
Depending on the results of the Combined First Screen, additional testing may be offered including a second trimester detailed anatomy ultrasound and prenatal diagnosis [chorionic villus sampling (CVS) or amniocentesis]. Genetic counseling is also recommended.
What are the Advantages of Having the Combined First Screen?
The Combined First Screen provides results earlier in pregnancy so that CVS is available as a diagnostic testing option.
What if a Pregnancy is More than 14 Weeks Along?
The patient may consider other testing options including the Quad Screen, ClariTest™ non-invasive prenatal screening (NIPS), ultrasound evaluation, and prenatal diagnosis.