A screen-negative result for prenatal screening indicates that the chance of the fetus having Down syndrome is low. Typically no follow-up testing is required unless there is an additional suspicion of a chromosome condition. A screen-negative result does not guarantee the absence of birth defects. In fact, other screening tests may be recommended, including additional ultrasound and/or non-invasive prenatal testing (NIPT) using a cell-free fetal DNA method.
A screen-positive result indicates that there is a higher risk or chance of the fetus having Down syndrome and follow-up testing such as CVS or amniocentesis is necessary. It is important to note that a screen-positive result does not necessarily mean that the fetus has Down syndrome or other trisomy. Many women with screen-positive results have normal babies.
In interpreting the test results, it is important to take into consideration that an underestimation of the gestational age can lead to a false-positive result or falsely high calculated risk.
Race is an important consideration for correct interpretation of several prenatal screening tests and medians should be adjusted for tests. Both MS-AFP and beta HCG values are approximately 10 to 15% higher in Black women than in Caucasian women. DIA medians are approximately 8% lower in Black women than in Caucasian women. PAPP-A values are approximately 25% higher in Black women.
Differences in test values for other races and ethnic groups (eg, Hispanic, Asian, Native American) also exist, although the differences tend to be small enough to not affect the risk calculations. However, values should be determined based on population tested, ie, if a specific race/ethnic group is the dominant population for the facility.