To the Point– Prenatal cfDNA screening results can potentially be confusing, especially when they come back abnormal, so be sure to ask your doctor or genetic counselor for clarification on any questions that you may have. And, prenatal cfDNA screening results are not “yes or no”. Diagnostic testing, such as amniocentesis, is recommended to provide a more definitive answer about whether the baby does or does not have the condition. Additional things to keep in mind regarding prenatal cfDNA screening results include the following:
Prenatal cfDNA is a screening test so results are not “yes or no” answers. For most labs, you will get a separate result for each of the conditions that they are screening for. For example, you may get a “positive/high risk “ result for Down syndrome but normal results for Trisomy 13, Trisomy 18, etc. Each lab does vary in the wording of their results. Here are some examples:
1) Positive (increased chance for a chromosome condition) or negative (decreased chance for a chromosome condition)
2) Aneuploidy detected (aneuploidy= extra or missing chromosome material detected) or no aneuploidy detected (no aneuploidy detected= no extra or missing chromosome material detected)
3) High risk, intermediate risk, or low risk
It is important to remember that extra or missing chromosome material may be detected in the mother’s blood due to reasons other than a chromosome condition in the baby. These reasons include: differences between the placenta and baby’s genetic makeup or subtle variations in the DNA that causes an unusual reading of the test.
Again, if your result for one of the chromosome abnormalities returns positive/aneuploidy detected/high-risk, this does not mean the baby has the condition; it means there is an increased chance that the baby has the condition. For more detailed information it will be very helpful to talk to a genetic counselor or other knowledgeable healthcare provider.
The statistical term that answers this question for a population just like you (same age or same background risk) is called Positive Predictive Value. PPV is calculated based on knowing the sensitivity, specificity and background risk of the condition. The lower your background risk is for a certain condition, the lower the PPV will be.
Here’s an example: during pregnancy the chance that a 20 year old is carrying a baby with Down syndrome is about 1 in 1,000 whereas the chance for a 40 year old woman is about 1 in 100. The chance that a prenatal cfDNA screen returning positive for a 20 year old woman is a true positive (baby actually has Down syndrome) may be about 50% with some prenatal cfDNA screenings, whereas for the 40 year old woman, the chance her positive result is a true positive (baby actually has Down syndrome) is actually closer to 90%.
Talk to your provider about what the results of your prenatal cfDNA screening really mean.
Of note, some laboratories are quoting PPVs that are very high and there is not literature to back up their claims. See this blog for more information on this topic or talk with your provider, such as a genetic counselor.
Labs may or may not be reporting this statistic that answers this question, which is called the Negative Predictive Value (NPV). The NPV for prenatal cfDNA screening is typically very good. The likelihood that any of these conditions is present in a pregnancy is low and most test results will come back indicating a low-risk. However, false negatives do occur with prenatal cfDNA screening in some cases.