This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (email@example.com) with numerous contributions by others.
The results may be presented in a number of ways. It is important to
understand what your numbers mean. Ask your caregiver to explain.
The triple screen might be given as a probability such as: "based on
maternal age your risk of Down's is 1/390 --- when the levels of AFP, HCG
and uE3 are also taken into account, your risk is 1/14000. Based on AFP
levels, your risk of neural tube defects is 1/1400."
Or the results might simply be positive/negative, or normal/abnormal. You
will probably want to know exactly what that means. Ask. In fact, there is
controversy as to what "abnormal" means. Try to find out what your lab
considers abnormal. They are working from the more detailed probability
I think that it is becoming more common for the results to be given as
probabilities, which are more meaningful than a simple "positive/negative"
which is impossible to interpret. Probabilities can still be kind of scary
to interpret though.
Addition by Dr. Tim Reynolds:
Probabilities can be very misleading and are often 'over-interpreted by
obstetricians as a way of avoiding doing amniocenteses. The usual risk cut
off in the UK is 1 in 300: Using this cut off about 1 in every 50 amnios
will yeild an abnormality. Some obstetricians use 1 in 150 as a cut off.
This halves the number of amnios they do. The rationale for using this cut
off is that the risk of abortion due to amnio is about 1 in 150. Since in
the 1 in 300 group you expect 1 in 50 amnios to give an abnormal result it
is evident that their argument is specious.