The risk of Down syndrome derived solely from a Combined test (eg. 1 in 20) is misleading when the test was performed after NIPT. If the NIPT result was positive for Trisomy 21 the risk will be about 900-fold higher (47 in 48) and if negative about 140-fold lower (1 in 2700).

For Down syndrome screening, NIPT is much more effective than the Combined test. Performing the Combined test following a positive NIPT is not recommended because it is extremely unlikely to detect additional true-positives but will substantially increase the net false-positive rate (ACOG Practice Bulletin 163, Obstet Gynecol 2016. [Epub ahead of print]). In healthcare systems were patients have choices in the tests they receive they should be counseled that there is essentially no benefit (with respect to aneuploidy detection) in receiving NIPT followed by conventional screening.

In the event that both were provided as described, it is advisable to carefully review the individual components of the Combined test to consider which components most likely contributed to the positive result. For example, a large nuchal translucency is associated with a much broader set of fetal abnormalities and therefore some additional testing may be indicated based on this consideration.