Question of the Month

Test your knowledge with our question of the month.

Question of the month: October 2019

Most NIPT technologies show very high sensitivities for trisomy 21 detection. Upon positive testing, pregnant women often immediately ask for an interruption. However, all professional guidelines urge to perform an invasive follow up test, preferably an amniocentesis, to confirm the presence of the trisomy in the fetus.

The reason to always perform an invasive test is that there is a reasonable chance that the fetus is normal. There is a biological and a technical reason for false positives: (1) The fetal faction of cfDNA is mainly derived from the placenta. Due to chromosomal mosaicism, the placental constitution is not always identical to the fetal constitution. (2) There can be some noise in the data which leads to a false positive result. So even despite the high sensitivity, the positive predictive value (PPV) – the probability that a positive is true positive- is still limited. This is especially true in a low risk population.