Post-test positive predictive values for trisomies 21, 18, and 13 based on pre-test risk are presented in Table 1. PPVs for trisomy 21 risks ranged from 99% for pre-test risks greater than or equal to 1/10 to 11% for pre-test risks less than or equal to 1/9750. Post-test PPVs for trisomy 18 ranged from 99% for pretest risks greater than or equal to 1/10 to 6.0% for pretest risks less than or equal to 1/9500. Post-test PPVs for trisomy 13 ranged from 98% for pretest risks greater than or equal to 1/10 to 4% for pre-test risks equal to 1/10,000. For all trisomies and all risk strata, the NPV was greater than or equal to 99%.
Post-NIPT PPV is highly dependent on the patient’s pre-test trisomy risk and should play a central role in accurate and informed pre- and post-test counseling. NIPT’s very low PPV in patients with low a priori trisomy risks must be considered in any discussion of expanding current guidelines to offer NIPT as a first line screen in this population.