The issue of Positive Predictive Value "PPV" is being mentioned more and more with respect to endorsement of low risk NIPT. A recent Mendelspod Podcast referenced it during a discussion about the early cancer diagnostic challenges that will confront Grail. Here is an example to clarify:
Most genetic counselors agree that NIPT is a very good test that has very strong specificity and sensitivity . The Negative Predictive Values "NPV" are excellent in all age groups. Unfortunately, the positive predictive values go down dramatically as you move from the high risk to the low risk population. Consider the following examples using the National Society of Genetic Counselors PPV Calculator:
Mother age 38 NIPT results (all tests included)
- T21 sensitivity 99.2
- T21 specificity 99.9
- PPV calculation 88% probability that a positive test result is a true positive. 12% probability that a positive result is false. All positive test results therefore must be confirmed by an invasive procedure.
- NPV calculation > 99%. Excellent level that indicates a negative is a negative which effectively eliminates any reason for an invasive procedure. Approximately 97% of tests return with a negative result which eliminates most invasive procedures saving lives and money.
Mother age 25 NIPT results
- T21 sensitivity 99.2
- T21 specificity 99.9
- PPV calculation 51% probability that a positive test result is a true positive. 49% probability that it is a false positive. This is the BEST PPV of the three trisomies normally inluded in the screens.
- NPV calculation > 99%. Test remains very reliable for negative test results.
Conclusion
The significant degradation of PPV in the average risk population is deservedly creating increased anxiety. Some mothers ignore medical advise to confirm a positive test result, choosing to immediately abort. This doesn't appear to be enough to ignore the benefits of testing but certainly demands more education. If the NIPT negative results remain close to the 97% level seen in the high risk cohort it remains a very effective screen that eliminates most unnecessary invasive procedures.
Disclosure: I am/we are long ILMN,SQNM.