The PPV reflects the rate of positive test results that are confirmed by chromosome analysis or after birth.
The risk of the presence of a fetal trisomy 21 during pregnancy is approximately 1:500 for the average population. Almost all of these cases are correctly detected by the Harmony test (detection rate 99.7%). However, if one in every 1680 Harmony test results for trisomy 21 is false positive, this corresponds to a total of approximately every fourth (1680/500) positive trisomy 21 result. The so-called “Positive Predictive Value” (PPV) for trisomy 21 is therefore approximately 80% for a middle-aged woman.
Since the risk of trisomy 21 depends to a large extent upon the age of the mother, this must be included in the calculation of the positive predictive value. For a 40-year-old woman with a risk of approximately 1:100 of giving birth to a child with trisomy 21, only one in every 17 (1680/100) positive Harmony test reports a false positive trisomy 21, thus the PPV of approxroximately 94% is correspondingly high.
The following Figure explains the calculation:
By comparison: the PPV of traditional first-trimester screening is only approximately 5 – 7%. This means that only 1 out of 18 abnormal results are confirmed by an invasive diagnosis; in 17 out of 18 cases, traditional first-trimester screening leads to an unnecessary invasive intervention with the risk of a miscarriage.