If you have been pregnant within the last few years, you were probably offered a noninvasive prenatal screening test in your first trimester. Billed as a highly accurate and safe alternative to invasive prenatal tests like amniocentesis, NIPT screens are actually non-diagnostic tests with a very poor positive predictive value (PPV). In simple terms, this means that a “positive” NIPT result is very often a false positive – in 50% of the cases or more, it turns out that the baby does not have the genetic syndrome for which the NIPT test was positive. But once the possibility of a diagnosis has been attached to a baby, parents are almost inevitably pressured to consent to the invasive diagnostic test that NIPT was supposed to replace, and/or will they will be pushed to abort based on an uncertain non-diagnostic test.
This handy positive predictive value calculator allows one to enter the mother’s age and condition for which the NIPT test was positive to determine the percentage of genuine positive results* for similar mothers. Try the PPV calculator for yourself – choose a Chromosome condition and set the mother’s age, then hit calculate (you can leave the Sensitivity and Specificity pre-filled percentages at the default numbers.) Then ask yourself why mothers are being offered a very expensive non-diagnostic screening test that is so often – and so tragically – wrong.
