Cell-free fetal DNA testing has changed the face of prenatal screening for aneuploidy. Initially, this technology was tested in “high-risk” subjects, and was found to have high sensitivity and specificity for Down syndrome. More recently, studies in “low-risk” women have been performed, and have confirmed high sensitivity and specificity. Given the rapid uptake of such testing by providers and the interest by patients, it is imperative that providers have the appropriate tools to accurately and effectively counsel patients.

In a Viewpoint in this edition of the American Journal of Obstetrics and Gynecology , Grace and colleagues describe the development of an online calculator for this purpose. This calculator uses a priori risk and the characteristics of a specific cell-free fetal DNA test (sensitivity and specificity) to calculate the positive predictive value (PPV). The PPV, and in this case perhaps 1-PPV, is what is most useful to counsel patients. It also highlights why cell-free fetal DNA testing is a screening test, and not a diagnostic test. An example using the calculator illustrates this latter point. Assume that you have a 24-year-old patient who has a cell-free fetal DNA test (sensitivity = 99.9%, specificity = 99.8%) done at 12 weeks’ gestation, and the result is screen positive for trisomy 21. Using the calculator, the PPV for this result is 38%, which means in fact that there is a 52% chance that the fetus is not affected by Down syndrome. If we change the maternal age to 40 years, and keep all of the other factors the same, the PPV increases to 90%. Even in this case of a 40-year-old with a screen-positive test for Down syndrome, the chance that the fetus is not affected by Down syndrome is still 10%. These 2 examples highlight exactly why cell-free fetal DNA is a screening test, and that positive results require confirmation with a diagnostic test such as chorionic villus sampling or amniocentesis.

I believe that this online calculator may be helpful in clinical practice, and will help with our counseling of patients with positive cell-free fetal DNA results. I encourage our readers to visit the calculator World Wide Web site ( www.mombaby.org/nips ).