New web-based calculator better interprets prenatal genetic test results

The calculator can estimate positive predictive value, or the likelihood that a positive result is a true positive, which is something most of the laboratories do not currently report.

New web-based calculator better interprets prenatal genetic test results click to enlarge Dr. Matt Grace

UNC OB-GYN's Division of Maternal-Fetal Medicine has unveiled a free, web-based calculator to help providers better counsel patients on the benefits and risks of the four most common screening tools for fetal genetic abnormalities available commercially in the U.S.

Using age- and gestational age-related risk tables for trisomies 21, 18 and 13 (commonly known as Down Syndrome, Edward Syndrome and Patau Syndrome) as well as the previously reported sensitivity and specificity of each cell-free fetal DNA testing platform, the calculator can estimate the positive predictive value (PPV) of each test, or the likelihood that a positive result is a true positive. Most of the laboratories do not currently report the PPV of their testing platforms.

The ease and accessibility of such a tool is especially important as the number of women interested in genetic testing for their pregnancies increases, said Dr. Matt Grace, a fellow in the Division of Maternal-Fetal Medicine and lead author of a paper on the topic that appears in the July 2015 issue of the American Journal of Obstetrics and Gynecology.

"When communicating the results of these tests, it's important for providers to help their patients interpret them. These tests are screening tests, and when a patient receives a positive result, we recommend follow up with a diagnostic test such as an amniocentesis to confirm or rule out a genetic abnormality," said Grace. "There are a number of reasons why a false positive might happen, and the evidence shows that some women are deciding to terminate their pregnancies without further confirmatory testing."

All four tests – known commercially as Harmony, Materniti 21, Panorama and Verifi – appear to perform better than previous genetic screening tests and have reduced the need for unnecessary and more invasive procedures for many women. The positive predictive value of the tests depend upon how high a patient's risk of having a baby with Down Syndrome is, which ultimately depends upon her age. The calculator offers a way for physicians and genetic counselors to better explain how the tests perform and to enhance their counseling of patients both before the testing and after.

"For instance, if our calculator shows that the positive predictive value of a test is 46 percent for a woman of a specific age and at a specific gestational age, there is still a 54 percent chance that the fetus does not have the abnormality," said Grace. "This is an opportunity for the provider to offer additional information and another level of care to the patient."

The calculator can be found at The UNC Center for Maternal and Infant Health website at mombaby.org/NIPS.

Also listed as authors on the paper are Emily Hardisty, Noah S. Green, Dr. Emily Davidson, Dr. Alison Stuebe and Dr. Neeta Vora.

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