www.clinicalomics.com July/August 2017 Clinical OMICs 23
misunderstanding about the difference between a diagnostic
test and a screening test. A lot of providers thought that cell-
free DNA was just as accurate as a diagnostic test and they were
counselling patients that their fetus was affected. In reality, they
all need confirmation with diagnostic testing because false pos-
itives and false negatives have been recorded," she explained.
This widespread lack of understanding is concerning con-
sidering that a 2014 study showed that 6.2% of women who
received a positive cfDNA screening test had a pregnancy ter-
mination without confirming the results with a diagnostic test.
To combat misunderstanding and make it easier for patients
and providers to interpret cfDNA test results, Dr. Vora and col-
leagues have developed an online calculator that takes into
account a number of factors before calculating maternal risk
for Down, Patau, and Edwards syndromes. These factors include
maternal age and gestational age, as well as the type of test
used (Harmony, Panorama, Verifi or Materniti21). It then calcu-
lates age-related risk, sensitivity (extent to which true positives
are not missed), specificity (extent to which positives really rep-
resent condition of interest), and PPV.
In addition to PPV, other factors can also affect cfDNA test
accuracy. These include the presence of placental mosaicism,
where the placenta may have a genetic abnormality not seen in
the fetus; high maternal BMI; multiple births and the presence of
a disappearing co-twin. n
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Dr. Vora's Cell Free DNA Calculator can be found at: www.med.unc.
edu/obgyn/Patient_Care/specialty-services/maternal-fetal-medicine/
nips_calc.html.