This well-written article in todays WSJ should be required reading for expectant parents considering prenatal testing to identify chromosomal abnormalities in their child. New testing that identifies placental (fetal) chromosomes in the maternal bloodstream is being advocated as a safe alternative to more invasive testing such as CVS or amniocentesis, with the additional advantage of being available as early as 10 weeks. But, false negative and false positive results can and do occur.
In one case published online by the American Journal of Obstetrics & Gynecology, a positive result from one of the new tests—together with preliminary results from another less-precise invasive measure—prompted a patient to terminate her pregnancy without undergoing a confirmatory amniocentesis. Testing of tissue from the aborted fetus showed the pregnancy was normal, the report said.
Though companies say patients should confirm positive results with invasive procedures, such case studies show “that message isn’t driven home enough,” said Athena Cherry, director of Stanford University Medical Center’s cytogenetics laboratory. In her lab, Dr. Cherry said she had found four out of six positive results for Trisomy 18, or Edwards syndrome, which the tests also detect, appeared to be false alarms after follow-up testing.
Bottom line – No test is perfect. Abnormal antenatal blood chromosome results should be confirmed with amniocentesis before acting on them.
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ACOG Committee Opinion and Press Release on Noninvasive Prenatal Screening with Cell Free DNA
Some mothers may accept the higher rate of false positives vs. waiting for amniocentesis, a more invasive test, followed by a second-term abortion. A 10-week abortion is less complicated than a 15-week abortion.
False negative is another issue entirely.