Prenatal testing can determine genetic abnormalities at early stages of pregnancy. These tests allow the pregnant woman the ability to explore options. Noninvasive testing, usually involving a blood test and/or an ultrasound examination), poses no risk to the mother or fetus. Invasive tests, such as amniocentesis or chorionic villus sampling come with a small risk of harm—or death—to the fetus. A new study compared invasive to noninvasive tests in regard to the ability to detect genetic abnormalities. The findings were published in the November edition of the journal Obstetrics & Gynecology by researchers at the University of California, San Francisco School of Medicine, San Francisco, and the Genetic Disease Screening Program, California Department of Public Health, Richmond, California.
The study authors note that noninvasive prenatal testing using cell-free DNA has a sensitivity for Down syndrome of greater than 99% with a false-positive rate of 0.15%. Because some professional societies recommend that noninvasive prenatal testing be reserved for high-risk women, many programs use noninvasive prenatal testing as a secondary screen for women at high risk based on age or traditional screening results. Down syndrome (trisomy 21), trisomy 13 and trisomy 18 are genetic defects with an extra chromosome. However, trisomy 21 accounts for only 50% of trisomies, while trisomies 13, 18, and 21 together comprise two-thirds. Less common chromosomal abnormalities are also identified when invasive diagnostic testing is conducted.
The objective of the study was to estimate how many additional chromosomal abnormalities can be detected by invasive testing compared to noninvasive prenatal testing in a high-risk prenatal population. The investigators reviewed all genetic results of invasive prenatal testing in singleton (one fetus) pregnancies conducted in response to a positive prenatal screen through the California Prenatal Screening Program from April 2009 through December 2012. Genetic abnormalities were categorized in regard to whether the abnormality is detectable by current noninvasive prenatal testing methods.
The researchers found that of 1,324,607 women who had traditional screening during the study period, 68,990 (5.2%) were screen-positive. Among the screen-positive women, 26,059 (37.8%) underwent invasive diagnostic testing and 2,993 had an abnormal result (11.5%). Among these, 2,488 (83.1%) were predicted to be detectable with current noninvasive prenatal testing methods, and 506 (16.9%) were considered not currently detectable. Trisomy 21 accounted for 53.2% of the abnormal results (1,592 pregnancies). Common genetic abnormalities, detectable by noninvasive prenatal testing, comprised a higher percentage of abnormal results in older women..
The authors concluded that for pregnant women with a positive genetic screen who underwent invasive testing, 16.9% of chromosome abnormalities are not currently detectable by noninvasive prenatal testing. Undetectable abnormalities range from relatively mild to those associated with significant disability. They note that their study provides important information to be considered by patients, healthcare providers, and screening programs in evaluating the use of traditional screening and invasive prenatal diagnosis compared with noninvasive prenatal testing.