Fetal DNA Test of Mother's Blood Reliable in Sex Detection

Nancy A. Melville

August 10, 2011

August 10, 2011 — The evaluation of cell-free fetal DNA obtained from a simple blood test of pregnant women shows high reliability in accurately identifying the sex of a fetus as early as 7 weeks' gestation, with the strongest accuracy in identification at 20 weeks, according to a new National Institutes of Health (NIH) meta-analysis published today in the Journal of the American Medical Association.

The cell-free fetal DNA test is already widely used in routine clinical care in countries, such as France, Spain, and the Netherlands, and is regarded as an important noninvasive alternative to amniocentesis for determining sex and sex-related problems, such as ambiguous genitalia, X-linked conditions, and single-gene disorders, such as congenital adrenal hyperplasia.

Despite its growing popularity, the accuracy of cell-free fetal DNA had yet to be formally assessed; therefore, researchers led by Stephanie A. Devaney, PhD, from the Genetics and Public Policy Center, Johns Hopkins University, Washington, DC, and the Department of Health and Human Services, NIH, Bethesda, Maryland, performed a meta-analysis of 57 studies and 80 data sets involving 3524 male-bearing pregnancies and 3017 female-bearing pregnancies.

According to results of the meta-analysis, overall performance of the tests showed a sensitivity of 95.4% (confidence interval [CI], 94.7% – 96.1%) and specificity of 98.6% (95% CI, 98.1% – 99.0%). The diagnostic odds ratio was 885; positive predictive value, 98.8%; negative predictive value, 94.8%; and area under curve, 0.993 (95% CI, 0.989 – 0.995), with significant interstudy heterogeneity .

The DNA methods and gestational age showed the greatest effects on test performance, with real-time quantitative polymerase chain reaction (RTQ-PCR) (sensitivity, 96.0%; specificity, 99.0%) showing superiority over conventional PCR (sensitivity, 94.0%; specificity, 97.3%).

Testing after 20 weeks' gestation using RTQ-PCR on the blood sample was the most optimal (sensitivity, 99.0%; specificity, 99.6%) and outperformed testing before 7 weeks (sensitivity, 74.5%; specificity, 99.1%), testing at 7 through 12 weeks (sensitivity, 94.8%; specificity, 98.9%), and testing at 13 through 20 weeks (sensitivity, 95.5%; specificity, 99.1%).

Testing performed before 7 weeks' gestation using blood and all tests using urine were found to be unreliable.

"The improved performance with later gestation is likely attributable to the increased concentration of cell-free fetal DNA within maternal blood as the fetus and placenta develop," the study authors wrote.

"This would explain the poor performance of the test prior to 7 weeks' gestation and the near-perfect performance in the third trimester."

Researchers first reported in 1997 the finding that cell-free circulating Y chromosome DNA sequences were present in the plasma of pregnant women, and since then many groups have validated that the sequences could be amplified and used to identify male fetuses, according to the study authors.

As the testing method has gained in popularity in Europe, the research has taken the shape of various methods, sex-specific markers, and sample types for all gestational ages.

Need for Amniocentesis Persists

Although the accuracy of the test is impressive, when it comes to this type of genetic testing, anything less than 100% can be problematic, explained Virginia R. Lupo, MD, chair of the Department of Obstetrics and Gynecology at Hennepin County Medical Center, in Minneapolis, Minnesota, when asked for independent comment by Medscape Medical News.

"Overall, the statistics are great, but if 95.4% of cases are picked up, that means 4.6% of cases of Down syndrome, for instance, aren't picked up, and for many people, that isn't good enough," she said.

The current criterion standard, amniocentesis, on the other hand, offers close to 100% accuracy and represents "the ultimate diagnostic test" for this purpose Dr. Lupo added. But, the fact that amniocentesis is invasive and is generally considered to run the risk of causing miscarriage in 1 of 200 pregnancies makes the idea of a simple blood test highly appealing.

"All of us who do amniocentesis respect the fact that putting a needle inside the uterus is pretty darn invasive, and we would be happy to have a test, such as a blood test, which has no risk at all," she said.

" I think it is incredibly interesting that this stuff is detectable so frequently, and it [i]s just a matter of the technology pushing through so we can get even more accurate testing on the DNA that is there and clean up the false positives and false negatives and wind up doing the amniocentesis much less frequency."

Dr. Lupo noted that a promising analogy is the shift away from the use of pneumoencephalograms.

"Thirty years ago, if you were worried that someone had [a] brain problem, you would do a spinal tap, take out the spinal fluid, put in air and put patients in a chair that moved to different positions, and do X-rays to see air fluid levels in the ventricles," she explained.

"People would get horrible headaches, and it was incredibly crude. These days, you do a painless MRI [magnetic resonance imaging] in 15 minutes and get incredible details," she said.

"There's no one who misses pneumoencephalograms, and similarly we're not going to miss amniocentesis. It will be pretty invasively archaic and something you [w]on't have to do anymore."

The study was supported by the National Human Genome Research Institute. One author reported having a sponsored research agreement with Sequenom Inc to study circulating cell-free fetal DNA. Another author served as a member of the scientific and clinical advisory boards of Verinata Health Inc, an early-stage biotechnology company, receiving honoraria from and holding stock options in this company. The remaining authors, including Dr. Devaney, as well as Dr. Lupo have disclosed no relevant financial relationships.

JAMA. 2011;306:627-636.

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