A new blood test for pregnant women, taken around the end of the first trimester of pregnancy, has proven much more effective than existing standard noninvasive screening tests at identifying trisomy 21 which accounts for 90% of Down's Syndrome cases, according to a study published in the New England Journal of Medicine (one of the most prestigious medical journals in existence).
The 15,803 women were tested using the old standard screening and the new test.
There were 38 pregnancies in the sample with this genetic defect.
The old standard screening missed 8 of 38 pregnancies with this genetic defect and had 854 false positives. Only 3.4% of positives on the old standard screening test were correct, and 21.1% of true positives were missed.
The new screening test missed no pregnancies with genetic defects and had 9 false positives (a more than 98% improvement in the false positive rate). Of those 9 false positives, 1 out of 9 were also false positives on the old standard screening test. 80.9% of positives on the new screening test were correct.
Among 11,994 women at low risk, there were 19 true positives and 6 false positives with the new screening test. For low risk women, 76.0% of positives on the new screening test were correct.
Among 3,809 women at high risk, there were 19 true positives and 3 false positives with the new screening test. For high risk women, 86.4% of the positives on the new screening test were correct.
Something on the order of 50% to 90% of pregnancies that are confirmed to have trisomy 21 are terminated.
The low accuracy of the old test led to many unnecessary follow up invasive diagnostic screening tests, and led many women, especially those with low risk, to skip screening tests all together. The new test makes a diagnostic screening test a very good idea in every case where it is positive and is appropriate for screening all pregnancies, even low risk pregnancies.
The new test also identifies two other kinds of genetic anomalies in the fetuses of pregnant women better than previous non-invasive standard screening tests:
There were 10 cases of trisomy 18 in the primary analysis population. Of these cases, cfDNA testing identified 9 and standard screening identified 8; cfDNA testing had 1 false positive result, for a false positive rate of 0.01% (95% CI, 0 to 0.04) and a positive predictive value of 90.0% (95% CI, 55.5 to 99.7), as compared with 49 false positive results on standard screening, for a false positive rate of 0.31% (95% CI, 0.23 to 0.41) and a positive predictive value of 14.0% (95% CI, 6.3 to 25.8) (P less than 0.001 for both comparisons).
Among the 11,185 women who underwent both cfDNA testing and standard screening for trisomy 13, there were 2 confirmed cases; of these cases, cfDNA testing identified 2 and standard screening identified 1. There was 1 false positive result on cfDNA testing and 28 false positive results on standard screening, for false positive rates of 0.02% (95% CI, 0 to 0.06) and 0.25% (95% CI, 0.17 to 0.36), respectively (P less than 0.001).Trisomy 18 causes Edward's Syndrome (which affects girl infants 80% of the time). The prognosis in a case of Edward's Syndrome is as follows (per Wikipedia):
In 2008/2009, 495 diagnoses of Edwards syndrome (trisomy 18) were made in England and Wales, 92% of which were made prenatally, resulting in 339 abortions, 49 stillbirths/miscarriages/fetal deaths, 72 unknown outcomes, and 35 live births. Because about 3% of cases with unknown outcomes are likely to result in a live birth, the total number of live births is estimated to be 37 (2008/09 data are provisional). Major causes of death include apnea and heart abnormalities. It is impossible to predict an exact prognosis during pregnancy or the neonatal period. Half of the infants with this condition do not survive beyond the first week of life. The median lifespan is five to 15 days. About 8% of infants survive longer than 1 year. One percent of children live to age 10, typically in less severe cases of the mosaic Edwards syndrome.Trisomy 13 causes Patau syndrome. Per Wikipedia, the prognosis is as follows:
In England and Wales during 2008–09 there were 172 diagnoses of Patau's syndrome (trisomy 13), with 91% of diagnoses made prenatally. There were 111 elective abortions, 14 stillbirth/miscarriage/fetal deaths, 30 outcomes unknown, and 17 live births. Approximately 4% of Patau's syndrome with unknown outcomes are likely to result in a live birth, therefore the total number of live births is estimated to be 18. . . . More than 80% of children with Patau syndrome die within the first year of life.