Combined use of genetic sonography and maternal serum triple-marker screening: an effective method for increasing the detection of trisomy 21 in women younger than 35 years.
OBJECTIVE
The current standard of practice is to screen women younger than 35 years for trisomy 21 with maternal triple-marker screening, followed by amniocentesis in high-risk (1:10-1:190) patients. Non-high-risk patients are not offered further diagnostic testing. This study was conducted to determine whether genetic sonography of fetuses considered to be at moderate risk (1:190-1:1000) after maternal triple-marker screening increases the detection for trisomy 21, is cost-effective, and reduces the number of amniocenteses required to detect a single fetus with trisomy 21.


METHODS
After triple-marker screening, mathematical modeling was used to classify 500,000 theoretical fetuses as high, moderate, or low risk (>1:1001-1:10,000) for trisomy 21. The sensitivity for genetic sonography varied between 60% and 90%, and false-positive rates varied between 5% and 25%. Two programs (I and II) were compared with the control program. The control program included patients with high-risk fetuses (1:10-1:190) who had amniocentesis. Program I consisted of patients in the moderate-risk group (1:191-1:1000) who had genetic sonography followed by amniocentesis only when an abnormal sonographic finding was present. Program II used an approach in which genetic sonography was done for both high- and moderate-risk fetuses, and amniocentesis was performed only when an abnormal sonographic finding was present.


RESULTS
When added to the control program, genetic sonography significantly increased the detection rate of Down syndrome (range, 68.1%-77.8% versus 49%), reduced the cost of detection, and resulted in a ratio of fetuses with Down syndrome detected to normal fetuses lost because of amniocentesis of greater than 1 (range, 2.1-4.2). Compared with the control program, program II significantly increased the detection of fetuses with trisomy 21 (range, 56%-72%) when the sensitivity of genetic sonography was >70%, reduced the cost of detection, and had ratios of trisomy 21 detection to normal fetuses lost because of amniocentesis of between 2.38 and 17.88.


CONCLUSIONS
Women younger than 35 years and classified as having moderate risk after triple-marker screening could undergo genetic sonography under 1 of 2 approaches, either of which would result in an increased detection rate of trisomy 21 and be cost-effective without increasing the loss rate of normal fetuses after genetic amniocentesis.
DeVore
G R
GR
The Perinatology Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, USA.
Romero
R
R
D000328:Adult; D000367:Age Factors; D004314:Down Syndrome / Q000175:diagnosis*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D011247:Pregnancy / Q000097:blood*; D012307:Risk Factors; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal