Three types of screening strategies are available: 1. Integrated screening combines results of irst-and secondtrimester tests. his includes a combined measurement of fetal NT and serum analyte levels at 11 to 14 weeks' gestation plus quadruple markers at approximately 15 to 21 weeks. n aneuploidy risk is then calculated from these seven parameters. As expected, integrated screening has the highest Down syndrome detection rate-94 to 96 percent, with a false-positive rate of 5 percent (see Table 14-4). If NT measurement is not available, then serum integrated screening includes all six serum markers to calculate risk. his screening, however, is less efective, and Down syndrome detection rates are 85 to 88 percent (Malone, 2005b).
2. Sequential screening involves performing first-trimester screening and informing the patient of the results. his is coupled with the understanding that if the calculated risk value lies above a speciied threshold, she will receive counseling and will be ofered diagnostic testing. here are two testing strategies in this category:
With stepwise sequential screening, women with firsttrimester screen results that confer risk for Down syndrome above a particular threshold are ofered invasive testing, and the remaining women receive secondtrimester screening. Using data from the First-and Second-Trimester Evaluation of Risk (FaSTER) trial, when the first-trimester threshold is set at approximately 1 :30, and the overall threshold is set at 1 :270, stepwise sequential screening resulted in a 92-percent detection rate of Down syndrome pregnancies, with a false-positive rate of 5 percent (see Table 14-4) (Cuckle, 2008).
With contingent sequential screening, women are divided into high-, moderate-, and low-risk groups. Those at highest risk for Down syndrome-for example, riski> 1 :30, are counseled and ofered invasive testing. Women at moderate risk, between 1 :30 and 1: 1500, undergo second-trimester screening, whereas those at lowest risk of < 1: 1500 receive negative screening test results and have no further testing (Cuckle, 2008). Using this strategy, more than 75 percent of those screened are provided with reassuring results almost immediately, while still maintaining a high detection rate of about 91 percent, with a 5-percent false-positive rate (see Table 14-4). his option is lso more cost efective because a second-trimester test is obviated in most patients.
In a population-based review of 450,000 pregnancies from the California Prenatal Screening Program, integrated screening detected 94 percent of trisomy 21 fetuses and 93 percent with trisomy 18 (Baer, 2015). Additionally, the screening result was abnormal in 93 percent of cases of trisomy 13, in 91 percent with triploidy, and in 80 percent with Turner syndrome. Women considering options of integrated screening and cellfree DNA screening may ind this information helpful.
his was introduced in 2011 and has completely changed the prenatal screening paradigm. he test works by identiying DNA fragments that are derived primarily from apoptotic trophoblasts, which are placental cells undergoing programmed cell death. hus, the term cell-free etal DNA is somewhat of a misnomer. Screening is not gestational-age dependent and can be performed at any time ater 9 to 10 weeks' gestation. Results are available in 7 to 10 days (American College of Obstetricians and Gynecologists, 2017 c). hree types of assays are currently available: wholegenome sequencing, which is also called massively parallel or shotgun sequencing; chromosome selective or targeted sequencing; and analysis of single nucleotide polymorphisms.