The Traditional screening method, maternal age, has been expanded. This brief summary will attempt to provide some clarification to the myriad of testing schemas available.
Screening in the first trimester primarily consists of a combination of an ultrasound measurement of the nuchal translucency (NT) and serum biochemical analysts HCG and Pregnancy Associated Plasma Protein-A (PAPPA-A). This should be performed at a gestational age of 10 3/7-13 6/7 weeks (36-84 mm CRL). Given a 5% false positive rate, the trisomy 21 detection rate is approximately 83% and 80% for trisomy 18. Although the first trimester screen can be performed without the ultrasound NT measurement, the detection rate falls to 70%. The first screen allows for invasive testing with a chorionic villus sampling but does not detect fetal anomalies such as ventral abdominal wall or neural tube defects.
The second trimester serum screening method includes maternal serum levels of hCG, aFP, uE3, and inhibin, the so call quadscreen. This test is performed between 15-20 6/7 weeks and given a 5% false positive rate, the detection rate for trisomy 21 is 81% and for trisomy 18 is 80%, virtually identical to the first screen. The added benefit to screening in the second trimester is the 80% detection rate for ventral abdominal wall and neural tube defects.
Screen has taken a confusing twist when the two methods for screening are combined in a two step process. This is called the integrated screen and is a combination of the NT measurement and PAPP-A in the first trimester and serum levels of hCG, aFP, UE3, and inhibin in the second trimester. Given a 5% false positive rate, the detection rate for trisomy 21 is 92% and trisomy 18, 80%. This has the greatest detection rate while maintaining the false positive rate at an acceptable 5%. However, remember that the results are given only after completion of the second step and this requires two separate clinic visits.
A new option being offered by some of the mainland labs include a sequential testing schema. The labs vary with respect to cut-off levels and what type of results are disclosed to the patient but the general premise is that a first screen is analyzed and if abnormal, disclosed to the patient. If the first screen falls into the intermediate or normal range the patient undergoes a second trimester screen. With the combination of these two components, the overall detection rate for trisomy 21 is 90% with an overall false positive rate of 5%. we are waiting for more information regarding this screening option before offering it to patients.
Advanced maternal age remains a valid indication for invasive testing. If first or second trimester screening is used in women of advanced maternal age, the detection rates (and false positive rate) are generally higher. Many patients find this individual risk assessment helpful before deciding whether to undergo an invasive diagnostic test. Secondly, although quadscreening is currently a covered benefit by most local insurers, it is unclear if first screening is a covered benefit.
Although confusing for patients and physicians, these new screening tests have ultimately allowed a greater detection rate while reducing the invasive diagnostic test. Secondly, although quadscreening is currently a covered benefit my most local insurers, it is unclear if first screening is a covered benefit.
Although confusing for patients and physicians, these new screening tests have ultimately allowed a greater detection rate while reducing the invasive testing and subsequent losses of normal pregnancies. If you have any questions please feel free to call Tammy Stumbaugh at our office.
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