A 3rd Generation Test
The Prenatest prenatal screening protocol is designed to provide patient specific risk for Down syndrome (trisomy 21), trisomy 18 and other chromosomal abnormalities (ex. trisomy 13). The Prenatest method combines maternal blood protein measurements with the ultrasound measurements of the fluid accumulation behind the neck of the foetus (nuchal translucency) and the presence of the nasal bone. It is the earliest and most effective Down syndrome screen available.
What is the detection rate?
The blood analysis, combined with a nuchal translucency and nasal bone ultrasound measurements, yields a detection rate of 95% and a false positive rate of 2% when the test is carried out during the first trimester of the pregnancy (between the 11th and 14th week)1.
When carried out during the second trimester of the pregnancy (between the 14th and 22nd week), the test detects 75% of trisomy 21 cases.2 At this stage of the pregnancy, the test consists of a blood analysis combined ideally with a fetal ultrasound that will determine the age of the foetus and will improve the accuracy of results.
Who should be screened?
Women age 35 and over and those with a previous child with Down syndrome are more likely to have a baby affected with Down syndrome. However, most cases of Down syndrome occur in women under age 35. Therefore, screening provides useful information for all pregnant patients.
At what gestational age should the test be performed?
The test should be performed between 11 weeks-1 day and 13 weeks-6 days of pregnancy. The test may also be carried out during the second trimester of the pregnancy, between the 14th and 22nd week (ideally between the 16th and 18th week). At this stage of the pregnancy, the test consists of a blood analysis of alpha-fetoprotein (AFP) and free Beta-hCG combined with a fetal ultrasound that will determine the age of the foetus. This 2nd trimester screening test is also useful in detecting open neural tube defects.
How is the test performed?
The test begins with an ultrasound examination between 11w1d and 13w6d gestation (CRL between 45 and 84 mm). CRL is measured for accurate pregnancy dating and nuchal translucency (NT) is measured. NT has been demonstrated to be a specific marker for chromosomal aneuploidy. The nasal bone can also be measured, which increases the accuracy of the test. At the time of the ultrasound exam a blood sample is collected via fingerstick and deposited on filter paper. The ultrasound data is entered onto the test requisition form and sent to Warnex along with the dried blood sample.
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Warnex analyzes the blood sample for free Beta-hCG and Pregnancy Associated Plasma Protein-A (PAPP-A) in the 1st trimester. Patient specific risk for Down syndrome and trisomy 18/13 is then calculated based on the patient's age, the foetus' nuchal translucency measurement, the presence or absence of the nasal bone, and the free Beta-hCG and PAPP-A measurements.
What are the advantages of the Prenatest method?
- The Prenatest method offers 95% detection in the first trimester1
- Provides you and your patient with information much earlier in pregnancy than all other Down syndrome screening tests
- Patients shown to be at increased risk will have added time to consider if they want to have a diagnostic procedure done such as amniocentesis or CVS
- Patients 35 and over who are reluctant to have invasive tests can obtain information on their risk non-invasively with the Prenatest screening
1 Cicero, S., et al. (2003) Ultrasound in Obstetrics and Gynecology 22(1): 31-5.
2 Macri, J.N., et al (1996) American Journal of Obstetrics and Gynecology 174(2): 566-572.
