Down syndrome screening
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Syed Hassan A. Kazmi BSc, MD [2]
Overview
Standard prenatal screens can discover Down syndrome. Genetic counseling along with genetic testing, such as amniocentesis, chorionic villus sampling (CVS), or percutaneous umbilical blood sampling (PUBS) are usually offered to families who may have an increased risk of having a child with Down syndrome, or where normal prenatal exams indicate possible problems. Genetic screens are often performed on pregnant women older than 30 or 35. During the first trimester of pregnancy, increased nuchal translucency in the fetus on ultrasound and decreased levels of pregnancy associated protein- A (PAPP-A) suggest the diagnosis of a chromosomal abnormality. Quad screen results during the second trimester of pregnancy may show increased beta-hCG, increased inhibin A, decreased alfa-fetoprotein (AFP) and decreased estriol.
Screening
Prenatal screening
- Pregnant women can be screened for various complications during pregnancy. Many standard prenatal screens can discover Down syndrome. Genetic counseling along with genetic testing, such as amniocentesis, chorionic villus sampling (CVS), or percutaneous umbilical blood sampling (PUBS) are usually offered to families who may have an increased chance of having a child with Down syndrome, or where normal prenatal exams indicate possible problems. Genetic screens are often performed on pregnant women older than 30 or 35.
- Amniocentesis and CVS are considered invasive procedures, in that they involve inserting instruments into the uterus, and therefore carry a small risk of causing fetal injury or miscarriage. There are several common non-invasive screens that can indicate a fetus with Down syndrome. These are normally performed in the late first trimester or early second trimester. Due to the nature of screens, each has a significant chance of a false positive, suggesting a fetus with Down syndrome when, in fact, the fetus does not have this genetic abnormality. Screen positives must be verified before a Down syndrome diagnosis is made. Common screening procedures for Down syndrome are given in Table 1.
| Screen | When performed (weeks gestation) | Detection rate | False positive rate | Description |
|---|---|---|---|---|
| Triple screen | 15–20 | 75% | 8.5% | This test measures the maternal serum alpha feto protein (a fetal liver protein), estriol (a pregnancy hormone), and human chorionic gonadotropin (hCG, a pregnancy hormone).[1] |
| Quad screen | 15–20 | 79% | 7.5% | This test measures the maternal serum alpha feto protein (a fetal liver protein), estriol (a pregnancy hormone), human chorionic gonadotropin (hCG, a pregnancy hormone), and high inhibin-Alpha (INHA).[1] |
| AFP/free beta screen | 13–22 | 80% | 2.8% | This test measures the alpha feto protein, produced by the fetus, and free beta hCG, produced by the placenta. |
| Nuchal translucency/free beta/PAPPA screen | 10–13.5 | 91%[2] | 5%[2] | Uses ultrasound to measure Nuchal Translucency in addition to the freeBeta hCG and PAPPA (pregnancy-associated plasma protein A). NIH has confirmed that this first trimester test is more accurate than second trimester screening methods.[3] |
Postnatal Screening
Persons with Down syndrome need to be closely screened for certain medical conditions. They should have:
- Eye exam every year during infancy
- Hearing tests every 6 - 12 months, depending on age
- Dental exams every 6 months
- X-rays of the upper or cervical spine between ages 3 - 5 years
- Pap smears and pelvic exams beginning during puberty or by age 21
- Thyroid testing every 12 months
References
- ↑ 1.0 1.1 For a current estimate of rates, see Benn, PA, J Ying, T Beazoglou, JFX Egan. "Estimates for the sensitivity and false-positive rates for second trimester serum screening for Down syndrome and trisomy 18 with adjustments for cross-identification and double-positive results". Prenatal Diagnosis. 21 (1): 46–51. PMID 11180240
- ↑ 2.0 2.1 Some practices report adding Nasal Bone measurements and increasing the detection rate to 95% with a 2% False Positive Rate.
- ↑ NIH FASTER study (NEJM 2005 (353):2001). See also J.L. Simplson's editorial (NEJM 2005 (353):19).