Down syndrome screening (trisomy 21) normal reference range
Jan 18, 2021
Source: Cainiu Health
Dr. Liu Yuanyuan
Introduction
Down Syndrome Screening (Triple/Quadruple Test) for Trisomy 21—Normal Risk Cutoff: The standard cutoff risk value for trisomy 21 in Down syndrome screening is 1:270; however, reference values may vary slightly among different hospitals. It is important to note that Down syndrome screening provides only a risk assessment—not a definitive diagnostic result. This screening test is routinely recommended for pregnant women around 4 months’ gestation to evaluate the risk of fetal Down syndrome (trisomy 21), enabling early detection and timely intervention if necessary.
Down syndrome screening for trisomy 21 assesses the fetal risk of developing trisomy 21 syndrome—also known as Down syndrome or congenital intellectual disability—a chromosomal genetic disorder caused by an abnormality of chromosome 21.
Normal Reference Range for Down Syndrome Screening (Trisomy 21)
The standard cutoff for Down syndrome screening (trisomy 21) is 1:270; however, reference values may vary slightly among hospitals. It is important to note that Down syndrome screening yields only a risk estimate—not a definitive diagnosis. This screening test is routinely performed during the second trimester (around 16–20 weeks’ gestation, though some guidelines recommend 15–20 weeks) to assess whether the fetus is at increased risk for Down syndrome. Early detection enables timely counseling and further diagnostic evaluation if needed. Generally, a risk value greater than 1:273 is considered high-risk. The optimal timing for screening is between 15 and 20 weeks of gestation; the commonly cited “normal” risk threshold is 1:308. If the calculated risk exceeds this threshold, additional testing—such as non-invasive prenatal testing (NIPT) or amniocentesis—is recommended. However, a high-risk result does not confirm Down syndrome; it merely indicates an elevated probability. Therefore, high-risk pregnant women should undergo confirmatory diagnostic testing—either NIPT or amniocentesis—to establish a definitive diagnosis.
Additional Information: Key Considerations for Down Syndrome Screening
1. Down syndrome screening estimates fetal risk by measuring maternal serum levels of free beta-human chorionic gonadotropin (free β-hCG) and alpha-fetoprotein (AFP), combined with maternal factors including gestational age at blood draw, estimated date of delivery, and maternal weight.
2. Screening can be performed in two phases: the first trimester (weeks 9–13) and the second trimester (weeks 14–22). Results are typically available within one week after blood sampling. A high-risk result does not equate to a diagnosis; definitive diagnosis requires invasive testing such as amniocentesis or fetal chromosomal analysis.
3. For asymptomatic individuals, Down syndrome screening serves as a risk stratification tool—identifying those at higher likelihood of carrying a fetus with Down syndrome—thereby guiding subsequent diagnostic evaluations.
The above outlines the normal reference range and key considerations for Down syndrome screening (trisomy 21). We hope this information is helpful.
