Down syndrome screening (trisomy 21) normal reference range

Down syndrome screening (also known as the “triple test” or “quad screen”) for trisomy 21 assesses the fetus’s risk of having Down syndrome (trisomy 21), a chromosomal genetic disorder also referred to as “Down syndrome” or “congenital intellectual disability.” The standard cutoff value is 1:270, though reference ranges may vary slightly among hospitals.

Borderline high risk: A screening result falling between 1/380 and 1/1000 indicates borderline high risk.

A result indicating a trisomy 21 risk greater than 1/380 or a trisomy 18 risk greater than 1/334 is considered high risk. High-risk individuals require further diagnostic testing for confirmation. Down syndrome screening does not require fasting; venous blood is drawn from the pregnant woman. The optimal timing for this test is between weeks 15 and 20 of gestation. Screening results are typically available within 2–3 weeks after blood draw. Down syndrome screening estimates the likelihood that the fetus has Down syndrome but cannot definitively diagnose it. A hospital’s screening result reflects only the probability—not certainty—of Down syndrome. A high-risk result does not guarantee the fetus has Down syndrome, nor does a low-risk result guarantee the absence of the condition.