The Health Technology Assessment Programme is currently reviewing two new approaches to screening, namely, repeated measure and cross trimester testing to evaluate their potential to meet the more challenging standard. However, further improvements in performance proposed by the Committee to meet a detection rate of 90% at a false positive rate of 2% or less are unlikely to be reached by single stage testing, and protocols which include some element of sequential testing are required. The NHS Fetal Anomaly Screening Programme Committee has set a current performance target for Down’s syndrome screening of at least 75% detection at a 3% or lower false positive rate and this can be achieved by CUB or integrated testing by setting a threshold (cut-off) risk of 1 in 150 at term. Typical screening performance is around 70% detection of Down’s syndrome pregnancies at a 5% false positive rate for second trimester quadruple marker screening, 90% detection at a 5% false positive rate for CUB screening and 90% detection at a 1-2% false positive rate for integrated screening. Screening performance can be optimized by applying appropriate correction factors for variables such as maternal smoking, ethnicity and assisted conception. Some of the screening policies which have been implemented in the UK are second trimester double, triple or quadruple marker tests, first trimester combined ultrasound and biochemical (CUB) screening, and integrated screening.

Early detection of Down’s syndrome pregnancies through screening will provide the option of early termination of pregnancy and better obstetric care to women with affected pregnancies. Down’s syndrome is a classic chromosomal disorder with an incidence rate of one in every 750 live births.

Sequential testing in pregnancy