Down's syndrome (trisomy 21) is one of the most common causes of serious congenital abnormality and severe mental handicap. In the absence of antenatal diagnosis about 1000 affected infants would be born annually in the UK representing 1 in 700 births.
Diagnosis is available antenatally by amniocentesis or chorionic villous biopsy. This is only generally acceptable provided that the group selected for the procedure is at high enough risk of Down's syndrome to warrant the cost and hazards (eg miscarriage). In the past selection has been largely on the basis of advanced maternal age. This has had limited impact since although birth prevalence increases with age, most affected infants are born to young women. For example in the UK, the oldest 5% of women include only about one third of affected pregnancies.
The discovery of multiple biochemical markers in maternal blood, led to a more effective screening process. The most popular screening test (‘triple test’) uses alphafetoprotein (AFP), unconjugated oestriol (uE3), and human chorionic gonadotrophin (hCG, or free beta-hCG) in conjunction with patient’s demographics and ultrasound data to estimate the individual risk of having a baby with Down’s syndrome.
The recommended screening test for Down’s syndrome is the first trimester combined. The combined screening is undertaken before 14 weeks + 1 day of pregnancy and uses ultrasound Nuchal Translucency (NT) measurement, plus serum biochemistry testing to measure free beta hCG and pregnancy-associated plasma protein A (PAPP-A). Second trimester test will always be required for those women who attend later in the pregnancy. The recommended second trimester screening test that will just meet the 2007 recommended outcome is the quadruple test that involves measuring Inhibin A in addition of the triple test. These recommendations are set out in the UK NSC Policy Recommendations 2007-2010: Model of best practice.
The antenatal laboratory in Leeds located in St James Hospital, Biochemical Genetics, block 46, currently offers the first trimester combined screening and the triple test for second trimester screening. From 1st of April 2011 the quadruple test will be offered to all Trusts in our region.
|Principal Clinical Scientist
||Mr Daniel Herrera