DOWN SYNDROME ANTENATAL RISK TESTING

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Specimen: 5 mL blood in a plain tube collected ideally at 15-16 weeks gestation.

Methods: Immunoassay. The tests usually performed are a-fetoprotein, unconjugated oestriol, human chorionic gonadotrophin (b subunit). This test grouping is sometimes referred to as the “triple test” or Maternal Serum Screen. The test protocol is still in the process of evolution towards greater sensitivity, to allow earlier testing.

Reference Intervals: Instead of a reference interval for each analyte, results are usually expressed as multiples of the median (MoM). The medians vary with gestational age and must be derived from local populations. Other factors that may influence the calculation of the MoMs include: maternal diabetes, maternal weight and multiple gestations. Methods and calibrators used on reference populations and patients must be comparable. The combined results are assessed and a risk factor for Down syndrome, derived from the biochemical results and maternal age, is reported. The calculation of a numerical value for Down syndrome risk is a complicated empirical process, using a purpose-designed computer program; the service should be set up only in laboratories with their own carefully established median values and a high throughput of tests.

Application: Following counselling, the test is offered for the identification of pregnancies at high risk of Down syndrome and some other chromosomal abnormalities.

Interpretation: Accurate dating of gestational age is mandatory. Increased risk is associated with high bhCG, low a-fetoprotein and low unconjugated oestriol. Because maternal age is a significant factor in the risk estimate calculation, women in their mid-30s or older are more likely to receive an “increased risk” result. If a high risk pregnancy is identified, the results do not constitute an actual diagnosis of Down syndrome. Definitive diagnosis is made by cytogenetic analysis of amniotic fluid aspirate, CVS or fetal blood. Results indicating a low risk pregnancy do not exclude Down syndrome. See CYTOGENETICS (prenatal and fetal) .

Reference: Ryall RG et al. Prenat Diag 1992; 12: 251-261.

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