Serum screening for Down's syndrome

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Serum screening for Down's syndrome

• Screening for Down's syndrome can be undertaken from 10 weeks + 0 days to 20 weeks + 0 days of pregnancy. The recommended method of screening for Down's syndrome is using the Combined Test
  
  
• Screening for Down's syndrome should be performed by the end of the first trimester (14 weeks and 1 day), but provision should be made to allow later screening (which could be as late as 20 weeks 0 days) for women booking later in pregnancy
  
  
• Tests should meet a detection rate (DR) of more than 90%, for a screen positive rate (SPR) of less than 2% (of affected pregnancies) for England for those undergoing combined screening
  
  
• The following tests meet current standards:
  • from 10 weeks + 0 days to 14 weeks + 1 day the combined test (maternal serum - hCG and PAPP-A and nuchal translucency scan) is the recommended screening strategy.
    • the blood sample (or serum sample) can be taken between 10 weeks and 0 days and 14 weeks and 1 day's gestation. The blood sample uses a combination of free beta-hCG and placenta associated plasma protein A (PAPP-A).
    • the NT and CRL (crown rump length) measurement can be taken between 11 weeks and 2 days and 14 weeks and 1 days gestation
      
      
  • from 14 weeks + 2 days to 20 weeks + 0 days, the quadruple test (maternal serum - hCG, AFP, uE3, inhibin A)) for those presenting later is the recommended screening strategy
    • the quadruple test screens for T21 only.
      • the quadruple test is offered when the:
        • NT measurement cannot be obtained
        • CRL measurement is greater than 84.0mm and the HC measurement is between 101.0mm and 172.0mm
          • the quadruple test uses maternal age and the following 4 biochemical markers measured from 14+2 to 20+0 weeks:
            • alpha-fetoprotein (AFP)
            • human chorionic gonadotropin (hCG) or free bhCG
            • inhibin-A
            • unconjugated oestriol (uE3)
          • combination of markers has a lower detection rate (DR) and a higher screen positive rate (SPR) than the combined test
          • for a woman presenting in the second trimester, an ultrasound scan is required to measure the HC to date the pregnancy and complete the quadruple test

Chance cut-off

• chance cut-off determines whether a woman has a lower chance or higher chance result.
• the chance cut-off is 1 in 150 at term for both the combined and quadruple tests, as defined by the NHS Fetal Anomaly Screening Programme (FASP)
  • a lower chance result is less than or equal to 1 in 151.
  • a higher chance result is greater than or equal to 1 in 150 (between 1 in 2 and 1 in 150)
• a woman with a higher chance result is offered a prenatal diagnosis (PND) test, such as CVS or amniocentesis.
• the cut-off is based on a chance at term, rather than a chance at the time of the screening test
  • the chance at the time of screening is not known because there is significant pregnancy loss between the time of screening and birth
  • this loss rate is not known

For more information then see www.fetalanomalyscreening.nhs.uk.

Reference:

• 1. NICE. Antenatal care: routine care for the healthy pregnant woman. England: NHS National Institute for Health and Clinical Excellence; 2008 Mar. Report No.: Clinical guidelines CG62
• 2. NHS Fetal Anomaly Screening Programme. Antenatal screening - working standards for Down's syndrome screening 2007. NHS FASP; 2007.
• 3. NHS Fetal Anomaly Screening Programme. Consent standards for screening fetal anomalies during pregnancy 2007. NHS FASP; 2007.
• 4. NHS Fetal Anomaly Screening Programme. NHS fetal Anomaly screening programme - screening for Down's syndrome: UK NSC Policy recommendations 2007-2010: Model of Best Practice. Department of Health; 2008.
• 5. Kirwan D, NHS FASP. 18+0 to 20+6 weeks fetal anomaly scan -National standards and guidance for England 2010. Exeter, England: NHS Fetal Anomaly Screening Programme; 2010.