Paper

Ethnic differences in the outcome of serum screening for Down's syndrome

BMJ 1996; 312 doi: 10.1136/bmj.312.7023.94 (Published 13 January 1996)
Cite this as: BMJ 1996;312:94
  1. Lucy Gilbert, senior registrara,
  2. Jon Nicholl, directorb,
  3. Susie Alex, senior house officera,
  4. Ian Smethurst, senior chief medical laboratory scientific officerc,
  5. Anthony Mander, consultanta,
  6. Anthony Andrews, consultantd,
  7. Janet Patrick, consultanta
  1. a Department of Obstetrics and Gynaecology, Oldham NHS Trust, Oldham OL1 2PN
  2. b Medical Care Research Unit, University of Sheffield, Sheffield S10 2RX
  3. c Department of Clinical Pathology, Oldham NHS Trust
  4. d Regional Cytogenetics Department, St Mary's Hospital, Manchester M13 0JH
  1. Correspondence to: Mrs L Gilbert, Women's Hospital, Doncaster Royal Infirmary, Doncaster DN2 5LT.
  • Accepted 1 November 1995

Serum screening for Down's syndrome was introduced in Oldham health district in January 1991 to reduce the false positive rate associated with screening based on age alone and to improve the detection rate.1 A retrospective analysis was undertaken to evaluate whether these objectives were achieved throughout the entire obstetric population in Oldham.

Subjects, methods, and results

All women booked at the Royal Oldham Hospital and the community clinic before 18 weeks' gestation are counselled and offered serum screening by the booking midwife, who also records their ethnic origin. The gestation is confirmed by an ultrasound scan. (alpha) Fetoprotein and intact human chorionic gondatrophin values are measured by enzyme immunoassay. The risk for Down's syndrome is calculated from the two analyte results (adjusted for weight) and the maternal age.2 Women who have a positive result (estimated risk of 1 in 300 or higher of a fetus with Down's syndrome) are counselled by a senior obstetrician and offered an amniocentesis. A trained interpreter is used if necessary.

Between 1 February 1991 and 31 October 1993, of the 9217 women offered serum screening 1655 (18%) were classified as “Indian Asians”—that is, from the Indian subcontinent. Pakistani and Bangladeshi women formed the largest subgroups, 956 (10.3%) and 605 (6.5%) respectively, with Indians and East African Indians accounting for the rest. Other ethnic minorities were counted together with the white population because of their small numbers: West Indian 51 (0.5%), Chinese 22 (0.2%), African 6 (<0.1%), Arab 9 (<0.1%), Mediterranean 6 (<0.1%). The outcomes of the pregnancies in the study population were determined from the obstetric, paediatric, and regional cytogenetics databases; the last includes all antenatal and postnatal karyotyping carried out in the North West region. In addition, all children with Down's syndrome known to social workers were identified. Proportions within ethnic groups were compared by using χ2 tests and 95% confidence intervals3 calculated for proportions and for the relative risks of positive screening.

Among the white and other women, serum screening improved the detection rate for Down's syndrome from a maximum of 25% (3/12) for a 7.8% amniocentesis rate if selection for amniocentesis had been based on age alone to 42% (5/12) for a 3.5% amniocentesis rate (table). It also extended the opportunity for screening to women under 35, in whom 80% of Down's pregnancies in these women occurred. In women of Asian origin, however, there was a significantly higher false positive screening rate (12.3% v 3.4%), seen both in those aged 35 and over and in those aged under 35, with no evidence of a higher incidence of Down's syndrome or an improved detection rate. The failure to detect either of the two babies with Down's syndrome born to serum screened Asian women was not due to their significantly lower uptake of amniocentesis. Both babies were born to women who gave a negative result on screening.

Comment

The apparent failure of serum screening in the Asian population may be due to ethnic influences (cultural and religious beliefs, uncertainty about age) as well as racial differences. The age specific risk estimates and the medians of the biochemical markers used in the United Kingdom are derived from largely white populations and extrapolated to other populations on the assumption that “there is no reason to believe that they cannot be generally applied.”1 Yet racial differences have been found in the age specific rates for Down's syndrome4 and in the medians of the biochemical markers.5

View this table:

Outcome of antenatal Down's syndrome screening. Values are numbers (percentages; 95% confidence intervals) unless otherwise indicated

This study suffers from the disadvantage that it is retrospective and confined to one health district. Nevertheless, it highlights the need for outcome data on serum screening to be collected by ethnic group, so that problems experienced by specific groups can be recognised and addressed. To date none of the published studies on serum screening has looked at the outcome by ethnic group. Ethnic minorities form a significant proportion of the obstetric population of some health districts, and to justify serum screening in these groups it must be based on validated risk estimations and be shown to be effective.

We thank I Basnett (Camden and Islington Health Authority); D E Mutton (Wolfson Institute of Preventive Medicine, St Bartholomew's Hospital); C Tyler (Public Health Resource Centre, Oldham, Rochdale and West Pennine Health Authorities); and, from the Oldham NHS Trust, M Scholes and S Cavanagh (midwifery department), J McAvedy (special care baby unit), R Subramanium and B Plant (ultrasound department), and T Windle and S Monteith (general manager and patient services manager).

Footnotes

  • Funding No additional funding.

  • Conflict of interest None.

References

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