Blood screening test for Down's syndrome no more effective than simply using age and scans

March 02, 2000

Six year survey of screening for Down's syndrome by maternal age and mid-trimester ultrasound scans

Antenatal screening for Down's syndrome using a blood test - serum screening, sometimes known as the "triple test" - is no more likely to detect the condition than using the mother's age and ultrasound scans, finds research in this week's BMJ. Furthermore, the method was introduced without any scientific evidence for its clinical effectiveness, say the authors.

Since 1992, serum screening has been widely accepted as the preferred method for antenatal detection of Down's syndrome, the risk of which rises with age, especially after 35. Despite its additional costs, the test was introduced on the assumption that it can detect around two thirds of all affected pregnancies compared with less than a third on the basis of age alone.

David Howe and colleagues from the Wessex Maternal and Fetal Medicine Unit at the Princess Anne Hospital, Southampton, assessed the antenatal detection rate of Down's syndrome among all women booked for delivery at the hospital over six years up to the end of 1998. Screening for Down's at the hospital is based primarily on older age, with amniocentesis offered to women 35 years and older. All women are also routinely offered an ultrasound scan at 19 weeks, followed by invasive testing, such as amniocentesis, if indicated by the scan.

Fifty seven cases of Down's syndrome were detected from among over 31,000 pregnancies - a rate of 1.7 in 1000 live births, and equivalent to national figures. Just over two thirds of all cases were detected antenatally. In women below the age of 35 this rate was 53 per cent, with most of the cases detected by scanning.

Although the antenatal detection rate was lower in younger women, say the authors, serum screening is also less effective in this age group. Among the 17 cases that were not detected antenatally, seven of the mothers had refused invasive testing and three cases occurred in twin pregnancies, where serum screening would not be effective.

None of the arguments advanced for the introduction of serum screening, including that it is cost effective, holds water, conclude the authors. Worryingly, they say, serum screening has not been submitted to the rigours of a comparative trial to produce clinical evidence of its effectiveness.
-end-
Contact:

Dr David Howe, Wessex Maternal and Fetal Medicine Unit, Princess Anne Hospital, Southampton. Email: dth@soton.ac.uk

BMJ

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