Combined risk assessment improves detection of Down's syndrome
Obstetrics and gynaecology
Obstetrics and gynaecology
The introduction of a combined risk assessment screening programme in Denmark significantly improved the detection of Down's syndrome as well as reducing the number of invasive diagnostic tests carried out.
An analysis of data from an audit of Down's syndrome screening compared universal ‘combined risk assessment' screening (a computation of maternal age, nuchal translucency scanning and two biochemical markers) with the estimated effects of screening using maternal age alone.
Nearly 65,000 women were pregnant in Denmark in 2006 of whom 54,843 underwent combined risk assessment for Down's syndrome. Using a 1:300 risk cut off, 1,899 were deemed ‘screen positive' and 1,704 (90%) had an invasive diagnostic test either chorionic villus biopsy (76%) or amniocentesis.
A total of 97 cases of Down's syndrome were identified in the ‘screen positive' group. Seven cases occurred in the ‘screen negative' group and 35 further cases occurred in the women who did not take up first trimester screening.
The chance of a Down's syndrome pregnancy in the screen positives was 1:20. The detection rate was 93% and the false-positive rate was 3.3%.
Had invasive testing been offered on grounds of maternal age the pick up figure would have dropped to 1:75.
Over 2005 and 2006 this translated into the detection of just 23 affected fetuses. Because invasive testing results in miscarriage in around 1%, the detection of these 23 cases would have been at the cost of losing 16 chromosomally normal fetuses. The calculated figures for universal screening in 2006 alone, which took into account the higher miscarriage rates in Down's syndrome pregnancies, were 193 cases detected with the loss of 31 normal fetuses. The combined screening programme has also resulted in a reduction of invasive procedures.
There is no doubt that if the birth of babies with Down's syndome is to be prevented using screening then biometric and biophysical markers are far superior to assessments based on maternal age alone.
Before the introduction of biometric and biochemical marker tests, screening for Down's syndrome was based solely on maternal age. We now have a plethora of tests each of which individually and in combination can be used to refine measurement of risk.
Combined risk assessment in the first trimester is not only a more effective screening method than maternal age alone, it also reduces the risk of miscarrying chromosomally normal fetuses.
• Ekelund CK, Jorgensen FS, Petersen OB et al. Impact of a new national screening policy for Down's syndrome in Denmark: a population based cohort study. BMJ 2008; 337: a2547
• Alfirevic Z. (editorial) Prenatal screening in Down's syndrome. BMJ 2009;338:b140
Dr Chris Barclay