Clinical trials: does your postcode matter?

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Research from Swansea University published in PLOS ONE suggests that clinical trials are drawing volunteers from a narrow demographic and this has the potential to damage the value of some clinical trials, and detract from their relevance to the wider population.

A team of medical, healthcare and statistics specialists, working with a local company analysed the recruitment of women into a randomised control trial of probiotics, called the PROBAT trial. Between 2005 and 2007, the trial recruited 454 mothers, but the fieldwork lead, Dr Sue Jordan, from the University’s College of Human and Health Sciences noticed a predominance of middle-class mothers in the research clinics even though a wider population had been targeted.

Supported by a team that included the University’s lead on clinical trials, Professor Ian Russell, Dr Jordan then set out to discover if this was the case.

Of the 454 originally recruited, 430 (94.7%) were followed-up at 6 months and 380 (83.7%) at 2 years. It was indeed the case that recruits were less deprived than the target population and, as the trial progressed, representation of the most deprived decreased.

The research suggests that the findings of trials that include conditions that vary across the socio-demographic spectrum could be affected if certain groups are excluded or are under- or over-represented.

Typically, the vulnerability of clinical trials to volunteer bias is under-reported. Volunteer bias is due to systematic differences between those who choose to participate in studies and those who do not.

Dr Jordan said: “Volunteer bias threatens the external validity, transferability, and utility of findings and detracts from their clinical value. When ‘hard to reach’ sections of the population are not included in a study, there can be no certainty that findings will be applicable to them. If a trial has been conducted in a population judged to be over-restricted, dissimilar or unrepresentative, there is a risk that findings may be dismissed as irrelevant. Prevention or vaccine trials are particularly vulnerable to such criticisms.

“Additionally and importantly, the incidence of disease in the recruited sample may be lower than accounted for in sample size calculations based on the incidence of disease in the whole population. This could leave the trial under-powered even when the target sample size has been recruited. “