What influences participants' treatment preference and can it influence outcome? Results from a primary care-based randomised trial for shoulder pain.

backgroundIn randomised clinical trials (RCTs), outcome may be influenced by the opinions of the participants about the efficacy of treatments.

aimTo examine how initial treatment preferences of participants in a shoulder pain trial affected functional outcome and future treatment preferences. DESIGN OF STUDY: Observational cohort study nested within a multicentre, pragmatic RCT of steroid injection versus physiotherapy for unilateral shoulder pain.

settingNine general practices in north Staffordshire.

methodTwo hundred and seven adults were randomised in the trial. Disability scores and preferences of the participants for the trial treatments were elicited at two points: prior to randomisation and 6 months post-randomisation. A good functional outcome was defined as at least a halving in the disability score at the 6 months follow-up point.

resultsPre-randomisation preferences were: 40% for injection and 20% for physiotherapy, and 40% gave no preference. A good outcome was achieved in a higher percentage of participants who gave a pre-randomisation treatment preference compared with those who did not (62% compared with 48% percentage difference = 14%; 95% confidence interval [CI] = -1 to 27%) with similar percentages in each preferred treatment group. However, receiving the preferred treatment did not confer any additional benefit in those who expressed a preference (receiving preferred treatment = 56%; not receiving preferred treatment = 69%). At 6 months post-randomisation, participants with a good, as opposed to poor, outcome were more likely to report as their preferred treatment the one to which they had been randomised, irrespective of pre-randomisation preference and whether the preferred treatment was received.

conclusionThis analysis suggests that preferences prior to treatment can affect outcome, but that treatment outcome is a stronger influence on post-treatment preferences. We present some empirical evidence to support the statement that treatment preferences can have important effects on the results of RCTs.