BACKGROUND: Invitations to screening programmes may include influences that are intending to increase the participation rates. This study had two objectives: (i) to assess if different categories of influences had a significant effect on the intention to participate in a screening programme for a fictitious disease and (ii) whether participants were aware of the influences, and if the intention to participate was associated to this awareness.

METHODS: A seven-armed randomized controlled trial. Six hundred passers-by were randomly allocated to receive one of seven pamphlets inviting to a fictitious screening programme (neutral, relative risk reductions, misrepresentation of harms, pre-booked appointment, recommendation of participation, fear appeals, all combined). Participants were surveyed to assess (i) intention to participate (ITP) in the screening programme and (ii) awareness of an exerted influence. Chi-squared test was used to calculate the effect of the influences on ITP and the association of ITP with indicating awareness of an exerted influence and correctly locating an influence.

RESULTS: Five hundred and eighty-nine participants were included for analysis. ITP was significantly increased (P < 0.05) in three pamphlets (misrepresentation of harms, fear appeals, all combined) [adjusted odds ratio (OR) 4.84, 95% confidence interval (CI): 2.54-9.23; OR 2.45, 95% CI: 1.31-4.59; OR 9.02, 95% CI: 4.44-18.34]. A percentage of 60.0-78.3 participants did not indicate awareness. Awareness was associated with a decreased ITP for those who could locate the influence (OR 0.39, 95% CI: 0.21-0.72) and those who failed to locate the influence (OR 0.47, 95% CI: 0.30-0.74).

CONCLUSION: The application of influences should be carefully considered for interventions where an informed choice is desired.