Background STIs are a considerable problem, with diagnoses concentrated among young people. Although the Theory of Planned Behaviour (TPB) predicts many health behaviours, there is a lack of research into how well it explains STI testing. The original TPB emphasised the importance of intentions as the ultimate influence on behaviour. Subsequent research focuses on implementation intentions—people who make explicit plans for how to implement their intentions are more likely to do so. Although some studies have analysed barriers and facilitators in STI testing, no published research has examined what impact developing implementation plans has. This study was designed to address this knowledge gap.

Objectives To assess an extended TPB and implementation intention interventions on STI testing; to carry out a qualitative study of barriers and facilitators in those who intend to test.

Methods 371 participants completed an online baseline questionnaire and were randomly allocated to a control, information or implementation plan condition to examine differential effects on intentions to undergo STI testing. 172 completed a 6-week follow-up questionnaire to assess actual testing behaviour. In-depth interviews were conducted with a purposive selection of the sample and analysed using interpretative phenomenological analysis.

Results Multivariate analysis revealed that TPB variables explained 20% of the variance in past testing behaviour (p<0.01) and 17% of variance in intention (p<0.01). No significant difference between intervention arms was found (p=0.90) (the intervention did not significantly strengthen intentions). Qualitative analysis revealed multiple perceived barriers to STI testing.

Conclusion Results provide support for the utility of an extended TPB in predicting past behaviour and intentions to undergo STI testing. Although the implementation plan intervention was ineffective, qualitative data helped explain why a significant intervention effect was not found.