A complex behaviour change intervention delivered by dental nurses: Mixed-methods fidelity assessment of the RETURN intervention

Introduction Behaviour change interventions delivered in dental settings could be useful in reducing oral health inequalities. Pragmatic randomised controlled trials testing interventions, however, are vulnerable to problems with internal and external validity. Intervention fidelity strategies are helpful to address methodological problems and to provide scientific assurances about trial results. This paper sets out the intervention fidelity assessment of the RETURN intervention which was delivered in dental settings to promote planned dental visits. Methods The assessment was guided by the National Institutes of Health behaviour Change Consortium intervention fidelity framework domains of training and delivery. A mixed-methods design was selected, using quantitative data collected during intervention delivery training, together with qualitative observations (n = 58), interviews (n = 13) and audio-recordings of intervention delivery sessions (n = 472). Data were analysed separately using descriptive statistics and multiple logistic regression for the quantitative data and reflexive thematic analysis for the qualitative data. Data were integrated to provide a comprehensive fidelity assessment. Results Dental nurses were successfully trained to deliver the RETURN intervention; training was successfully standardised, and skills drift minimised. Training presented challenges, and not all nurses achieved competency sign-off to deliver the intervention independently. Nurse characteristics such as dental nursing experience, wider trial procedures, and confidence were all found to impact training success. The RETURN intervention was judged to have been delivered with high levels of fidelity, despite few interventions reaching the pre-determined fidelity threshold. Fidelity levels of between 75%-85% were achieved across intervention domains. Interventionist, intervention dose and intervention topic (dental visiting barrier) were all found to have a relationship with fidelity levels. Conclusions Dental nurses can be trained to deliver a brief behaviour change intervention alongside their usual clinical roles, and this can be delivered with a high degree of intervention fidelity. The results from this fidelity assessment provide assurances about the scientific validity of the RETURN trial results. Recommendations about the suitability of dental nurse interventionists within future dental trials are discussed. Trial registration ISRCTN: 84666712, registered: 12/04/2021