Background Implementation fidelity evaluation is crucial in complex interventions, for understanding why they do or do not work, as highlighted by recent MRC guidance on process evaluations, yet such evaluations are often not reported. The PACE-UP trial used a pedometer and physical activity (PA) diary either by post or combined with 3 practice nurse consultations to increase walking in 45–74 year olds from seven South West London practices. We used the modified Conceptual Framework for Implementation Fidelity, assessing adherence (coverage, frequency, duration, content) and moderating factors (quality of delivery, participant responsiveness).

Methods We assessed recruitment (coverage). For the nurse group we assessed: frequency (sessions attended); duration (session length, by self-report and audio-recording); content (nurse checklists); quality; and participant responsiveness for different trial aspects. For both nurse and postal groups we assessed the number returning completed PA diaries (participant responsiveness).

Results Trial recruitment was 9% (1033/11015). 74% (258/351) of nurse group participants attended all 3 consultations and 4% (15/351) attended none. Planned consultation durations were 30 min (session 1) and 20 min (sessions 2 and 3). Mean durations for session 1 were 30 (range 20–60) minutes for self-report, 26 (range 12–57) minutes for audio-recorded; and for sessions 2 and 3, 23 (range 5–60) minutes for self-report, 17 (range 9–29) minutes for audio-recorded. Content adherence was high, mean completion at session 1 of 11/11 items and sessions 2 and 3 of 5/6 items. Quality of delivery: 89% (232/262) felt heard, understood and respected by the nurse. Participant responsiveness: 79% (208/262) reported the consultation number was just right; 76% (198/262) felt able to maintain PA changes; and across both groups 79% (549/692) returned completed PA diaries.

Discussion Coverage was low, indicating poor population penetration. However, other adherence aspects, quality of delivery and participant responsiveness demonstrate high implementation fidelity, suggesting strong internal validity for PACE-UP trial outcomes. We have demonstrated a useful model for evaluating implementation fidelity and will discuss this in the light of new MRC guidance on process evaluation in complex interventions.