Objectives Assessing whether interventions are implemented as intended (fidelity) is critical to establishing efficacy in clinical research yet rarely applied in advance care planning (ACP) interventions. We aimed to develop and implement a fidelity audit tool for an ACP intervention.

Methods We developed a fidelity audit tool assessing: (A) content; (B) quality (general communication, eliciting EOL preferences and prognostic communication); and (C) family/caregiver involvement. We audited (double-coded) 55 audio-recordings of ACP discussions delivered to advanced cancer patients and caregivers, within a clinical trial.

Results Fidelity to content was high: mean=9.38/11 but lower for the quality of general communication (mean=12.47/20), discussion of patient preferences (mean=4.67/7), prognosis (mean=3.9/6) and family/caregiver involvement (mean=2.67/4). Older patient age and caregiver religiosity were associated with higher fidelity. Higher fidelity to content was associated with the trial primary outcome of family caregiver report of patient wishes being discussed and met.

Conclusions Fidelity to content, but not quality, of the ACP intervention is strong. Communication skills training is critical for ACP interventionists. Adherence was higher with older patients and religious carers, factors that may influence acceptance of death and readiness to undertake ACP, making the discussion easier.

Trial registration number ACTRN12613001288718.