Adherence to adjuvant endocrine therapy in women with breast cancer is low. We conducted a 24-1 fractional factorial pilot optimization trial to test four intervention components supporting medication adherence [text messages, information leaflet, acceptance and commitment therapy (ACT), self-management website], in the preparation phase of the multiphase optimization strategy. Guided by the National Institute of Health Behavior Change Consortium fidelity framework, we investigated fidelity of design, training, delivery, receipt, and enactment of four intervention components. Women prescribed adjuvant endocrine therapy (n = 52) were randomized to one of eight experimental conditions comprised of combinations of the four intervention components (ISRCTN: 10487576). We assessed fidelity using self-report data (4 months post-randomization), trial data, ACT session observations, behavior change technique (BCT) coding, and interviews with participants (n = 20) and therapists (n = 6). Design: Each intervention component targeted unique behavior change techniques with some overlap. Training: All 10 therapists passed the competency assessment. Delivery: All leaflets (27/27) and website (26/26) details were sent, and ACT procedural fidelity was high (85.1%-94.3%). A median of 32.5/41 (range 11-41) text messages were delivered, but a system error prevented some messages being sent to 22 of 28 participants. Receipt: Most participants [63.0% (ACT, leaflet) to 71.4% (text messages)] read all or at least some of the intervention components they were randomized to receive. Enactment was reported most positively for ACT. All intervention components demonstrated adequate fidelity. We have provided an exemplar for assessing fidelity using the National Institute of Health Behavior Change Consortium framework in the preparation phase of multiphase optimization strategy.

Assessing multidimensional fidelity in a pilot optimization trial: A process evaluation of four intervention components supporting medication adherence in women with breast cancer