The impact of treatment preferences: A narrative review.

Attending to treatment preferences is an element of person-centred care, reported as beneficial in improving treatment adherence, satisfaction, and outcome, in practice. The results of preference trials were inconsistent in supporting these benefits in intervention evaluation research. Informed by the conceptualisation of treatment preferences positing their indirect impact on outcomes, this narrative review aimed to summarise the evidence on the effects of preferences on enrolment; withdrawal or attrition; engagement, enactment, and satisfaction with treatment; and outcomes. The search yielded 72 studies (57 primary trials and 15 reviews). The results of vote counting indicated that (1) offering participants the opportunity to choose treatment enhances enrolment (reported in 87.5% of studies), and (2) providing treatments that match participants' preferences reduces attrition (48%); enhances engagement (67%), enactment (50%) and satisfaction with (43%) treatment; and improves outcomes (35%). The results are attributed to conceptual and methodological issues including less-than-optimal assessment of treatment preferences, which contributes to ill-identified preferences, accounting for withdrawal, low enactment, and limited satisfaction with treatment. These treatment processes, in turn, mediate the impact of treatment preferences on outcomes. It is important to refine and standardise the methods for assessing preferences and to examine their indirect impact (mediated by treatment processes) on outcomes in future preference trials to validly identify their benefits.