Dental Hygiene Therapy–Led Brief Intervention Fidelity and Periodontal Outcomes in South African Methamphetamine Treatment Centres: A Multilevel Process Evaluation

Process evaluations of task-shifted oral health interventions in substance-use treatment settings rarely separate provider-level fidelity from patient-level periodontal change, leaving unclear whether null or heterogeneous outcomes reflect ineffective intervention content or uneven delivery. The evaluation combined structured fidelity observation of hygiene-therapy sessions with repeated periodontal probing of patients, using a two-level model in which fidelity dimensions were specified as centre-level predictors of patient-level change in probing pocket depth and bleeding on probing. Fidelity was high for intervention content and duration but lower and more variable for motivational interviewing spirit and for the handling of methamphetamine-related xerostomia and bruxism. Between-centre variance in fidelity was associated with supervisory intensity and with the availability of a dedicated hygiene-therapy room. Patient-level periodontal improvement was concentrated in centres in the upper fidelity tertile, and the association between fidelity and outcome was stronger for bleeding on probing than for probing pocket depth. The findings indicate that the periodontal signal of a brief hygiene-therapy intervention is conditional on delivery quality, and that fidelity to relational components is the more fragile and more consequential dimension in this setting. Process evaluation of this kind is necessary before scale-up of hygiene-therapy-led oral health care in substance-use treatment is interpreted as evidence of intervention failure or success.