Community Dental Hygiene Therapist Supervision Ratio and Fluoride Varnish Application Fidelity in South African Primary Care Clinics: A Multilevel Variance Decomposition of Task-Shifting Saturation

Task-shifting of fluoride varnish application to community dental hygiene therapists is central to South African primary care oral health strategy, yet the supervision ratio at which therapist practice fidelity begins to degrade remains unspecified. The framework treats supervision ratio as a continuous clinic-level exposure and decomposes observed application fidelity into between-therapist, between-clinic and residual within-clinic components, permitting the identification of a saturation threshold beyond which additional task-shifting yields diminishing fidelity returns. Using a real DHS Program indicator on injection provision by health care providers across 14 African countries (2021-2025; N = 14; mean 91.964; SD 12.468; range 60.90 in Malawi to 99.70 in Kenya) as a comparative benchmark for task-shifting performance, the analysis illustrates how variance decomposition can inform supervision-ratio policy. The framework proposes that fidelity loss under high supervision ratios is concentrated at the clinic level rather than the therapist level, implying that supervision-ratio thresholds should be set with attention to clinic organisational capacity rather than therapist caseload alone. The contribution is a transferable decomposition method and a set of testable propositions linking supervision ratio to fidelity saturation in task-shifted oral health services.