Dental hygiene therapy–led fluoride varnish substitution under sodium-fluoride stockouts and caries arrest in Malawian health centres: an instrumental-variable analysis of consumable shortages

Sodium-fluoride stockouts disrupt caries-preventive care in Malawian health centres, yet the effect of substituting fluoride varnish for sodium-fluoride mouthrinse on caries arrest has not been quantified. The analysis uses the DHS Program indicator 'Current users most recent supply or information from a government health center' (FP_SRCM_W_GHC) across 36 African countries and 150 country-survey observations from 1990 to 2025 (mean 25.261, SD 18.895, range 1.50 in Morocco to 51.89 in Zambia) to characterise the supply environment in which substitution decisions are made. The framework specifies the exclusion restriction, the monotonicity condition and the first-stage and reduced-form equations, and derives the Wald estimand for the complier subpopulation. Because the DHS indicator measures government-source supply rather than fluoride-specific stockouts, the analysis is presented as a methodological proposal for a trial that would link facility stock records to caries-arrest outcomes; no causal estimate is reported. The contribution is a testable identification strategy that treats consumable shortages as a source of quasi-random variation in therapist behaviour, and a set of falsification tests that would distinguish substitution effects from confounding by facility quality.