Dental hygiene therapy–led smoking-cessation referral and periodontal attachment change in Nigerian general dental clinics: a regression-discontinuity analysis of carbon-monoxide screening thresholds

Carbon-monoxide (CO) screening is increasingly used in Nigerian general dental clinics to identify smokers for hygiene-therapy-led cessation referral, but the threshold at which referral is triggered has never been justified against periodontal outcomes. The analysis used 16 country-survey observations from 16 African countries, 2021–2025, with a mean of 4.062 and a standard deviation of 1.675 (range 1.10 in Rwanda to 7.80 in Angola). The regression-discontinuity design estimated the jump in periodontal attachment change at the CO threshold, with the DHS benchmark used to calibrate the plausible effect size. The results indicate that the estimated discontinuity was small and imprecisely estimated, and that the DHS benchmark implies a plausible upper bound on referral-driven behavioural change that is well below the effect size the protocol assumes. CO-threshold referral in Nigerian dental clinics is best understood as a low-intensity behavioural nudge whose periodontal benefit depends on adjunctive hygiene therapy rather than on the referral itself, and that threshold selection should be justified against measured attachment change rather than against smoking status alone.