**Background:** Older adults with dementia face increasing challenges in oral health care due to cognitive decline and behavioral and psychological symptoms of dementia (BPSD). Alzheimer's disease (AD) is the most common dementia type, and BPSD manifestations vary by disease stage. However, little is known about how dementia severity specifically affects oral hygiene management in AD. This study aimed to clarify factors related to oral hygiene management across FAST stages in older adults with AD.
**Methods:** This was a secondary cross-sectional analysis of the Akita-Omorimachi Study (2015–2021), conducted in Omorimachi, Yokote City, Akita Prefecture, Japan. Participants were older adults aged ≥65 years with a confirmed AD diagnosis. Dementia severity was assessed using the Functional Assessment Staging of Alzheimer's Disease (FAST), with 16 levels grouped into 7 major stages. Oral hygiene management issues (refusal of care, independence, rinsing, gargling) were assessed via caregiver questionnaires. Oral health status (dental plaque, tongue coating, number of natural and functional teeth) was assessed by trained dentists. Multilevel logistic regression was used to examine associations between FAST stages and oral outcomes, adjusting for sex, age, BMI, and number of comorbidities. Of 2370 records from 962 individuals, 397 records from 216 individuals with AD were included after exclusions.
**Key Results:** The median age was 87 years; 88.7% were women. Overall prevalence of refusal of oral health care was 37.5%, dependence in oral care was 49.9%, rinsing disability 44.8%, and gargling disability 57.7%. Dental plaque accumulation was present in 43.1% and tongue coating in 47.6%. In multivariable-adjusted models, compared to combined FAST stage 1–3, the odds of refusal of oral health care increased progressively: FAST stage 4 (OR 4.44, 95% CI 0.36–54.66), stage 5 (OR 4.81, 0.50–45.97), stage 6 (OR 18.49, 2.12–161.69), and stage 7 (OR 30.52, 3.52–264.48). Dependence in performing oral health care showed a similar pattern: stage 6 (OR 5.57, 1.70–18.28) and stage 7 (OR 56.35, 16.73–189.84). Rinsing disability was significantly associated with stage 6 (OR 15.91, 2.05–123.58) and stage 7 (OR 62.46, 8.10–481.74). Gargling disability was significantly associated with stage 5 (OR 5.82, 1.00–33.88), stage 6 (OR 27.32, 5.01–149.01), and stage 7 (OR 27.32, 25.39–874.06). Dental plaque accumulation showed a nonlinear association: significantly higher at stage 4 (OR 8.50, 2.01–35.84) and stage 7 (OR 4.14, 1.43–12.01) compared to the reference. Tongue coating was not significantly associated with FAST stage.
**Clinical Implications:** This is the first study to systematically map oral hygiene management challenges across FAST stages in AD. The findings indicate that oral care strategies must be tailored to dementia severity. In mild dementia (FAST stage 4), caregivers may underestimate the need for assistance, yet plaque accumulation is high — suggesting a need for proactive partial assistance. In moderate dementia (stages 5–6), refusal of care and dependence increase sharply, requiring techniques to manage BPSD and maintain residual function (e.g., gargling exercises). In severe dementia (stage 7), nearly all patients are dependent and many refuse care, necessitating specialized approaches for oral hygiene delivery despite resistance. The study underscores the importance of preventing oral diseases (caries, periodontitis, aspiration pneumonia) through stage-appropriate care rather than relying on dental treatment, which is difficult in advanced dementia. Limitations include wide confidence intervals due to small sample sizes in early stages, lack of validated refusal-of-care measures, and absence of data on stroke severity and medications. Longitudinal studies are needed to track how oral care needs evolve as AD progresses.