Global Prevalence of Onychomycosis in Adults with Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis

Background/Objectives: Onychomycosis is a common fungal nail infection that may contribute to foot morbidity in adults with type 2 diabetes mellitus (T2DM), particularly when neuropathy, peripheral vascular disease, nail dystrophy, local trauma, tinea pedis, or poor glycaemic control coexist. This systematic review and meta-analysis aimed to estimate the prevalence of onychomycosis in adults with T2DM and to explore sources of between-study heterogeneity. Methods: MED-LINE/PubMed, Scopus, Web of Science, LILACS, and EMBASE were searched from 1 January 2000 to 31 January 2026. Observational studies reporting onychomycosis prevalence in adults with T2DM were included. Pooled prevalence was estimated using a random-effects model, and subgroup, sensitivity, and exploratory meta-regression analyses were performed according to diagnostic method, sampling type, geographic region, publication year, and mean age. Results: Eighteen studies, including 6764 participants, were included. The random-effects pooled prevalence was 31.4% (95% CI: 21.7–42.0%), with extreme heterogeneity (I2 = 98.8%). Prevalence ranged from 3.8% to 71.7% across studies. Laboratory- or histology-confirmed studies showed a lower pooled prevalence than clinically diagnosed studies, suggesting that clinical diagnosis alone may overestimate onychomycosis prevalence. Exploratory meta-regression indicated that diagnostic method was the main methodological factor associated with prevalence differences, whereas publication year, sampling type, and mean age did not explain heterogeneity. Conclusions: Onychomycosis is frequent among adults with T2DM, but prevalence estimates vary substantially across studies. The pooled estimate should be interpreted cautiously because of extreme heterogeneity and differences in diagnostic definitions. Future studies should use standardised clinical and mycological or histological diagnostic criteria and report diabetes-related risk factors to improve comparability.