Determinants of polydoctoring among patients with multimorbidity: a Nationwide Longitudinal Fixed-Effects Study in Japan.

Abstract

Objective: To identify which chronic conditions and patient characteristics contribute to increases in the number of regularly visited facilities (RVF), a measure of polydoctoring, among adults with multimorbidity in Japan.

Design: Retrospective cohort study using individual fixed-effects Poisson regression models to estimate within-person associations between the onset of each chronic condition and changes in RVF. This approach controlled for all time-invariant personal characteristics. Effect modification by age, geographic region and baseline multimorbidity was also assessed.

Setting: A nationwide health claims database covering multiple insurance systems in Japan, including municipal National Health Insurance, Employees' Health Insurance and the Long-Life Medical Care System.

Participants: 4 696 790 adults with two or more chronic conditions and two or more consecutive years of follow-up.

Results: The onset of examined chronic conditions was associated with changes in RVF, with substantial variation in effect size across conditions. The strongest association was observed for malignancy (RR=1.102; 95% CI 1.101 to 1.103), followed by osteoporosis (RR=1.065; 95% CI 1.064 to 1.065), arthritis (RR=1.061; 95% CI 1.060 to 1.061), obesity (RR=1.049; 95% CI 1.044 to 1.055), stroke (RR=1.044; 95% CI 1.041 to 1.048) and cardiovascular disease (RR=1.043; 95% CI 1.042 to 1.044). Associations between disease onset and RVF were consistently attenuated among individuals aged ≥75 years and those with higher baseline multimorbidity while differences between urban and rural residents were minimal.

Conclusion: Substantial heterogeneity was observed in the magnitude of RVF increases across chronic conditions, with particularly strong associations for malignancy, osteoporosis and arthritis. Attenuated effects among older adults and individuals with higher multimorbidity suggest that patient capacity and treatment burden may influence patterns of healthcare utilisation. These findings underscore condition-specific drivers of care fragmentation and may inform more tailored care-coordination strategies for people living with multimorbidity.