Nine‐Year Trajectories of Multisite Pain in Middle‐Aged and Older Chinese Adults: A Group‐Based Trajectory Analysis of the CHARLS 2011–2020

BACKGROUND: Chronic pain is a leading cause of disability, yet the longitudinal course of multisite pain-the number of anatomical sites affected-has not been characterised in a nationally representative Asian population. Most prior evidence is cross-sectional or Western. METHODS: We analysed five waves (2011-2020) of the China Health and Retirement Longitudinal Study, including 23,376 adults aged ≥ 45 years with multisite-pain data (painful sites, 0-15) in ≥ 2 waves. Trajectories were identified by a single-stage zero-inflated Poisson finite-mixture growth model fitted directly to the repeated counts, with pre-specified enumeration criteria. Site-level concordance and baseline factors associated with class membership were also examined. RESULTS: Five trajectories were identified (APPA 0.74-0.94): persistent minimal (44.9%), low fluctuating (18.6%), low-rising (16.9%), moderate persistent (10.0%) and high increasing (9.6%); two were non-monotonic. Among participants reporting pain at consecutive waves, at least one site recurred in 65%-98% of wave pairs, most often the lower back, knees and leg, but the identical site set recurred in fewer than 12%. Higher depressive symptoms (odds ratio per 5 CES-D-10 points up to 1.65), greater comorbidity (up to 1.57 per condition), female sex (up to 1.95) and lower education were associated, in a graded manner, with higher-burden trajectories. Refitting the model with sampling weights reproduced the five-class structure. CONCLUSIONS: Middle-aged and older Chinese adults follow heterogeneous, often non-monotonic multisite-pain courses, in which a recurring anatomical core is accompanied by movement of pain between sites. Depressive symptoms, multimorbidity, female sex, and lower education mark higher-burden trajectories. SIGNIFICANCE: Pain-site counts are widely used as a proxy for pain burden, but whether they track a stable anatomical problem or shifting symptoms has not been shown. Coupling trajectory modelling with site-level concordance in a national cohort reveals that a rising count reflects accumulation around a recurring low-back and lower-limb core rather than migration between unrelated sites, giving the count a concrete anatomical meaning for the first time and supporting its use to stratify risk early, before burden accumulates.