Chronic kidney disease (CKD) is frequently accompanied by disturbances in calcium, phosphate, parathyroid hormone, and bone metabolism, collectively known as CKD - Mineral and Bone Disorder (CKD-MBD). These abnormalities contribute substantially to reduced bone quality and increased osteoporosis risk. While calcium is a cornerstone biomarker in CKD-MBD, its longitudinal patterns after CKD diagnosis and their implications for bone health remain poorly understood. Static calcium measurements may not capture dynamic physiological changes that could influence skeletal outcomes. This study aimed to identify distinct 3-year calcium trajectory patterns after CKD diagnosis and to evaluate their association with subsequent osteoporosis risk in a large, real-world population.

We conducted a population-based retrospective cohort study using the Hong Kong’s Clinical Data Analysis and Reporting System (CDARS), a comprehensive electronic medical record database covering all public hospitals and representing over 80% of the Hong Kong population. In addition, CDARS provides high-quality, longitudinal laboratory data that are ideal for trajectory modelling. Patients with incident CKD between 2010 and 2012 with at least one calcium measurement within 3 years of diagnosis were included. Latent class mixed modelling (LCMM) was applied to uncover hidden patterns in calcium trajectories. The primary outcome was incident osteoporosis following the 3-year trajectory period. Associations between trajectory classes and osteoporosis were examined using Cox proportional hazards regression, adjusted for demographics, comorbidities, and laboratory values.

Among patients with CKD, three distinct calcium trajectory patterns were identified: (1) Fast Decline, (2) Rising at Low Level, and (3) Rising at High Level. Using “Fast Decline” as the reference group, the adjusted hazard ratios for osteoporosis were 0.6374 (p < 0.05) for the “Rising at Low Level” class and 0.6347 (p < 0.05) for the “Rising at High Level” class. Patients exhibiting calcium instability, either progressive decline or persistently at lower levels despite upward trends, were consistently at greater risk of osteoporosis than those maintaining higher, more stable calcium levels.

Among patients with CKD, longitudinal calcium regulation follows three distinct trajectory patterns, each conveying different risks for osteoporosis. Trajectories marked by declining or low calcium levels were associated with significantly higher osteoporosis incidence, underscoring the importance of early recognition of calcium instability in CKD-MBD management. Trajectory-based monitoring may enhance risk stratification, support timely bone health interventions, and improve long-term musculoskeletal and endocrine outcomes in CKD populations.