Longitudinal patterns of C-reactive protein values in calcium pyrophosphate deposition disease

OBJECTIVE: Calcium pyrophosphate deposition (CPPD) disease is a crystalline arthritis with inflammatory and non-inflammatory manifestations. We investigated longitudinal patterns of C-reactive protein (CRP) in patients with different inflammatory CPPD phenotypes. METHOD: Participants in a single-centre CPPD registry were categorized as having acute calcium pyrophosphate (CPP) crystal arthritis only (acute CPP), chronic CPP inflammatory arthritis only (chronic CPP), or both acute & chronic CPP. We included patients with one CRP (mg/L) value from the date of CPPD diagnosis to March 2024. We used generalized estimating equation (GEE) linear regression to compare log-transformed CRP values by inflammatory CPPD phenotype, accounting for within-subject correlation. Longitudinal CRP was plotted with cubic splines for each phenotype. A mixed-effects model adjusted for age and sex estimated the effect of CPPD phenotype on log-transformed CRP over time. RESULTS: Among 54 patients with 264 CRP values, mean ± sd age was 68.2 ± 8.8 years, 63% were female, and median (interquartile range) follow-up was 4.7 (2.7-7.9) years. Longitudinal CRP values fluctuated most widely in the acute CPP group. CRP was significantly higher in the acute & chronic CPP than in the chronic CPP group in a GEE model (p = 0.01). CRP was non-significantly higher over time in the acute & chronic CPP group (p = 0.15) and the acute CPP group (p = 0.12) compared with the chronic CPP group in the mixed-effects model. CONCLUSION: We identified distinct patterns of CRP according to CPPD clinical phenotype. CRP was higher over time in patients with acute CPP crystal arthritis compared with chronic CPP inflammatory arthritis only.