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

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.

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.

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.

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.