Real-World Distributions and Concordance of C-Reactive Protein and Erythrocyte Sedimentation Rate Across Rheumatic Diseases

Objective: To characterize real-world distributions of C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) across major rheumatic diagnoses, and to quantify concordance/discordance patterns and combined CRP-ESR inflammatory phenotypes. Methods: We retrospectively extracted all CRP and ESR tests performed in a tertiary university rheumatology hospital (January 2018-December 2023), including ICD-10-coded diagnoses. Analyses were conducted at measurement level and patient level (medians across repeated tests). CRP and ESR were expressed as raw values and multiples of ULN, and categorized into severity strata. CRP and ESR datasets were merged by patient identifier and calendar date to define same-day pairs; paired analyses used Spearman correlations and ULN-based phenotype classes. Sensitivity analyses tested alternative pairing windows, first-pair-only analyses, phenotype persistence rules, and tertile/quartile discordance definitions. Results: Among 16921 patients with ≥1 CRP and 17126 with ≥1 ESR, CRP was more disease-discriminative and only negligibly age-related, whereas ESR increased modestly with age and showed marked sex shifts across severity categories. Inflammatory burden was highest in gout and rheumatoid arthritis, intermediate in psoriatic arthritis and ankylosing spondylitis, and lower in connective tissue diseases and osteoarthritis; CRP distributions were more strongly right-tailed than ESR. Merging yielded 44427 same-day CRP-ESR pairs from 16824 patients (99.1% match). CRP and ESR were moderately correlated at measurement and patient levels, yet discordance was common: 27.3% of pairs showed isolated elevation of a single marker. Conclusions: In routine rheumatology care, CRP and ESR provide complementary information. Clinically relevant CRP-ESR dissociation is frequent, persists at the patient level, and follows diagnosis-dependent phenotype patterns.