<b>Comparative Usefulness of CRP and ESR in Patients With Rheumatoid Arthritis</b>

Background: Disease activity monitoring in rheumatoid arthritis (RA) commonly relies on Disease Activity Score-28 (DAS28) calculated with either erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP), yet these indices may not be interchangeable and can influence treat-to-target decisions. Objective: To compare the usefulness of CRP and ESR by assessing agreement and discordance between DAS28-ESR and DAS28-CRP in patients with established RA. Methods: This cross-sectional observational study was conducted at the Department of Medicine, Combined Military Hospital, Multan (October 2024–March 2025). Adults aged 30–65 years with RA duration ≥1 year receiving non-biologic DMARDs with or without low-dose corticosteroids and without major comorbid confounders were enrolled consecutively. DAS28 was calculated using paired same-day joint counts, patient global assessment, and laboratory ESR and CRP. Agreement between DAS28-ESR and DAS28-CRP categories (remission/low/moderate/high) was evaluated using Cohen’s kappa, with discordance proportions and McNemar testing for high disease activity (HDA) versus non-HDA; ESR–CRP correlation was assessed by Spearman’s rho. Results: Ninety-two patients were analyzed (median age 51.0 years; 71.7% female). DAS28-ESR classified more patients as HDA than DAS28-CRP (27.2% vs 12.0%). Overall categorical agreement was 46.7% with fair concordance (κ=0.265; 95% CI 0.12–0.41), and discordance occurred in 53.3%, predominantly ESR-higher/CRP-lower. Agreement for HDA versus non-HDA was moderate (κ=0.47) with significant paired discordance (McNemar p=0.001). ESR and CRP correlated strongly (ρ=0.871; p<0.001). Conclusion: Despite strong ESR–CRP correlation, DAS28-ESR and DAS28-CRP show only fair categorical agreement, with DAS28-ESR more frequently classifying high disease activity; the indices are not interchangeable and discordant cases require clinical correlation.