Threshold Effect of C-Reactive Protein-Albumin-Lymphocyte (CALLY) Index on Disease Activity in Rheumatoid Arthritis: Unveiling a Nonlinear Association.

The relationship between the C-reactive protein (CRP)-albumin-lymphocyte (CALLY) index and disease activity of rheumatoid arthritis (RA) has not been explored at present. This study included 1058 RA patients and used a multiple linear regression model to evaluate the association between the CALLY index and 28 joint disease activity scores (DAS28) and further explored its potential nonlinear relationship using a two-stage segmented linear regression model. Multivariate adjusted analysis showed that CALLY was significantly negatively correlated with DAS28-erythrocyte sedimentation rate (ESR) (β = -0.119, 95% confidence interval [CI]: -0.145 to -0.093) and DAS28-CRP (β = -0.201, 95% CI: -0.226 to -0.177) (both p  < 0.001), and there was a dose-response relationship (trend p  < 0.001). Segmented regression analysis revealed a significant nonlinear correlation between the two, with inflection points of 0.499 and 0.555, respectively. Below the inflection point, CALLY has a significant negative impact on disease activity (DAS28-ESR: β = -2.102, 95% CI: -2.498 to -1.706); DAS28-CRP: β = -2.311, 95% CI: -2.591 to -2.031); after exceeding the inflection point, this negative correlation effect significantly weakens but still maintains statistical significance. The results of this study indicate a significant nonlinear relationship between the CALLY index and the DAS28 score, especially at low CALLY levels. This study suggests the potential of CALLY as a novel composite biomarker reflecting the inflammatory status and disease severity of RA.