This study aims to explore the correlation between inflammatory markers, including C-reactive protein (CRP) to albumin ratio (CAR) and albumin to fibrinogen ratio (AFR), as well as systemic immune-inflammation index (SII) and systemic immune response index (SIRI) in RA and knee joint pathologies assessed via Magnetic Resonance Imaging (MRI).

51 RA patients with knee joint symptoms from the Third Affiliated Hospital of Sun-Yat sen University between July 2022 and March 2024 were recruited to undergo contrast-enhanced 3.0 T MRI scans. The RA MRI scoring system (OMERACT 2016 RAMRIS) was used to evaluate knee joint pathologies, including inflammation (synovitis, bone marrow edema) and structural impairment (joint space, bone erosion). All MRI parameter evaluations were conducted using the ITK-SNAP 3.5 Workstation. Associations between CAR, AFR, SII, and SIRI and markers of systemic inflammation, joint damage, disease activity, and autoantibodies were analyzed by Spearman’s rank correlation coefficients, the “corrplot” package in the R software was employed to visualize the correlation coefficients through a heatmap. Differences between groups were analyzed using the Mann–Whitney U-test and the one-way ANOVA test.

The results notably revealed a positive correlation between SII, CAR, and synovial volume, while AFR exhibited a negative correlation. Moreover, CAR showed a positive association with bone erosion, whereas AFR displayed a negative association (Figure 1). Although no disparities were observed among the four indices in mild, moderate, and severe synovitis, CAR, SII and SIRI levels elevated with disease progression, whereas AFR levels declined; however, this pattern was absent in bone marrow edema (Figure 2C-2D). Additionally, CAR and AFR correlated with rheumatoid factor (RF), whether it was utilized as a numerical or categorical variable (Figures 1 and 2A). CAR significantly elevated in anticitrullinated protein antibodies (ACPA) -positive group (Figure 2B). CAR, AFR, SII and SIRI were associated with systemic inflammatory indicators (CRP, ESR) and disease activity score in 28 joints (DAS28-ESR, DAS28-CRP) (Figure 1).

CAR and AFR may be more prognostically useful markers of radiological joint impairment in RA than blood cell-derived indices (SII and SIRI). Both are associated with localized inflammation and bone destruction in knee joints.