Non-linear Association between Atherogenic Index of Plasma and Disease Activity in Rheumatoid Arthritis: A Cross-sectional Study

Objective: To explore the relationship between plasma atherogenic index (AIP) and disease activity of rheumatoid arthritis (RA). Method: This cross-sectional study included a total of 1088 RA patients. According to the third quartile of AIP, patients were divided into three groups (T1, T2, T3), and a multiple linear regression model was used to analyze the association between AIP and 28 joint disease range of motion scores (DAS28-ESR). In addition, a two-stage linear regression model is used to explore potential nonlinear relationships. Result: After adjusting for confounding factors, multiple linear regression showed that, with T1 group as a reference, the DAS28-ESR scores of T2 group (β=0.159, 95% CI: 0.041~0.277) and T3 group (β=0.192, 95% CI: 0.071~0.314) were significantly higher (trend P-value=0.002). The analysis of AIP as a continuous variable also yielded consistent conclusions (β=0.312, 95% CI: 0.080~0.545). However, the two-segment linear regression revealed a significant nonlinear relationship (log likelihood ratio P＜0.001), with an inflection point of AIP=0.175. Before the inflection point (AIP<0.175), AIP was positively correlated with DAS28-ESR (β=0.927, 95% CI: 0.543-1.311); After the inflection point (AIP>0.175), AIP showed a negative correlation with DAS28-ESR (β=-1.010, 95% CI: -1.682~-0.339). Conclusion: There is a significant nonlinear correlation between AIP and RA disease activity. When AIP is lower than 0.175, it is positively correlated with disease activity; But when AIP exceeds this critical value, the correlation reverses. This result suggests that lipid metabolism may play a complex role in RA inflammation.