Rheumatoid Factor Titer as an Indicator of the Risk of Rheumatoid Arthritis Activity: Dose-Effect Analysis with the Restricted Cubic Spline Model

Rheumatoid factor (RF) titer is known to be correlated to rheumatoid arthritis (RA) activity, but the ideal cut-off titer of RF remains unclear. Here, the relationship between RF titer and RA activity was investigated in order to determine the ideal RF value indicative of the risk of RA activity. Clinical data from 2044 eligible patients were collected from the First Affiliated Hospital of Anhui Medical University from February 2022 to October 2023. A restricted cubic spline (RCS) model was used to evaluate the relationship between RF titer and RA activity. Data from a total of 2044 patients with RA were collected and analyzed. Multivariate logistic regression analysis revealed that higher RF levels were significant predictors of the risk of RA activity calculated according to the disease activity score 28 (DAS28)-erythrocyte sedimentation rate (ESR) (OR = 2.020, 95% CI = 1.457-2.801, P < 0.001) and DAS28-C reactive protein (CRP) (OR = 1.526, 95% CI = 1.092-2.131, P = 0.013), after the results were adjusted for potential covariates. The relationship between log2RF and the risk of RA activity was non‑linear in the RCS model (P < 0.05). The cutoff value of RF titers for determining the risk of RA activity was 65.80 IU/mL. When RF exceeded the cutoff value, the risk of RA activity based on DAS28-ESR increased by 99.2% and the risk of RA activity based on DAS28-CRP increased by 62.8% (P < 0.001). The risk of RA activity increased non-linearly with the continuous change in RF titer.