How to Define Boolean Low Disease Activity in Rheumatoid Arthritis: Experience from a Large Real-world Cohort

The aim of this work is to propose Boolean-defined low disease activity (LDA) and to test its utility in rheumatoid arthritis (RA).

We used data from a longitudinal academic clinical database of RA in Peking University First Hospital over a decade. The initial proposal of Boolean-defined LDA was proposed with ascending thresholds from 2 to 5 in steps of 1 (referred to as Boolean-LDA2/3/4/5). Agreement and residual swollen joint count (SJC) pattern with the index-based [Simplified Disease Activity Index (SDAI) and Clinical Disease Activity Index (CDAI)] LDA was analyzed. To confirm discovery, we randomly classified RA patients in a 3:2 ratio into either analysis cohort or validation cohort.

In total, 4881 visits of 672 patients were included in the analysis cohort. Of these visits, the frequencies of achieving LDA were 71.9% (SDAI), 73.6% (CDAI), 52.8% (Boolean-LDA2), 65.2% (Boolean-LDA3), 73.5% (Boolean-LDA4), and 80.7% (Boolean-LDA5). High consistency and similar SJC pattern with SDAI-LDA or CDAI-LDA were observed in Boolean-LDA3 (kappa = 0.796, 0.771). Further analysis found meeting SDAI-LDA but not Boolean-LDA3 was largely attributable to higher patient's global assessment (PGA) scores (62.9%). In further modification of Boolean-LDA3, better agreement with SDAI-LDA or CDAI-LDA was reached when exclusively increasing PGA cutoffs to 4.0, 4.5 or replacing PGA by evaluator's global assessment (EGA) with cutoff to 3.0. These findings were further replicated in randomly generated validation cohort of 449 patients with 3306 clinic visits.

Using cutoff of 3 to Boolean-LDA provides great clinical utility with index-based LDA, especially when exclusively increasing PGA cutoffs to 4.0, 4.5 or replacing PGA by EGA with cutoffs to 3.0. This may deserve being considered in clinical practice.