To test whether rheumatoid arthritis (RA) trials treatment efficacy versus control is better detected for patients with lower tender joint counts (TJC) or swollen joint counts (SJC) than for higher counts.
Using data from six large multicentre trials (N = 2002) and an intent-to-treat approach at 6 months, two subtrials were created within each trial, the lower disease activity group (defined by TJC less than overall median) and the higher disease activity group. The same approach was used for SJC. Active treatment was tested for treatment control differences using several RA trial outcome measures: ACR20, EULAR response, ACRHybrid. Sample sizes needed for higher TJC and SJC RA trials versus lower TJC and SJC trials were compared.
Subtrials of subjects with lower TJC were found to have much higher sensitivity to change than those of subjects with higher TJC across all trials and outcome measures. A trial with lower TJC patients would require a smaller sample size than those with higher TJC patients. Results were not consistent for SJC subgroups. Three reasons were found for sensitivity to change of lower TJC: compared with higher TJC, those with lower TJC showed greater response to active treatment.
with higher TJC on control treatment had greater percentage improvement and more variable responses than those in the lower TJC group.
In RA trials, patients with lower disease activity within the range of current trial eligibility are more likely to show treatment efficacy than patients with higher disease activity. Lowering thresholds especially for TJC in trials may make it easier to detect treatment effects in RA.