Background/Purpose: The
patient rating of global rheumatoid arthritis (RA) disease activity (PtGA) is a key variable in composite measures of disease
activity as well as definition of remission (REM). In multimorbid RA patients
(RA-MM) the perception of RA disease activity as measured by the PtGA might be impacted by the burden of multiple diseases,
leading to higher levels of PtGA. In this study, we
aimed to quantify the difference in PtGA between
RA-MM and RA-only patients and to determine the factors contributing to this difference.

Methods: We compared mean
levels of PtGA between RA-MM and RA-only patients
followed in a longitudinal RA cohort at an academic medical center. Multimorbidity
was defined as having at least one chronic condition in addition to RA and
assessed by the counted multimorbidity index (cMMI ³ 2) [1]. The mean
difference in PtGA (ΔPtGA)
between RA-MM and a cohort of RA-only patients, matched on swollen and tender
joint count (SJC, TJC), evaluator global of disease activity, and disease
duration, was assessed. The unique contribution of specific components to the
explained variation of ΔPtGA in the matched
cohort was calculated as semi-partial R² in linear
regression models and summarized as the percentage of the total R².

Results: Out of a total of 1,040
RA patients, 575 (55.5%) were multimorbid; RA-MM reported higher (worse) PtGA, with a positive linear relationship between number of
morbidities and PtGA (linear trend p<0.01). This
relationship remained significant after adjusting for disease activity, age and
disease duration (Figure 1).   In the matched cohort of 294 RA-MM
patients and 294 RA-only patients, the mean PtGA (in
mm) was significantly higher for RA-MM (30.5±24.3) versus RA-only patients
(25.6±22.9), with a mean difference of 4.9±26.7 (p<0.01, paired t-test).
Variables contributing to this difference were fatigue (18.2% of total R²),
pain (16.8% of total R²) and modified health assessment
questionnaire (mHAQ; 8.7% of total R²). Age,
gender and level of education were not significantly contributing to the
difference in PtGA between RA-MM and RA-only.

Conclusion: RA-MM patients had
higher levels of PtGA compared to RA-only patients.
The most relevant determinants of this difference in PtGA
between RA-MM and RA-only patients are differences in fatigue and pain.

Figure 1. This figure demonstrates higher
mean values of patient global of disease activity (PtGA)
in groups of patients with higher number of morbidities assessed by the counted
multimorbidity index (cMMI). Panel A
shows unadjusted analyses of variance (ANOVA) testing for linear trend
p<0.001. Panel B shows adjusted
estimated marginal means generated by a general linear model considering all
covariates set to the cohort’s mean (tender joint count= 7.9; swollen joint
count = 7.1; evaluator global = 32.2; Age = 56.8; Disease duration = 13.7years).