Background/Purpose:   Multimorbidity is increasingly prevalent in western populations and has the potential to influence disease specific outcomes.  Rheumatoid arthritis (RA) is a chronic inflammatory arthritis which has known associations with a number of specific other morbidities such as cardiovascular disease and cancer.  However, the effect of the burden of multiple chronic diseases on RA specific disease outcomes is less well described, and may be of particular interest in patients taking expensive biologic drugs.  The aim of this study was to describe the relationship between multimorbidity and disability and disease activity over time in a large cohort of patients with RA taking biologic drugs.

Methods:   The British Society for Rheumatology Biologics Register for RA (BSRBR-RA) recruited patients with a physician diagnosis of RA from across the UK starting biologic therapy.  This study included only patients starting their first biologic, to exclude morbidity acquired as a consequence of previous biologic treatment.  At baseline assessment, data were collected on demographics, multimorbidity, medication and disease specific variables, including disease activity (DAS28 score) and disability (HAQ).  HAQ and DAS28 were collected 6 monthly for the first three years.  The association at baseline between multimorbidity burden (assessed using the rheumatic diseases comorbidity index (RDCI) (ref)) and the outcomes HAQ and DAS28, adjusting for age, gender, smoking status, year of recruitment, disease duration and rheumatoid factor (RF) positivity was assessed using linear regression models; subsequently mixed effects models were used to investigate the same associations over time.  Sensitivity analyses were performed to examine these associations in the subgroup of patients taking tumour necrosis factor inhibitors (TNFi). 

Results: A total of 13957 patients were included, 10367 (76%) were female, median (IQR) age was 57 years (49-65) and 8655 (64%) were RF positive.  Median (IQR) RDCI at baseline was 1 (0-2).  Multimorbidity burden was significantly associated with disability but not disease activity at baseline(table 1); throughout follow up it was associated with both outcomes, adjusted beta (95% confidence interval (CI)) 0.17 (0.16,0.18) and 0.37 (0.35,0.40) for HAQ and DAS28 respectively.  Similar results were seen when the analysis was confined to TNFi users alone.

Conclusion: A higher burden of multimorbidity in patients with RA starting their first biologic drug is associated with more disability and higher disease activity over time. The presence of one or more comorbid conditions is an important factor to consider when assessing outcomes in these patients.