Development of a multimorbidity index: Impact on quality of life using a rheumatoid arthritis cohort

Abstract

Objective

To develop a multimorbidity index (MMI) based on health-related quality of life (HRQol).

Methods

The index was developed in an observational RA cohort. In all, 40 morbidities recommended as core were identified using ICD-9 codes. MMIs of two types were calculated: one by enumerating morbidities (MMI.count) and the other by weighting morbidities based on their association with HRQol as assessed by EQ-5D in multiple linear regression analysis (using β-coefficients; MMI.weight). MMIs were compared to the Charlson comorbidity index (CCI) and externally validated in an international RA cohort (COMORA Study).

Results

In all, 544 out of 876 patients were multimorbid. MMI.count was in the range 1–16 (median = 2) and MMI.weight in the range 0–38 (median = 1). Both indices were more strongly associated with EQ-5D than CCI (Spearman: MMI.count = −0.20, MMI.weight = −0.26, and CCI = −0.10; p < 0.01). R² obtained by linear regression using EQ-5D as a dependent variable and various indices as independent variables, adjusted for age and gender, was the highest for MMI (R²: MMI.count = 0.05, MMI.weight = 0.11, and CCI = 0.02). When accounting for clinical disease activity index (CDAI) R² increased: MMI.count = 0.18, MMI.weight = 0.22, and CCI = 0.17, still showing higher values of MMI compared with CCI. External validation in different RA cohorts (COMORA, n = 3864) showed good performance of both indices (linear regression including age, gender, and disease activity R² = 0.30 for both MMIs).

Conclusion

In our cohort, MMI based on EQ-5D performed better than did CCI. Findings were reproducible in another large RA cohort. Not much improvement was gained by weighting; therefore a simple counted index could be useful to control for the effect of multimorbidity on patient’s overall well-being.

Introduction

In the past decade, clinical and scientific interests in comorbidity and multimorbidity have increased [1], [2], [3], [4]. However, the concepts of comorbidity and multimorbidity are used interchangeably [5]. Both concepts refer to being afflicted by more than one disease at the same time, but approach the patient from different perspectives [6]. As inflammatory rheumatic conditions are systemic diseases, a high prevalence of coexisting conditions can be observed. The average rheumatoid arthritis (RA) patient has 1.6 additional conditions, increasing with age, disease duration, and/or disease activity [7], [8], [9]. Compared with the concept of comorbidity, where the index disease is at the center of interest, multimorbidity constitutes a more holistic, patient-centered concept [6].

To date, no gold standard exists on how to measure multimorbidity. A systematic literature review on assessing comorbidity and multimorbidity identified 39 different indices showing heterogeneity in terms of types and numbers of conditions included and outcomes the indices are based on. One of the most common indices used is the Charlson Comorbidity index (CCI) [10], which was originally developed as a prognostic index to predict 1-year mortality in a breast-cancer patient cohort. Research using a morbidity index based on mortality but studying outcomes different from death therefore might have misleading findings [11].

In chronic diseases, like RA, health-related quality of life (HRQoL) is the main outcome, associated with physical function, pain, and global health. It reflects patients’ overall well-being, incorporating a multidimensional patient-centered concept. In a previous work we showed that an increasing number of morbidities leads to a decrease of HRQoL [12]. As rheumatology patients are typically afflicted by more than one disease, considering multimorbidity is of special importance when deciding on diagnostic or therapeutic strategies. Multimorbidity can cause polypharmacy, and an increasing treatment burden, which might also impact patients’ overall HRQoL. Therefore, an index reflecting multimorbidity that is based on HRQoL might be helpful to better address the disease-related aspects of patients’ overall well-being. This could also be useful for application in both clinical trials and epidemiological studies.

The purpose of this work was to create a multimorbidity index (MMI) based on HRQoL. We developed the MMI in RA patients, reflecting a typical cohort with a chronic condition. In further studies the new developed index should be validated in patients with different chronic rheumatic diseases and other conditions.