Original Article
The development of a comorbidity index with physical function as the outcome

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Abstract

Background and Objectives

Physical function is an important measure of success of many medical and surgical interventions. Ability to adjust for comorbid disease is essential in health services research and epidemiologic studies. Current indices have primarily been developed with mortality as the outcome, and are not sensitive enough when the outcome is physical function. The objective of this study was to develop a self-administered Functional Comorbidity Index with physical function as the outcome.

Methods

The index was developed using two databases: a cross-sectional, simple random sample of 9,423 Canadian adults and a sample of 28,349 US adults seeking treatment for spine ailments. The primary outcome measure was the SF-36 physical function (PF) subscale.

Results

The Functional Comorbidity Index, an 18-item list of diagnoses, showed stronger association with physical function (model R2 = 0.29) compared with the Charlson (model R2 = 0.18), and Kaplan-Feinstein (model R2 = 0.07) indices. The Functional Comorbidity Index correctly classified patients into high and low function, in 77% of cases.

Conclusion

This new index contains diagnoses such as arthritis not found on indices used to predict mortality, and the FCI explained more variance in PF scores compared to indices designed to predict mortality.

Introduction

Physical function, health status, and perceived quality of life are important indicators, from the patient's perspective, of the success of medical and surgical interventions. As a result, condition-specific and generic measures of health are used ubiquitously to evaluate medical and surgical interventions [1]. However, in many types of research it is essential to adjust for other diseases, called comorbid diseases, in addition to the disease of concern, which may be related to the outcome(s) of interest. This is of particular importance in research conducted in older populations where many chronic illnesses may be present in the same patient [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Without this adjustment outcomes cannot be attributed to investigative interventions as the patients themselves may differ substantially in prognostic expectations due to their initial comorbid illnesses [2].

Prior comorbidity indices have been developed primarily to predict mortality or administrative outcomes such as length of stay in acute care or disease-specific populations [2], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. These indices typically include diagnoses, often asymptomatic, such as hypertension, that are important in predicting mortality, and exclude diagnoses, such as arthritis, that impact physical function but are unlikely to result in short-term mortality. Research using indices designed to predict mortality have concluded that comorbid illnesses have little relationship with physical disability [39], [40], a finding that seems intuitively false but underscores the need to consider the purpose for which an index was designed.

The purpose of this study was to develop a self-administered, general population index of comorbid diseases with physical function as the outcome of interest. The underlying premise was that diagnoses associated with physical function would be, at least in part, different from those associated with mortality, and therefore, an index designed with physical function as the outcome would perform better than indices designed with mortality as the outcome of interest.

Section snippets

Methods

The Functional Comorbidity Index was developed in two stages.

Results

Forty unique diagnoses were identified in both literature review and focus groups (Table 1). Table 2 shows the demographic characteristics of participants in the CaMos and NSN patients. The average age of CaMos participants was 62 years (range 25 to 103 years), while the average age of NSN patients was 49 years (range18 to 97 years). Six thousand seven hundred thirty-four (71.5%) of the CaMos patients and 19,362 (68.3%) of the NSN patients had at least one comorbid illness and the mean number

Discussion

The Functional Comorbidity Index was developed specifically for use in the general population with physical function, not mortality, as the outcome of interest. The Functional Comorbidity Index can be used to adjust for the effect of comorbidity on physical function in the same manner that other indices are used to adjust for the effect of comorbidity on mortality. The Functional Comorbidity Index contains diseases such as visual impairment, osteoporosis, and arthritis, which do not appear in

Acknowledgments

The authors would like to thank the Canadian Multicentre Osteoporosis Study (CaMos) Research Group and the National Spine Network for allowing the use of their databases.

This project was supported by the Canadian Arthritis Network and the Canadian Institutes for Health Research.

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