Review of methods used to measure comorbidity in cancer populations: no gold standard exists.

OBJECTIVE: This article reviews methods used to measure comorbidity in the context of cancer; summarizing methods, identifying contexts in which they have been used, and assessing the validity, reliability, and feasibility of each approach. STUDY DESIGN AND
SETTING: Studies describing methods to measure comorbidity in epidemiological studies related to cancer were identified. Data relating to content, face, and criterion validity, reliability, and feasibility were collected.
RESULTS: Two thousand nine hundred seventy-five abstracts were reviewed and 21 separate approaches identified. Content and face validity varied but tended to be higher for measures developed for cancer populations. Some evidence supporting criterion validity of all approaches was found. Where reported, reliability tended to be moderate to high. Some approaches tended to score well on all aspects but were resource intensive in terms of data collection. Eight indices scored at least moderately well on all criteria, three of which demonstrated usefulness in relation to non-site specific cancer (Charlson Comorbidity Index, Elixhauser approach, and National Cancer Institute [Combined] Index).
CONCLUSIONS: No gold standard approach to measuring comorbidity in the context of cancer exists. Approaches vary in their strengths and weaknesses, with the choice of measure depending on the study question, population studied, and data available.