Study design: Secondary analysis of prospective cohort study.
Background: Common comorbidity indices were developed to predict mortality and may not be optimal for functional outcomes.
Objective: Determine how well the Functional Comorbidity Index (FCI) predicts outcomes in older adults with back pain versus the Quan/Charlson and Elixhauser comorbidity indices.
Methods: We included 5155 adults ≥65 years with new primary care visits for back pain. Comorbidity was measured using diagnosis codes 12 months prior to the new visit. Outcomes of functional limitation (Roland Morris Disability Questionnaire [RMDQ]), health-related quality-of-life (EQ5D), and total health care use (sum of Relative Value Units [RVUs]) were measured 12 months the new visit. We compared multivariable models containing preselected prognostic factors.
Results: Spearman's Correlation Coefficients among the indices were ≥0.70. Multivariable models for RMDQ had similar R2 and mean squared error (MSE) of prediction when using the FCI (R2=0.190; MSE= 6.19), Quan/Charlson (R2=0.184; MSE=6.20), or Elixhauser (R2=0.189; MSE=6.19). Multivariable models for EQ5D showed small differences in R2 and MSE when using the FCI (R2=0.157; MSE= 0.163), Quan/Charlson (R2=0.148; MSE=0.164), or Elixhauser (R2=0.154; MSE=0.163). Multivariable models for health care use had similar Akaike's information criterion (AICs) when using the FCI (AIC=10.04), Quan/Charlson (AIC=10.04), or Elixhauser (AIC=10.01).
Conclusion: All indices performed similarly in predicting outcomes. There does not seem to be an advantage to using one index over another for older adults with back pain. There is still a need to develop better function-based risk adjustment models that improve prediction of functional outcomes versus standard comorbidity indices.
Level of evidence: Level 2 Prognostic Evidence. J Orthop Sports Phys Ther, Epub 23 Jul 2019. doi:10.2519/jospt.2019.8764.
Keywords: comorbidity measures; risk adjustment; spine.