Abstract
Background The Charlson Comorbidity Index (CCI) is frequently used to control for confounding by comorbidities in observational studies, but its performance as such has not been studied. We evaluated the performance of CCI and an alternative summary method, logistic principal component analysis (LPCA), to adjust for comorbidities, using as an example the association between insurance and mortality.
Materials and Methods Using all admissions in the National Trauma Data Bank 2010-2015, we extracted mortality, payment method, and 36 ICD-9-derived comorbidities. We estimated ORs for the association between uninsured status and mortality before and after adjusting for CCI, LPCA, and separate covariates. We also calculated standardized mean differences (SMDs) of comorbidity variables before and after weighting the sample using inverse probability of treatment weights (IPTW) for CCI, LPCA, and separate covariates.
Results In 4,936,880 admissions, most (68.3%) had at least one comorbidity. Considerable imbalance was observed in the unweighted sample (mean SMD=0.086, OR=1.17), which was almost entirely eliminated by IPTW on separate covariates (mean SMD=0.012, OR=1.36). The CCI performed similarly to the unweighted sample (mean SMD=0.080, OR=1.25), while 2 LPCA axes were better able to control for confounding (mean SMD=0.04, OR=1.31). Using covariate adjustment, the CCI accounted for 56.1% of observed confounding, whereas 2 LPCA axes accounted for 91.3%.
Conclusions The use of the CCI to adjust for confounding may result in residual confounding, and alternative strategies should be considered. LPCA may be a viable alternative to adjusting for each comorbidity when samples are small or positivity assumptions are violated.
Author contributions: AR and MB designed the study, AR carried out statistical analysis, AR composed the first draft of the manuscript, and AR and MB reviewed and edited the final manuscript.
Footnotes
Audrey.renson{at}nyumc.org, marc.bjurlin{at}nyumc.org
Author Disclosure Statement The authors have no conflicts of interest, financial or otherwise, to declare.
Abbreviations
- CCI
- Charlson Comorbidity Index
- PCA
- principal components analysis
- LPCA
- logistic principal components analysis
- IPTW
- inverse probability of treatment weighting
- SMD
- standardized mean difference
- NTDB
- National Trauma Data Bank
- ICD-9-CM
- International Classification of Diseases, Ninth Revision, Clinical Modification
- PVD
- peripheral vascular disease
- COPD
- chronic obstructive pulmonary disorder
- ADD/ADHD
- attention deficit disorder/attention deficit hyperactivity disorder
- ACS
- American College of Surgeons
- CHF
- congestive heart failure