Background and objectives: To develop a comorbidity scoring system that out-performs the Charlson index.
Methods: Population-based cohorts of medical (n=326,456), procedural (n=349,686), and psychiatric (n=16,895) inpatients in Western Australia were followed for 1-year mortality, 30-day readmissions, and length of stay (LOS) using data linkage. Conditions were identified at index admission and over the preceding 12 months. A Multipurpose Australian Comorbidity Scoring System (MACSS) was developed, based on the most frequent 102 comorbid conditions associated with a rate ratio (RR) > or = 1.1 of death or readmission or a LOS difference > or =0.5 days. The performance of MACSS and the Charlson index in predicting mortality, readmission, and LOS, and in controlling confounding by comorbidity, was compared in five test scenarios involving asthma, myocardial infarction, mastectomy, transurethral prostatectomy, and major depressive illness.
Results: MACSS performed better than the Charlson index on all three outcomes in all five clinical groups. It reduced the failure of the Charlson index to discriminate on mortality and readmission outcomes by 5-40%, improved R(2) in LOS models by up to fourfold and often doubled the correction of originally confounded effect measures.
Conclusion: The use of the MACSS and similar alternatives to the Charlson index are a new methodologic standard for adjustment of comorbidity risk.