THE IMPORTANCE OF COMORBIDITIES IN EXPLAINING DIFFERENCES IN PATIENT COSTS

OBJECTIVES. The authors examine to what extent comorbidities contribut e to differences in patient hospital costs. METHODS. The medical recor d data for this study were collected from 15 metropolitan Boston hospi tals for 4,439 patients admitted mostly in 1985 for one of eight commo n conditions. Massachusetts hospital discharge abstract data for 1985 and 1993 also were used. Comorbidities were identified from the medica l record for the 15-hospital data set and from discharge abstracts for all cases. Stepwise regression models were used to develop comorbidit y scores. RESULTS. Across all conditions, the medical record-based com orbidity score increased the R(2) value from .42 in a model with diagn osis-related groups alone to .50. In condition-specific analyses, incl uding the comorbidity score increased the R(2) by more than 50% in six of eight conditions, and was more important than several other dimens ions of severity in explaining condition-specific costs. When comorbid ities were determined from discharge abstract data rather than medical records, only approximately half as much comorbidity was found. Also, there was much less explanatory power: the all-condition R(2) only we nt from .42 to .44. However, a comorbidity score developed from statew ide hospital discharge abstract data was more useful in explaining var iations in charges in the eight condition-specific analyses conducted on patients 65 years and older. CONCLUSIONS. Comorbidities, particular ly when determined from the medical record, are important determinants of patient costs.