Validity of three measures of severity of AIDS for use in health services research studies

Policy makers and hospital managers often use severity adjustments as a control for patient mix differences when evaluating outcomes of care. Unfortunately, few indices are carefully examined and therefore the evaluations based on these methods of severity adjustment are suspect. This paper examines the accuracy of three indices for measuring the severity of illness of AIDS patients. We examine the Diagnosis-Based Severity Index (DBSI), a modified version of DBSI referred to as MDBSI and the Composite Laboratory Index (CLI) in predicting survival of AIDS patients at one medical centre. We analysed the correlation between indices and months of survival. We also examined the percentage of variance in survival months explained by each index separately and together. Finally, we used survival analysis to examine whether DBSI classifies patients in groups with distinct patterns of survival. Only patients who had died were included in the analysis so that information on the patients' full course of illness was available. Of the 91 patients abstracted, 81 cases had date of AIDS, date of death, and the CLI. These 81 cases were the focus of the analysis. Both CLI and DBSI were predictive of months of survival but were not correlated to each other. Predictions of months of survival were improved if both indices were used together rather than separately. Survival analysis confirmed that patients classified by DBSI had distinctly different survival patterns. Each index measures different aspects of the severity of the patient's condition and when possible both indices should be used together. When laboratory data are not available, e.g. in Medicaid administrative files, the use of DBSI may be reasonable.