Development and Testing of a New Measure of Case Mix for Use in Office Practice

Case mix has been shown to be of critical importance in studies of effectiveness and quality of care using health outcomes. How these variables are defined, combined, and used to adjust or increase precision in tests for differences in health outcomes has been a source of controversy. Because existing measures were developed to adjust mortality and have marginal relevance for the adjustment of functional status outcomes, especially in ambulatory settings, the authors developed a measure of case (or patient) mix that is specifically designed to adjust functional status outcomes measured in office practice or out-of-hospital settings. This measure, developed as part of Type II Diabetes Patient Outcomes Research Team project, uses patients' reports of symptoms and conditions, as well as patients' ratings of symptom intensity to characterize total disease burden. It differs from other measures of case mix in lack of dependence on diagnoses. Separate measures were developed for each of 15 different disease categories (e.g., chronic lung disease) grouped by body system affected. Within each measure, questionnaire items were combined to rate the severity of that disease on a 1 to 4 scale, according to definitions provided by clinicians. A single, global measure was developed by aggregating the 15 measures, weighted according to the expected impact of each disease category on functional outcomes and disability. In a sample of 1,738 patients, significant relationships were observed between the global case mix measure and functional status, disability days, and service utilization. For example, scores from the Role Function Due to Physical Health Scale from the Medical Outcomes Study Short Form 36 (SF-36),-scored to range from 0 to 100, with higher scores as healthier, responses were 84.0, 68.0, 43.0, and 25.0 for patients classified as least, minimal, moderate, and severely ill, respectively, on the global measure. This and all relationships between case mix and health outcomes studies remained significant after controlling for age, gender, and socioeconomic status. This newly developed case mix measure, based primarily on patient-reported symptom severity, is reliable, valid, and feasible for use in assessing patient mix in ambulatory settings. Further testing in more varied patient samples and use in longitudinal studies, specifically for predictions of transitions in health outcomes, will determine its generalizability for use in widespread clinical studies of m