Definition of Variables
We used NIS variables to identify patient age, sex, and other patient level variables, such as admission type (elective vs. non-elective), admission day (weekend vs. weekdays); hospital level variables, such as hospital bed size (small vs. medium vs. large), hospital teaching status (non-teaching vs. teaching), and hospital region (Northeast vs. Midwest vs. South vs. West). We divided age into 4 subgroups: 18 to 49 years of age, 50 to 64 years of age, 65 to 79 years of age, and 80 years of age and older. We defined the severity of comorbid conditions by using the Deyo modification of the Charlson Comorbidity Index. This index contains 17 comorbid conditions with differential weights. The score ranges from 0 to 33, with higher scores corresponding to greater burden of comorbid diseases. Facilities were considered to be a teaching hospital if they had an American Medical Association–approved residency program, were a member of the Council of Teaching Hospitals, or had a full-time equivalent interns and residents to patient’s ratio of 0.25 or higher15. The bed size cutoff points divided into small, medium, and large have been done so that approximately one-third of the hospitals in each region, location, and teaching status combination would fall within each bed size category16. Co-morbidities associated with hospitalization for HIV infection were identified using AHRQ comorbidity measures, i.e., by using ICD-9-CM diagnoses and the Diagnosis Related Group (DRG) in effect on the discharge date17.