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.