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The Johns Hopkins ACG (originally "Ambulatory Care
Group") Case-Mix System grew out of clinical observations
made by Barbara
Starfield, MD, MPH. Research by Dr. Starfield and her
colleagues in the early 1980s examined the relationship between
morbidity or "illness burden" and health care services
utilization among children in managed care settings. Dr.
Starfield theorized that children using the most health care
resources (in other words, children with the highest health
care expenditures) were not those with a single chronic illness,
but rather children with multiple, seemingly unrelated conditions.
To test her hypothesis, she grouped illnesses within pediatric
HMO populations into five discrete categories:
- Minor illnesses which are self-limited if treated appropriately,
e.g., the flu, or chicken pox,
- Illnesses which are more major but also time limited
if treated appropriately, e.g., a broken leg or pneumonia,
- Medical illnesses which are generally chronic and which
remain incurable even with medical therapy, e.g., diabetes
or cystic fibrosis,
- Illnesses resulting from anatomical problems that are
generally not curable even with adequate and appropriate
intervention, e.g., cerebral palsy or scoliosis, and
- Psychosocial conditions, e.g. behavior problems or depression.
Dr. Starfield's research supported her hypothesis: namely,
clustering of morbidity is a better predictor of health services
resource use than the presence of specific diseases. This
finding forms the basis of the ACG system; it remains the
fundamental concept that differentiates ACGs from other case-mix
adjustment methodologies.
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