Case-mix grouping and DRGs: making the principal diagnosis.

Labelling a hospital admission with a single disease code based on the selection of the patient's principal diagnosis poses problems for clinicians, administrators and epidemiologists. The use of a case-mix grouping classification, Diagnosis Related Groups (DRGs), allows the modification of the principal diagnosis by other factors affecting the length of stay of the patient in hospital. This system requires as its entry point a decision on which diagnosis is the principal one. The definition of principal diagnosis in Australia differs from that used in the United States where DRGs were developed. We describe a study to determine how often the use of the Australian definition leads to the allocation of a different DRG.