How much change in the Case Mix Index is DRG creep?

We re-abstracted a nationally representative sample of 7,887 Medicare charts to determine how much of the change in Medicare's Case Mix Index between 1986 and 1987 was true change in the complexity of cases and how much was upcoding or ‘DRG creep’. About two-thirds of the change is true. Most of the remaining third is attributable to a general change in the completeness of coding; some is attributable to changes in the Grouper program. Thus, most of the additional $1 billion paid to hospitals because of the Case Mix Index change appears justified by the additional complexity of patients hospitalized.