Case mix index (CMI) is the average diagnosis-related group weight for all of a hospital's Medicare volume. It can be used to adjust the average cost per patient (or day) for a given hospital relative to the adjusted average cost for other hospitals by dividing the average cost per patient (or day) by the hospital's calculated CMI. The adjusted average cost per patient would reflect the charges reported for the types of cases treated in that year. If a hospital has a CMI greater than 1.00, their adjusted cost per patient or per day will be lowered and conversely if a hospital has a CMI less than 1.00, their adjusted cost will be higher.
A link to the 2009 spreadsheet of the CMI for all US providers is called FY 2009 Final Rule Case Mix Index at the HHS webpage [1]
An analysis of that file shows that there are 3619 hospital records. The number of cases for the hospitals ranges from a low of 1 to a high of 36,282 cases at Florida Hospital in Orlando, FL (Medicare ID 100007). That hospital has a Case Mix Index of 1.57. The mean number of cases across all the hospitals in the database is 3,098 with a standard deviation of 3,102. As far as the Case Mix Index, the average is 1.37 with a minimum of .58 and a max of 3.73 and a standard deviation of 0.31.
A mix of cases in a hospital reflects the diversity, clinical complexity and the needs for resources in the population of patients in a hospital. Patients that have been treated in hospitals are classified in groups where other patients have the same condition, (based on main and secondary diagnosis, procedures, age) complexity (comorbidities) and needs. These groups are known as Diagnosis Related Groups (DRG) in the USA.
Each DRG has a specific measurement that is related to the amount of resources needed to treat that condition in a specific kind of patient. The different mix of all the DRGs in a hospital or hospital departments is called the Case mix index.
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