The Centers for Medicare and Medicaid Services (CMS) maintains a list of relative value units (RVUs) for inpatient hospital care. These RVUs are also known as the Diagnosis-Related Group (DRG) weight. DRGs are used to determine how much Medicare reimburses a hospital for providing care, as patients within each DRG category are expected to use a similar amount of hospital resources. A DRG is assigned by a hospital based on the diagnoses and procedures noted by physicians in a patient’s medical record.
Under the conventional DRG system, hospitals were reimbursed the same amount for treating a patient within that DRG, whether that patient was extremely sick or relatively uncomplicated. Recognizing that it was more expensive for hospitals to treat sicker patients, in 2008, CMS changed from DRGs to MS-DRGs, which stand for Medicare-Severity Diagnosis Related Groups. The MS-DRG reflects varying resource intensity to treat a condition through its classification of inpatient admissions into three mutually-exclusive categories: a base MS-DRG; a MS-DRG with care complications or comorbidities (CC); or a MS-DRG with major care complications or major comorbidities (MCC). Note that while these MS-DRGs all reference the same underlying condition, each of the three MS-DRGs will have its own MS-DRG number. A three-digit code is used to represent DRGs and MS-DRGs, but there is no way to match DRGs in the old system to the MS-DRGs, as a given DRG may have been split into two or more MS-DRGs. There were approximately 475 DRGs under the old system, and there are currently approximately 750 MS-DRGs.
The latest DRG relative value units, along with information on the average length of stay for that MS-DRG, can be found on CMS' web site, in Table 5 entitled, “List of MS-DRGs, Relative Weighting Factors and Geometric and Arthritic Mean Length of Stay.” MS-DRG's weights can be merged to a utilization file using the MS-DRG number. Therefore, in order to be able to use the MS-DRG weights, your dataset must include MS-DRG code numbers. If you are using these data to determine length of stay for your population of interest, you must choose between the average length of stay and geometric mean length of stay. The average length of stay (ALOS) is the simple arithmetic mean, and is calculated by adding up the length of stay for all patients with that MS-DRG and dividing by the number of patients. The geometric length of stay (GLOS) is calculated by multiplying all of the lengths of stay for all patients with that MS-DRG and taking the nth root of that number, where n is the number of patients. The GLOS has the effect of reducing the influence of outlier values, or patients with very high or very low length of stay.
An example of the type of data available from CMS can be seen in the DRG weight files for 1983-2007 and the MS-DRG weight files for 2008-2013 below. However, please refer to the files on the CMS website for the most up-to-date information. It is important to make sure that the DRG or MS-DRG weight file is the same year as the utilization data you are using. HERC also has historical DRG files since 1983; you can email email@example.com to obtain these.