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The case mix of a patient population is a description of that population based on any number of the following characteristics:

Purposes

To determine reimbursement.
DRGs
APCS
MPFS
To describe a population being served.
To identify differences in practice patterns or coding complexity.
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Case-mix methodologies are used for a number of purposes with the most common usage being for reimbursement. Case-mix methodology is used to determine relative weights and base payments for DRGs and APCs. Case-mix methodology is used with the MPFS when calculating the Geographic Practice Cost Index (GCPI).

Case-mix methodology can be used for statistical purposes to describe or identify a particular patient population.

Case-mix methodology can also be used to identify differences in coding practice patterns and the complexity of the coding because surgical cases are generally more resource extensive than medical cases.

Severity of Illness Classifications

Extent of the disease
Risk of mortality
Need for intervention
Urgency of care
Intensity of resources
Difficulty of treatment
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The most common case mix methodology is to group cases by diagnoses and procedures. The major criticism to this type of grouping is that the diagnoses and procedures do not accurately measure the severity of an illness. The APR-DRG case-mix methodology was developed to address those concerns.

Severity of Illness Classfications

Adjusted Clinical Groups
National Association of Children’s Hospital and Related Institutions (NACHRI)
Medical Outcomes Study Short Form Health Survey
Atlas System
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Severity of Illness Classifications includes the following:

Adjusted Clinical Groups - classifies individuals into groups that are likely to have similar resource requirements.

National Association of Children’s Hospitals and Related Institutions (NACHRI) - classifies diseases based on the progression of the disease, the anticipated course of the disease, and treatment goals.

Medical Outcomes Study Short Form Health Survey - classifies by severity of the disease and health status measures. Also uses patient feedback to obtain a total picture of the disease process.

Atlas System - uses information abstracted from the medical record from all points of care including ancillary service data excluding diagnosis data.

Diagnostic/Procedural/Severity Classifications

Resource Utilization Groups (RUGs)
Functional Related Group System (IRFPPS)
Home Health Resource Grouping System (HHPPS)
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Some case mix classifications use a combination of diagnoses and procedures and severity of illness in their methodology. These systems are:

Resource Utilization Groups - used in the skilled nursing facility prospective payment system (SNFPPS). Includes clinical factors such as cognition, sensory deficits and psychological well-being

Functional Related Group System - used in the inpatient rehabilitation prospective payment system (IRFPPS). Includes clinical factors such as activities of daily living (ADLs) and spinal cord injury data.

Home Health Resource Grouping System - used in the home health prospective payment system (HHPPS). Includes measuring the extent of pain, respiratory status, and integrity of the Integumentary system. Also uses social factors such as residential and caregiver demographics.

Risk Adjustment

Age and gender adjustments
Clinical risk adjustments
Situation-specific adjustments
Adjustments for specific clinical conditions
Adjustments by specialty type
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Any method used to compare the severity of illness in one group of patients in comparison to the severity of illness in another group of patients. Used make a fair comparison of diverse populations and treatment patterns. Types of risk adjustments include:

Case-Mix Index

Classification by Disease Conditions and/or Procedures
Determines patterns of resource use
Used to determine anticipated reimbursement.
Comparison to other facilities
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The most commonly used case mix index uses disease staging as the basis for classifying patient with similar conditions to determine patterns of resource use. This type of case mix is also used by a facility to determine anticipated reimbursement and to compare the results to other facilities.

The case mix index is the average of the relative weights of all cases treated at a given facility. The theoretical average CMI is 1.000. CMI’s over 1.000 signify more complex cases and CMI’s less than 1.000 signify less complex cases. Surgical cases have a higher CMI than medical cases.

Factors that influence Case-Mix

Changes in Relative Weights
Changes in Services
Accuracy in Documentation and Coding
Accuracy in DRG/APC assignment
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Factors that influence a facility’s case mix index include:

Changes made in relative weight values

Changes in the type of services offered or provided by the facility

Accuracy of documentation and coding in DRG or APC assignment