The diagnosis categories used in the Inpatient Hospitalization MICA are based on the principle diagnosis, which is submitted as the first of 23 possible diagnoses coded on the discharge record. As defined by the Uniform Hospital Discharge Data Set (UHDDS), the principal diagnosis represents the "condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care."

For data prior to the fourth quarter of 2015, diagnoses and procedures associated with hospitalizations are classified in accordance with the Ninth Revision of the International Classification of Diseases (ICD-9). This classification is the result of close collaboration among many nations and non-governmental organizations, under the auspices of the World Health Organization. Its original use was to classify causes of mortality. Later, it was extended to include diagnoses in morbidity. For example, the "clinical modification" of the ICD is used in categorizing hospital diagnoses. In practice, the ICD has become the international standard diagnostic classification for all general epidemiological, as well as health management, purposes. The ninth revision of the ICD has been used to classify disease and procedures beginning January 1, 1979, through September 30, 2015.

Clinical Classification Software

The procedure and diagnostic groupings used in the hospitalization Profiles and MICA are based on the Clinical Classification Software (CCS). CCS is a tool for clustering patient diagnoses and procedures into a manageable number of clinically meaningful categories. It was developed by the Agency for Healthcare Research and Quality (AHRQ - formerly known as the Agency for Health Care Policy and Research (AHCPR). The tool is updated periodically. It was originally named the Clinical Classifications for Health Policy Research (CCHPR). The CCS compresses the 12,000 diagnosis codes of the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) into 259 mutually exclusive categories. In its multi-level mode, the system further categorizes these 259 into some 26 broader groupings. The 3,500 procedure codes of the ICD are similarly compacted into 231 groups. The system can be used with any data that are coded using the ICD-9-CM. It has been shown to be helpful in understanding and analyzing patient data by managed care plans, insurers and researchers.

Further information on the clinical classification, as well as the list of diagnoses and procedures utilized by CCS, can be found at