Major diagnostic category

The Major Diagnostic Categories (MDC) are formed by dividing all possible principal diagnoses (from ICD-9-CM) into 25 mutually exclusive diagnosis areas.

The diagnoses in each MDC correspond to a single organ system or etiology and, in general, are associated with a particular medical specialty. MDC 1 to MDC 23 are grouped according to principal diagnoses. Patients are assigned to MDC 24 (Multiple Significant Trauma) with at least two significant trauma diagnosis codes (either as principal or secondaries) from different body site categories. Patients assigned to MDC 25 (HIV Infections) must have a principal diagnosis of an HIV Infection or a principal diagnosis of a significant HIV related condition and a secondary diagnosis of an HIV Infection.

MDC codes, like DRG codes, are primarily a claims and administrative data element unique to the United States medical care reimbursement system.