<P> The Major Diagnostic Categories (MDC) are formed by dividing all possible principal diagnoses (from ICD - 9 - CM) into 25 mutually exclusive diagnosis areas . MDC codes, like diagnosis - related group (DRG) codes, are primarily a claims and administrative data element unique to the United States medical care reimbursement system . DRG codes also are mapped, or grouped, into MDC codes . </P> <P> The diagnoses in each MDC correspond to a single organ system or cause 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 . </P>

How many major diagnostic categories are there in the ms-drg system