Skip to main content

MDC

Last updated: 06-16-2025
ColumnData TypeDescription
Download CSV

What are Major Diagnostic Categories (MDCs)?​

Major Diagnostic Categories (MDCs) are broad groupings of diagnoses that represent the primary reason for hospital inpatient admission. Each MDC corresponds to a body system or a specific medical condition, such as diseases of the circulatory system or pregnancy and childbirth.

MDCs are used as the first level of classification in the Medicare Severity Diagnosis Related Groups (MS-DRGs) system. There are typically 25–27 MDCs in each version, with some additional special assignment codes (e.g., MDC 0 or 99) used in certain cases.

Every MS-DRG is assigned to a single MDC based on the principal diagnosis and procedure codes on a claim. The MDC acts as the “parent” category that determines the general clinical grouping of the DRG.

Example:
MDC 05 — Diseases & Disorders of the Circulatory System
→ DRG 280 (Acute Myocardial Infarction), DRG 291 (Heart Failure)

MDC assignment is derived from:

  • The principal ICD-10-CM diagnosis code
  • In some cases, the ICD-10-PCS procedure code

The official CMS MS-DRG Grouper processes these codes to assign an MS-DRG, which in turn is associated with one MDC.

On what kind of claims are MDC Codes found?​

MDCs are not explicitly included on the claim record.

However:

  • They are inferred from the DRG, which is found on institutional (inpatient) claims.
  • MDCs are used internally in the MS-DRG classification logic but are not stored as a discrete field on standard claim files.

If you need to assign an MDC:

  • Use the DRG on the claim to look up the corresponding MDC in a reference file.
  • Or run the claim through a Grouper if DRG is not available.

Do MDC Code Values change as MS-DRG versions change?​

Yes. With each annual update to the MS-DRG Grouper (effective October 1), CMS may:

  • Add or revise DRG-to-MDC mappings
  • Change logic for how ICD codes map into the system
  • Add new special MDCs (e.g., MDC 0 for Pre-MDC transplant and ECMO cases)

How do MDC Codes Compare to Other Diagnostic Groupers like CCSR?​

MDCs and other groupers such as CCSR (Clinical Classifications Software Refined) serve similar purposes—organizing diagnosis data into higher-level clinical categories—but they differ in key ways:

FeatureMDCCCSR
Maintained ByCMSAHRQ (Agency for Healthcare Research and Quality)
Based OnDRG logic and inpatient claimsICD-10-CM diagnosis codes
Used InInpatient hospital reimbursementClinical analytics, dashboards, utilization studies
Claim RequirementRequires grouped DRGCan be applied directly to ICD-10-CM diagnoses
GranularityBroad (25–27 categories)More specific (approx. 530 categories)
Body System vs. ClinicalBody system-basedClinically nuanced (e.g., separates anxiety vs. depression)

In summary:

  • MDCs are tightly coupled to the MS-DRG reimbursement logic
  • CCSR and other tools are often used for analytic segmentation across all claim types, not just inpatient

Code Structure​

  • MDC codes are 2-digit numeric codes:
    • 01–25: Body systems (e.g., Nervous System, Digestive System)
    • 00: Pre-MDC (transplants, ECMO)
    • 99: Invalid or ungroupable
  • Each MDC corresponds to a clinical area and includes many DRGs

Notes for Analysts​

  • MDCs are useful for high-level rollups of inpatient utilization and cost
  • Each inpatient claim with a valid DRG can be mapped to exactly one MDC
  • Some MDCs are mutually exclusive (e.g., Pregnancy vs. Digestive)
  • Be cautious when using MDCs to analyze transfers or partial payments—MDCs do not reflect final payment details

Key Use Cases for MDC Codes​

  • Clinical dashboards that track inpatient service lines
  • Financial modeling based on DRG families
  • Policy analysis involving MS-DRG changes or population segmentation
  • Case mix analysis for health systems or payers

Tuva Seed File Update Process​

Note: This is the maintenance process used by Tuva to maintain the current codeset in the Tuva package. Tuva users do not need to complete this step unless they are leveraging a different version of codes or are no longer updating to the current version of the project, but need an updated terminology set.

  1. Navigate to the CMS MS DRG website
  2. Under the section "MS-DRG Definitions Manual and Software", click on "V42 Definitions Manual Table of Contents - Full Titles - HTML Version"
    • The version (e.g. V42) will change with each new release.
  3. Click on the hyperlink "Design and Development of the Diagnosis Related Group (DRGs)"
  4. Scroll through the PDF to find the "Major Diagnostic Categories" table
  5. Copy and paste the code list into any text editor
  6. Format the codes as a CSV and save
  7. Import the CSV file into any data warehouse
  8. Upload the CSV file from the data warehouse to S3 (credentials with write permissions to the S3 bucket are required)
-- example code for Snowflake
copy into s3://tuva-public-resources/terminology/mdc.csv
from [table_created_in_step_7]
file_format = (type = csv field_optionally_enclosed_by = '"')
storage_integration = [integration_with_s3_write_permissions]
OVERWRITE = TRUE;
  1. Create a branch in docs. Update the last_updated column in the table above with the current date
  2. Submit a pull request

The below steps are only required if the headers of the file need to be changed. The Tuva Project does not store the contents of the mdc file in GitHub.

  1. Create a branch in The Tuva Project
  2. Alter the headers as needed in MDC file
  3. Submit a pull request