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A Diagnosis Related Group (DRG) is a statistical inpatient classification methodology that groups acute-care hospital stays into clinically coherent categories with similar expected resource consumption. First developed by health services researchers at Yale University and enacted by Congress in 1983 for the Medicare Inpatient Prospective Payment System (IPPS) under Social Security Act § 1886(d) and 42 CFR Part 412, DRGs replaced retrospective cost-based hospital reimbursement with a single bundled prospective payment per discharge.
Whereas outpatient and capitated Medicare Advantage risk models rely on longitudinal Hierarchical Condition Category (HCC) coding, acute inpatient reimbursement and hospital mortality profiling in the United States rely on two primary DRG architectures: Medicare Severity DRGs (MS-DRGs) and All Patient Refined DRGs (APR-DRGs).
What Is a Diagnosis Related Group and How Does Grouping Work?
After a patient is discharged from an acute-care inpatient stay, health information management coders and clinical documentation integrity (CDI) specialists abstract the clinical record into an electronic UB-04 (CMS-1450 / X12 837I) institutional claim. Automated DRG “grouper” software assigns exactly one DRG per hospital stay using six standardized data elements:
- Principal Diagnosis (ICD-10-CM): Defined by the Uniform Hospital Discharge Data Set (UHDDS) as the condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care. The principal diagnosis routes the stay into one of 25 mutually exclusive Major Diagnostic Categories (MDCs) organized by organ system or etiology (plus Pre-MDC high-resource transplant/tracheostomy categories).
- Operating Room & Surgical Procedures (ICD-10-PCS): Within the assigned MDC, the presence or hierarchy of ICD-10-PCS procedures partitions the stay into either a Surgical or Medical DRG track.
- Secondary Diagnoses (Complications and Comorbidities): Co-existing conditions present on admission or developing during the stay that increase hospital length of stay or treatment intensity.
- Present on Admission (POA) Indicators: Mandatory 1-character flags (
Y,N,U,W, or1) attached to every principal and secondary diagnosis on inpatient claims. Under Present on Admission (POA) rules and the Deficit Reduction Act of 2005, preventable Hospital-Acquired Conditions (HACs) not present on admission cannot shift a claim into a higher-paying severity tier. - Patient Age and Sex: Used where pediatric vs. adult physiology or reproductive anatomy subdivides specific base categories.
- Discharge Status / Disposition: Distinguishes routine home discharges from transfers to another acute-care facility, skilled nursing facility, hospice, or leaving against medical advice (AMA), or in-hospital expiration.
MS-DRG vs. APR-DRG: Side-by-Side Comparison
Hospital finance teams, state Medicaid agencies, and clinical epidemiologists must distinguish between the federal CMS MS-DRG system and the all-payer APR-DRG system developed by 3M Health Information Systems (now Solventum) in collaboration with the National Association of Children’s Hospitals and Related Institutions (NACHRI):
| Comparison Dimension | MS-DRG (Medicare Severity DRG) | APR-DRG (All Patient Refined DRG) |
|---|---|---|
| Governing Authority & Regulation | Centers for Medicare & Medicaid Services (CMS) under 42 CFR Part 412 (IPPS). | Solventum (formerly 3M HIS); widely adopted by state Medicaid programs, commercial payers, and AHRQ databases. |
| Target Patient Population | Designed for the elderly and disabled Medicare fee-for-service population; limited granularity for pediatrics, neonates, and obstetrics. | Built for all-payer populations, including birthweight-stratified neonatal categories, pediatric conditions, and high-risk obstetrics. |
| Severity Tier Structure | Up to 3 tiers per base DRG: (1) Without CC/MCC, (2) With CC (Complication or Comorbidity), and (3) With MCC (Major Complication or Comorbidity). | Every base APR-DRG is assigned two independent 4-level subclasses (Levels 1–4: Minor, Moderate, Major, Extreme): Severity of Illness (SOI) and Risk of Mortality (ROM). |
| Secondary Diagnosis Logic | Single-diagnosis threshold: A single qualifying MCC (or CC) is sufficient to shift the stay into the highest severity tier for that base DRG. | Multi-condition interaction model: Reaching SOI or ROM Level 3 (Major) or Level 4 (Extreme) requires multiple interacting secondary comorbidities across organ systems. |
| Quality & Research Applications | Drives Medicare IPPS payment weights, Case Mix Index (CMI), Geometric Mean Length of Stay (GMLOS), and HAC Reduction Program benchmarking. | Powers risk adjustment in the AHRQ HCUP National Inpatient Sample (NIS) and AHRQ Inpatient Quality Indicators (IQIs) via ROM 1–4 and SOI 1–4 subclasses. |
How Is a Diagnosis Related Group Used in Clinical Research and Healthcare?
Each DRG carries an annually recalibrated Relative Weight (RW) published by CMS (in Table 5 of the annual IPPS Final Rule) representing the expected costliness of an average case in that DRG relative to the national average Medicare inpatient case (`RW = 1.0000`). A hospital’s bundled base payment is computed by multiplying its facility-specific base payment rate (adjusted for the regional area wage index, indirect medical education [IME] for teaching hospitals, and disproportionate share hospital [DSH] adjustments) by the assigned DRG’s Relative Weight.
In health services research and clinical informatics, DRGs serve three analytical functions:
- Case Mix Index (CMI) Calculation: A hospital’s or clinical service line’s CMI is the arithmetic mean of the DRG relative weights across all discharges in a defined period (
CMI = ∑ RW / Total Discharges), providing a standardized metric of acute inpatient clinical acuity. - Geometric vs. Arithmetic Mean Length of Stay (GMLOS vs. AMLOS): CMS publishes both the GMLOS (which dampens extreme right-skewed outlier stays and governs per-diem transfer payment reductions under 42 CFR § 412.4) and AMLOS for every MS-DRG, enabling operations researchers to benchmark excess hospital days.
- Risk-Adjusted Inpatient Mortality Profiling: Because MS-DRG payment weights measure resource consumption rather than physiological death risk, observational studies in the AHRQ HCUP databases use the APR-DRG Risk of Mortality (ROM 1–4) subclass alongside Elixhauser comorbidity measures to adjust in-hospital mortality models.
Frequently Asked Questions
What is a diagnosis related group?
A Diagnosis Related Group (DRG) is an inpatient classification system that categorizes hospital stays into clinically similar groups expected to consume comparable hospital resources. Originally developed at Yale University and adopted by Medicare in 1983 under the Inpatient Prospective Payment System (IPPS, 42 CFR Part 412), a DRG assigns a fixed relative payment weight to an admission based on the principal ICD-10-CM diagnosis, ICD-10-PCS procedures, secondary comorbidities/complications, age, sex, and discharge status.
How is a diagnosis related group used in clinical research and healthcare?
Hospitals and payers use DRGs (specifically MS-DRGs for Medicare and APR-DRGs for Medicaid and commercial all-payer claims) to calculate bundled inpatient reimbursement and track hospital Case Mix Index (CMI). Clinical and health services researchers use DRG relative weights, Geometric Mean Length of Stay (GMLOS), and APR-DRG Severity of Illness (SOI 1–4) and Risk of Mortality (ROM 1–4) subclasses to risk-adjust inpatient mortality, readmission rates, and hospital resource utilization across observational cohorts.
