APR-DRG vs MS-DRG matters because the grouper you use determines how you document, code, and get paid. MS-DRGs drive Medicare reimbursement for most U.S. hospitals. APR-DRGs capture severity of illness and risk of mortality in ways MS-DRGs don't, making them the standard for state Medicaid programs, pediatric hospitals, and quality reporting. If you're coding inpatient cases, you need to know which system applies to your payer mix and how each one translates clinical complexity into revenue.
This post walks you through the functional differences between both methodologies, shows real-world examples of how severity levels shift payment, and explains what your coding and CDI teams need to do differently under each system.
What MS-DRGs are and how they work
Medicare Severity Diagnosis-Related Groups (MS-DRGs) are the payment classification system CMS uses to reimburse acute inpatient hospital stays under Medicare Part A. Introduced in 2008 to replace the older CMS-DRG system, MS-DRGs group cases based on principal diagnosis, procedures performed, presence of major complications or comorbidities (MCCs) and complications or comorbidities (CCs), patient age, and discharge status.
Each MS-DRG has a fixed relative weight that reflects the average resources required to treat that type of case. CMS publishes updated weights every federal fiscal year, typically in August. Hospitals multiply the MS-DRG weight by their base rate and applicable adjustments to calculate payment.
MS-DRGs recognize three tiers of case complexity: cases without CCs or MCCs, cases with CCs, and cases with MCCs. A sepsis case coded to MS-DRG 871 (Septicemia without MV >96 hours with MCC) pays significantly more than MS-DRG 872 (Septicemia without MV >96 hours without MCC), even when the principal diagnosis and procedure codes are identical. The difference comes down to whether your coders captured an MCC-level secondary diagnosis with proper documentation.
MS-DRGs do not measure severity of illness or risk of mortality as separate dimensions. They capture comorbidities that complicate care, but they don't differentiate a moderately ill pneumonia patient from a critically ill one unless that higher severity manifests as a separately codable MCC.
What APR-DRGs are and how they differ structurally
All Patient Refined Diagnosis-Related Groups (APR-DRGs) were developed by 3M Health Information Systems to address the limitations of earlier DRG models. Unlike MS-DRGs, APR-DRGs assign every case both a severity of illness (SOI) level and a risk of mortality (ROM) level, each rated on a scale from 1 to 4. This two-dimensional framework allows APR-DRGs to differentiate patients with the same base diagnosis and procedure but very different clinical intensity.
A pneumonia patient coded to APR-DRG 139 (Other pneumonia) could fall into SOI level 1 (minor) or SOI level 4 (extreme), depending on factors like respiratory failure, sepsis, organ dysfunction, age, and multiple comorbidities. Each SOI level within that APR-DRG carries its own relative weight. Payers using APR-DRGs typically assign higher reimbursement to higher SOI levels.
APR-DRGs are the standard in several contexts. Many state Medicaid programs use them for inpatient hospital reimbursement. Pediatric and neonatal hospitals prefer APR-DRGs because the system includes specific groupings for low birth weight, prematurity, and congenital conditions that MS-DRGs handle poorly. Quality measurement and public reporting programs, including CMS's Inpatient Quality Reporting program, use APR-DRGs for risk adjustment and outcome benchmarking.
APR-DRGs also include more base groups than MS-DRGs. The current MS-DRG system contains around 760 groups. The APR-DRG system includes over 300 base DRGs, each subdivided by four SOI and four ROM levels, producing more than 2,400 possible payment categories.
How severity of illness levels affect payment and coding strategy
The practical impact of SOI levels shows up in real dollars. Consider a patient admitted with acute myocardial infarction. Under MS-DRGs, this case might group to MS-DRG 282 (Acute myocardial infarction, discharged alive with MCC) with a national relative weight of approximately 1.4688 in fiscal year 2026. If the same patient has no MCC documented, the case groups to MS-DRG 284 (Acute myocardial infarction, discharged alive without CC/MCC) with a weight around 0.7785. That's an 89% increase in payment simply from capturing an MCC.
Under APR-DRGs, that same AMI case groups to APR-DRG 174 (Acute myocardial infarction). But now the grouper evaluates all secondary diagnoses, age, and clinical indicators to assign an SOI level. If the patient is 82 years old with heart failure, chronic kidney disease stage 4, and diabetes with complications, the case might reach SOI level 3 or 4. The relative weight difference between SOI 1 and SOI 4 in a single APR-DRG can exceed 300% in some payer contracts.
This means your coding strategy must adapt to the grouper your payers use. For MS-DRG payers, coders focus on identifying and documenting MCCs and CCs that CMS recognizes. For APR-DRG payers, coders need to capture every secondary diagnosis that contributes to clinical complexity, even if it wouldn't qualify as an MCC under MS-DRG logic. Conditions like anemia, malnutrition, delirium, and functional status can shift APR-DRG SOI levels without necessarily being MCCs.
Your CDI team's query strategy should align with your payer mix. If 60% of your inpatient volume comes from MS-DRG payers and 30% from APR-DRG Medicaid plans, you're coding for two systems simultaneously. Documentation needs to support both. That's where many hospitals lose revenue because they optimize for Medicare and leave Medicaid SOI levels undercoded.
Example: Pneumonia with sepsis and organ dysfunction
A 76-year-old patient is admitted with pneumonia, develops sepsis, and requires vasopressors for septic shock. The patient also has acute kidney injury requiring short-term dialysis and chronic conditions including COPD and atrial fibrillation.
Under MS-DRGs, this case groups to MS-DRG 870 (Septicemia with mechanical ventilation >96 hours with MCC) if intubated, or MS-DRG 871 (Septicemia without MV >96 hours with MCC) if not. The MCCs here include acute kidney injury with dialysis and septic shock. The weight for MS-DRG 871 is approximately 1.7264.
Under APR-DRGs, the case groups to APR-DRG 720 (Septicemia and disseminated infections). Given the combination of septic shock, AKI with dialysis, respiratory failure, and age, the case likely reaches SOI level 4 (extreme). The weight for APR-DRG 720, SOI 4 typically ranges from 4.0 to 6.0 depending on the payer's fee schedule. That's more than double the MS-DRG weight for the same clinical scenario.
The coding requirement is identical in terms of ICD-10-CM codes. But the financial outcome depends entirely on which grouper the payer applies.
Example: Pediatric asthma exacerbation
A 4-year-old child is admitted with status asthmaticus requiring continuous nebulizers and supplemental oxygen. The child has no other comorbidities and responds to treatment within 48 hours.
Under MS-DRGs, this case groups to MS-DRG 203 (Bronchitis and asthma with CC/MCC) or MS-DRG 204 (Bronchitis and asthma without CC/MCC). If the status asthmaticus qualifies as a CC, the case gets MS-DRG 203 with a weight around 0.8821. If not, MS-DRG 204 pays approximately 0.6346.
Under APR-DRGs, the case groups to APR-DRG 141 (Asthma). The SOI level depends on clinical indicators like respiratory distress severity, oxygen requirement, and treatment intensity. A straightforward status asthmaticus case might be SOI level 2 (moderate). If the child required ICU-level monitoring or had hypoxemia documented, it could reach SOI level 3. The weight difference between SOI 2 and SOI 3 in pediatric APR-DRGs can be 50% to 80%.
Pediatric hospitals using APR-DRGs capture revenue that MS-DRGs don't recognize because MS-DRGs weren't designed for children. This is why children's hospitals lobby state Medicaid programs to adopt APR-DRG payment models.
Which payers use which system
Medicare fee-for-service uses MS-DRGs exclusively for acute inpatient hospital reimbursement. If your hospital operates under the Inpatient Prospective Payment System (IPPS), Medicare pays you based on MS-DRG weights published annually by CMS at CMS.gov.
Medicaid programs vary by state. Approximately 18 states use APR-DRGs for Medicaid inpatient reimbursement as of 2026. Others use MS-DRGs, per diem rates, or cost-based reimbursement. Your state Medicaid agency website or fee schedule will specify which grouper applies.
Commercial payers use both systems depending on their contracts. Some follow Medicare and use MS-DRGs. Others negotiate APR-DRG-based rates, especially for pediatric or high-acuity cases. Your revenue cycle team should confirm the grouper in each payer contract because assumptions lead to undercoding.
Medicare Advantage (MA) plans technically use MS-DRGs for inpatient claims, but they also submit diagnosis data to CMS for risk adjustment under the CMS-HCC model. This creates a hybrid coding priority where both MS-DRG optimization and HCC capture matter.
Critical access hospitals (CAHs) and Medicare-dependent hospitals may be reimbursed under cost-based or blended payment models rather than pure DRG systems, but they still assign DRGs for data reporting and quality measurement.
How this affects your CDI and coding operations
If your hospital serves MS-DRG payers only, your CDI program should focus on MCC and CC capture, principal diagnosis accuracy, and procedure code completeness. Your coders need to know which conditions CMS classifies as MCCs versus CCs and query physicians when documentation is unclear.
If you serve APR-DRG payers, your CDI program must broaden its scope. Every secondary diagnosis contributes to SOI calculation. Conditions that wouldn't move an MS-DRG needle can shift an APR-DRG from SOI 2 to SOI 3. Your CDI specialists need to query for malnutrition, pressure ulcers, functional dependence, acute confusion, and other conditions that MS-DRG-focused workflows might skip.
Many hospitals serve both payer types. That means your coding team is essentially working two jobs. A coder might finish assigning codes for a case, run the MS-DRG grouper, verify the MCC captured, then run the same case through an APR-DRG grouper to check the SOI level for a Medicaid secondary payer. This doubles the work and increases the risk of missed revenue if your team lacks training on both systems.
Outsourcing partners like MedCodex Health manage this complexity by maintaining coding teams trained on both MS-DRG and APR-DRG logic. When your payer mix includes both Medicare and Medicaid, having coders who understand how the same diagnosis set behaves in two different groupers prevents revenue leakage. You can learn more about dual-grouper workflows at MedCodex Health's inpatient coding services.
Query strategy for dual grouper environments
Your physician query program must account for both systems. For MS-DRGs, queries focus on clarifying whether a condition meets MCC or CC criteria. For APR-DRGs, queries need to capture clinical details that support higher SOI levels even if the condition isn't an MCC.
For example, if a patient has documented "renal insufficiency," that's not specific enough for optimal coding under either system. A query asking the physician to specify acute kidney injury (with stage if possible) or chronic kidney disease (with stage) will improve both MS-DRG and APR-DRG outcomes. But an APR-DRG-focused query might also ask about dialysis status, fluid overload, or electrolyte imbalances because those details contribute to SOI scoring.
Your physician query management process should include templates and training that address both grouper requirements without creating redundant queries or alert fatigue for your medical staff.
Common coding errors that hurt both groupers
Some mistakes harm your revenue regardless of which DRG system applies. Principal diagnosis misassignment is the most expensive. If your coder lists pneumonia as principal when the patient was actually admitted for sepsis, both the MS-DRG and the APR-DRG will be wrong and your payment will drop.
Incomplete secondary diagnosis coding costs money in both systems, but it hurts APR-DRG reimbursement more because SOI levels depend on the full clinical picture. If your coder captures the sepsis and the pneumonia but misses the acute kidney injury, malnutrition, and anemia, the MS-DRG might still land on an MCC and pay correctly. The APR-DRG will understate SOI and you'll lose 20% to 40% of the payment you earned.
Procedure code omissions also affect both groupers. MS-DRGs assign different groups based on whether specific procedures were performed. APR-DRGs use procedure codes to determine whether a case belongs in a medical or surgical DRG and sometimes to adjust SOI. Missing a procedure code can drop your case into a lower-paying DRG in either system.
Using outdated grouper software is another silent revenue killer. CMS updates MS-DRG weights and logic annually. 3M updates APR-DRG definitions periodically. If your coding system runs on last year's grouper version, you're assigning the wrong DRG to a percentage of cases and your payers are adjusting your claims on the back end.
FAQ
What is the main difference between APR-DRG and MS-DRG?
MS-DRGs classify inpatient cases into payment groups based on diagnosis, procedures, and presence of complications or comorbidities. APR-DRGs add two additional dimensions: severity of illness (SOI) and risk of mortality (ROM), each rated on a scale from 1 to 4. This allows APR-DRGs to differentiate patients with the same diagnosis but very different clinical complexity, resulting in more precise payment and risk adjustment.
Do Medicare and Medicaid use the same DRG system?
Medicare uses MS-DRGs for acute inpatient hospital reimbursement under the Inpatient Prospective Payment System. Medicaid payment systems vary by state; approximately 18 states use APR-DRGs for Medicaid inpatient reimbursement, while others use MS-DRGs, per diem rates, or cost-based models. Hospitals serving both programs often need to code and group cases under both systems.
How do severity of illness levels impact APR-DRG payment?
Each APR-DRG base group is subdivided into four severity of illness (SOI) levels: minor, moderate