DRG Coding and Inpatient Bill Audits: What Patients Need to Know in 2026

What is a DRG, and why does it control your inpatient bill?

When you're admitted to a hospital as an inpatient, the hospital doesn't bill Medicare (or most commercial insurers) for each individual service the way an outpatient clinic does. Instead, at discharge, a coder reviews your medical records and assigns a single classification code called a Medicare Severity Diagnosis-Related Group (MS-DRG). That one code sets the payment the hospital receives for your entire stay.

Section 1886(d) of the Social Security Act specifies that the Secretary shall establish a classification system — referred to as DRGs — for inpatient discharges and adjust payments under the Inpatient Prospective Payment System (IPPS) based on appropriate weighting factors assigned to each DRG. In practice, this means CMS sets base payment rates prospectively for inpatient stays, generally based on the patient's diagnosis, the services or treatment provided, and the severity of illness — and, subject to certain adjustments, a hospital receives a single payment for each case depending on the payment classification assigned at discharge.

There are 772 MS-DRGs for FY 2026, and the patient's principal diagnosis, secondary diagnoses, procedures performed, sex, age, and discharge status all determine MS-DRG assignment. Under 42 CFR § 412.60(c), CMS establishes a methodology for classifying specific hospital discharges within DRGs which ensures that each hospital discharge is appropriately assigned to a single DRG based on essential data abstracted from the inpatient bill. The classification is based on the patient's age, sex, principal diagnosis, secondary diagnoses, procedures performed, and discharge status — and each discharge is assigned to only one DRG regardless of the number of conditions treated or services furnished during the patient's stay.

Why does this matter to you as a patient? Because your cost-sharing — deductibles, coinsurance, and out-of-pocket exposure — is calculated as a percentage of what the hospital is paid. A DRG assigned to a higher-severity tier generates a larger payment and a larger bill for you. If the DRG is wrong, everything downstream is wrong too.

How does the MS-DRG weighting system work in 2026?

Each MS-DRG carries a relative weighting factor. A DRG with a weight of 2.0 pays approximately twice as much as one with a weight of 1.0. CMS assigns, for each DRG, an appropriate weighting factor that reflects the estimated relative cost of hospital resources used with respect to discharges classified within that group compared to discharges classified within other groups. This base payment rate is multiplied by the MS-DRG relative weight to produce the final payment amount.

Within most clinical categories, MS-DRGs are split into three tiers based on complication and comorbidity documentation: MCC (Major Complication or Comorbidity), CC (Complication or Comorbidity), and no CC/MCC. The MCC tier pays the most. CMS considers up to 25 diagnoses and 25 procedure codes for MS-DRG assignment. That means a coder adding even one secondary diagnosis code — such as a comorbidity like malnutrition or sepsis — can shift a case from the no-CC tier to the MCC tier, dramatically increasing the payment.

CMS reviews MS-DRG classifications annually and makes adjustments to reflect changes in treatment patterns, technology, and any other factors that may change the relative use of hospital resources. For FY 2026 specifically, CMS deleted 6 MS-DRGs and finalized 5 new MS-DRGs, decreasing the total number of MS-DRGs by 1, for a total of 772 for FY 2026. This means DRG definitions you may find from prior years could be out of date — always check against the current FY 2026 grouper tables on the CMS website.

What do DRG billing errors actually look like?

HHS-OIG has documented DRG coding errors in multiple audits. A particularly clear example: prior OIG audits found that hospitals did not fully comply with Medicare requirements for inpatient claims paid with certain MS-DRGs that required enrollees to have received 96 or more consecutive hours of mechanical ventilation. Medicare payments to hospitals for inpatient claims with certain MS-DRGs that required more than 96 consecutive hours of mechanical ventilation did not fully comply with Medicare requirements. For 233 of 250 sampled claims, payments complied — but for the 17 remaining claims, they did not. Specifically, hospitals used incorrect procedure or diagnosis codes. On the basis of sample results, OIG estimated that Medicare improperly paid hospitals $79.4 million during the audit period.

A broader pattern has also been documented. The HHS Office of Inspector General found that a significant rise in high-severity inpatient admissions with no corresponding increase in length of stay indicates a possible pattern of upcoding by hospitals. High-severity admissions accounted for 40% of inpatient stays and nearly half ($54.6 billion) of all Medicare spending on inpatient stays by FY19, averaging $15,500 per billed stay. MS-DRGs with a higher proportion of significantly shorter stays — a potential flag for upcoding — include heart failure and shock (MS-DRG 291), pneumonia (MS-DRG 193), and renal failure (MS-DRG 682), according to OIG.

HHS-OIG has also maintained an ongoing series of hospital compliance reviews. This review series focuses on hospitals with claims that may be at risk for overpayments. Prior OIG reviews and investigations have identified areas at risk for noncompliance with Medicare billing requirements, and OIG will review Medicare payments to acute care hospitals to determine compliance with selected billing requirements and recommend recovery of overpayments. If you'd like to understand how hospitals inflate individual line items that feed into a DRG determination, our post on what upcoding is and how hospitals inflate your bill covers the mechanics in detail.

What are the most common DRG coding errors patients should watch for?

The table below summarizes the most common DRG-related billing error types, what they look like on your records, and how to spot them. This is an original synthesis of documented patterns from HHS-OIG audit findings and CMS guidance — not hypothetical scenarios.

For a worked example of how phantom line items can feed into a DRG determination, see our guide on phantom charges on your hospital bill and how to dispute them.

How do I find out which DRG was assigned to my stay?

Your DRG code won't appear on the summary statement most patients receive. You need to dig one layer deeper. Here's how:

1. Request your itemized bill. Ask the hospital's billing department in writing for a complete itemized bill (sometimes called a "UB-04" or "CMS-1450"). This lists every charge and should include the DRG code in the billing fields. Patients have the right to access their medical records, including current medical records, upon an oral or written request, in the form and format requested by the individual, if it is readily producible in such form and format.
2. Check your Explanation of Benefits (EOB). If you have Medicare or commercial insurance, your EOB will typically show the MS-DRG or DRG number used to calculate the payment. For Medicare, your Medicare Summary Notice (MSN) will reflect the payment determination. Our guide on reading a Medicare Summary Notice versus a commercial EOB walks through both documents line by line.
3. Request your medical records. Ask for the discharge summary, admission history and physical (H&P), operative reports if applicable, and nursing flow sheets. Under 42 CFR § 482.13, hospitals must not frustrate legitimate efforts to access medical records.
4. Look up your DRG. The CMS MS-DRG Definitions Manual (currently V43.1 for FY 2026, effective April 1, 2026) is publicly available on the CMS website. Look up your assigned DRG number to see exactly which diagnosis and procedure codes are required to support that assignment.

One important distinction: make sure you were actually admitted as an inpatient, not placed under "observation status." An inpatient is an individual admitted to a hospital for bed occupancy for purposes of receiving inpatient hospital services with the expectation that he or she will remain at least overnight. The difference matters enormously for what you owe. Our post on observation versus inpatient admission billing explains the Medicare 2-Midnight Rule and its financial consequences in full.

What is the formal process for disputing a DRG assignment?

Under federal regulations, there is a formal DRG review mechanism — though it's primarily framed around hospital-initiated reviews rather than patient-initiated ones. Under 42 CFR § 412.60(d), a hospital has 60 days after the date of the notice of the initial assignment of a discharge to a DRG to request a review of that assignment and may submit additional information as part of its request. The intermediary (Medicare Administrative Contractor) reviews the hospital's request and any additional information and decides whether a change in the DRG assignment is appropriate.

As a patient, your practical leverage is through the appeals and grievance process. Under 42 CFR § 482.13, the hospital must establish a process for prompt resolution of patient grievances and must inform each patient whom to contact to file a grievance. The hospital's governing body must be responsible for the effective operation of the grievance process. If your insurer has paid based on an incorrect DRG, you can also file an appeal with your insurer directly — insurers have their own processes for claim reconsideration when incorrect coding is identified.

For Medicare patients, the Medicare appeals process begins with a redetermination request to the Medicare Administrative Contractor (MAC). You must file within 120 days of receiving your Medicare Summary Notice. If the redetermination is unfavorable, you can escalate to a Qualified Independent Contractor (QIC) reconsideration, then to an Administrative Law Judge (ALJ) hearing. OIG itself has recommended that hospitals strengthen processes to ensure that diagnosis codes and HCPCS codes are supported in the medical records, and provide additional training to coding staff on DRG and HCPCS code assignments — the same documentation standards you should be checking as a patient.

What about the hospital price transparency rule — can it help me audit a DRG?

Yes, but indirectly. Since January 2021, CMS has required hospitals to publish machine-readable files listing their standard charges, including payer-specific negotiated rates. These files include DRG-level pricing for inpatient stays. Under the IPPS and LTCH PPS payment systems, CMS sets base payment rates prospectively for inpatient stays, generally based on the patient's diagnosis, the services or treatment provided, and the severity of illness — and a hospital receives a single payment for each case depending on the payment classification assigned at discharge. By looking up the DRG code from your EOB in the hospital's machine-readable price file, you can verify whether the payment amount billed to your insurer matches the hospital's own published rate for that DRG.

Our detailed guide on how to find and use a hospital's machine-readable price file explains exactly how to locate the file, parse the DRG columns, and compare against what you were billed. If the billed amount materially exceeds the hospital's own published rate for your assigned DRG, that's a direct audit flag worth pursuing. Keep in mind that outlier payments — additional amounts paid when a case is extraordinarily costly — are governed by 42 CFR § 412.80 and can legitimately push a payment above the base DRG rate.

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