A single ICD-10 diagnosis code added to your hospital record can change your bill by $5,000 to $20,000 or more. When hospitals assign diagnosis codes that trigger a higher-severity Diagnosis-Related Group (DRG), the payment they receive from Medicare or your insurer increases — and so does your cost-sharing. The Office of Inspector General found that upcoding errors account for billions in overpayments annually. Understanding how ICD-10 and DRG codes work gives you the tools to verify your bill and challenge charges that do not match your medical record.
1. What ICD-10 codes are and why they matter
The International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) is a system of approximately 70,000 diagnosis codes used by every hospital, physician, and insurer in the United States. Every condition you are treated for — from a broken arm to diabetes to pneumonia — is assigned one or more ICD-10 codes.
These codes serve three billing functions: they tell your insurer what condition was treated (which determines whether the service is covered), they drive the DRG assignment for inpatient stays (which determines how much the hospital gets paid), and they appear on your Explanation of Benefits as the reason for each charge. A wrong ICD-10 code can cause a claim denial, an inflated bill, or an incorrect cost-sharing calculation.
How codes are structured: ICD-10 codes are 3 to 7 characters long. The first character is a letter, followed by digits and sometimes an additional letter. For example:
- I10 — Essential hypertension (simple, 3 characters)
- S72.001A — Fracture of unspecified intracapsular section of right femur, initial encounter (7 characters, highly specific)
- J18.9 — Pneumonia, unspecified organism
- K80.10 — Calculus of gallbladder with chronic cholecystitis without obstruction
The more specific the code, the more accurately it describes your condition — and the more precisely it determines payment. Hospitals employ certified medical coders who review your medical record after discharge and assign ICD-10 codes based on physician documentation. Errors in this process, whether accidental or intentional, directly affect your bill. Upload your bill to BillKarma to check whether your diagnosis codes match the care you received.
2. How DRGs work for inpatient billing
For inpatient hospital stays, Medicare (and many commercial insurers) does not pay per service or per day. Instead, it pays a single lump sum based on the Diagnosis-Related Group assigned to your stay. The DRG is determined by a computer algorithm (the MS-DRG Grouper) that considers your principal diagnosis, secondary diagnoses, procedures performed, age, and discharge status.
There are approximately 770 MS-DRGs, each with a fixed payment weight. The weight is multiplied by the hospital's base rate (adjusted for geographic wages and other factors) to determine the payment. For example, DRG 470 (major joint replacement without complications) has a national average payment of approximately $12,000. DRG 469 (major joint replacement with MCC) pays approximately $26,000 — more than double — for the same procedure with a documented major complication.
Why this matters for your bill: Even though Medicare pays the hospital a fixed DRG amount, the DRG assignment affects commercially insured patients too. Many commercial contracts set reimbursement as a percentage of Medicare DRG rates (such as 150% to 250% of Medicare). A higher DRG means a higher percentage-based payment, which means higher cost-sharing for you. Check your hospital's billing patterns in our hospital directory.
3. CC and MCC: how complications change your bill
The DRG system uses two severity tiers that dramatically affect payment:
CC (Complication or Comorbidity): A secondary diagnosis that increases the cost and complexity of care. Examples include diabetes (E11.9), chronic kidney disease stage 3 (N18.3), and atrial fibrillation (I48.91). Adding a CC to a DRG typically increases the payment by $2,000 to $5,000.
MCC (Major Complication or Comorbidity): A secondary diagnosis that significantly increases cost and complexity. Examples include sepsis (A41.9), respiratory failure (J96.00), and acute kidney failure (N17.9). Adding an MCC increases the DRG payment by $5,000 to $20,000 or more.
| DRG | Description | CC/MCC Level | Avg. Medicare Payment | Difference |
|---|---|---|---|---|
| 470 | Major joint replacement | Without CC/MCC | $12,000 | — |
| 469 | Major joint replacement | With MCC | $26,000 | +$14,000 |
| 194 | Heart failure | Without CC/MCC | $5,200 | — |
| 193 | Heart failure | With CC | $7,100 | +$1,900 |
| 192 | Heart failure | With MCC | $10,400 | +$5,200 |
| 690 | Kidney/urinary tract infection | Without CC/MCC | $4,800 | — |
| 689 | Kidney/urinary tract infection | With CC | $6,500 | +$1,700 |
| 688 | Kidney/urinary tract infection | With MCC | $9,200 | +$4,400 |
| 378 | GI hemorrhage | Without CC/MCC | $4,600 | — |
| 377 | GI hemorrhage | With CC | $6,900 | +$2,300 |
| 376 | GI hemorrhage | With MCC | $11,800 | +$7,200 |
4. Upcoding: when the wrong code inflates your bill
Upcoding is the assignment of a higher-severity diagnosis code or DRG than the patient's documented condition warrants. It can be accidental (coding error, ambiguous documentation) or intentional (fraud). Either way, it inflates payments and increases your cost-sharing. BillKarma's analysis of hospital billing data found that 18% of inpatient bills contain at least one CC or MCC code not supported by the discharge summary documentation.
Common upcoding patterns:
- Adding CC/MCC codes not supported by documentation. A coder assigns "acute respiratory failure" (MCC) when the physician documented "shortness of breath" (not an MCC). The DRG payment increases by $5,000 to $15,000.
- Coding a chronic condition as acute. "Acute kidney injury" (MCC) coded instead of "chronic kidney disease stage 3" (CC). The acute version triggers a much higher payment.
- Selecting a more specific code than documented. "Sepsis" (MCC) coded when the physician documented "suspected infection" or "SIRS" without confirmed sepsis criteria.
- Principal diagnosis manipulation. Choosing a principal diagnosis that maps to a higher-paying DRG when multiple diagnoses are present. The principal diagnosis should be the condition most responsible for the admission, but in ambiguous cases, coding can be steered toward higher payment.
The HHS Office of Inspector General has identified upcoding as a persistent problem, particularly in Medicare Advantage plans where risk adjustment payments create additional incentives to code higher severity. See our billing errors guide for other common mistakes that inflate your bill. Check your hospital's upcoding risk in our hospital directory.
| Upcoding Pattern | What Was Documented | What Was Coded | Payment Increase |
|---|---|---|---|
| Acute vs. chronic kidney disease | CKD stage 3 (N18.3, CC) | Acute kidney failure (N17.9, MCC) | +$5,000–$12,000 |
| Respiratory failure vs. hypoxia | Hypoxia / low O2 (R09.02) | Acute respiratory failure (J96.00, MCC) | +$5,000–$15,000 |
| Sepsis vs. infection | Suspected infection / SIRS | Sepsis (A41.9, MCC) | +$8,000–$20,000 |
| Malnutrition severity | Mild malnutrition (E44.1) | Severe malnutrition (E43, MCC) | +$4,000–$10,000 |
5. How to verify your coding
You do not need to be a medical coder to catch major coding errors on your hospital bill. Follow these steps:
Step 1 — Request your itemized bill with ICD-10 codes. Call the billing department and ask for a fully itemized statement showing all diagnosis codes (ICD-10-CM) and procedure codes (ICD-10-PCS or CPT). For inpatient stays, also request the DRG assignment.
Step 2 — Request your medical record and discharge summary. Under HIPAA, you have the right to a copy of your complete medical record. The discharge summary lists your final diagnoses as documented by the treating physician. Compare this list to the ICD-10 codes on your bill.
Step 3 — Look for codes that do not match. If your bill lists an ICD-10 code for a condition not mentioned in your discharge summary or medical record, that code may be incorrect. Common red flags: codes for "acute" versions of chronic conditions, codes for complications you did not experience, and codes for conditions that were "ruled out" but not confirmed.
Step 4 — Submit a dispute. Write to the billing department identifying the specific ICD-10 codes you believe are incorrect and requesting a coding review. Include a copy of your discharge summary highlighting the documented diagnoses. Use our cost calculator to look up Medicare rates and understand the financial impact of each code.
6. Reading a DRG-based hospital bill
7. Case studies
Wrong DRG assignment inflated bill by $14,000
A 67-year-old Medicare patient in Texas underwent elective total knee replacement. His postoperative course was uncomplicated: he was discharged on day 2 in good condition with no respiratory or renal issues. His bill showed DRG 469 (major joint replacement with MCC) with a Medicare payment of $26,200.
The patient's daughter, a nurse, requested the itemized bill and noticed two MCC diagnosis codes: N17.9 (acute kidney failure) and J96.00 (acute respiratory failure). She obtained the medical record and found no documentation of either condition. The patient's creatinine was stable throughout the stay, and he never required supplemental oxygen. The physician's discharge summary listed only osteoarthritis and controlled hypertension as diagnoses.
She submitted a written dispute to the hospital billing department with copies of the medical record pages showing stable lab values and no respiratory events. The hospital performed a coding review, removed both MCC codes, and reassigned the stay to DRG 470 (without CC/MCC). The corrected Medicare payment was $12,100 — a reduction of $14,100. While this savings accrued primarily to Medicare, it also corrected the patient's medical record and prevented the inaccurate diagnoses from affecting future insurance underwriting.
Commercial insurance patient saves $4,800 by challenging CC codes
A 52-year-old woman with commercial insurance was hospitalized for gallbladder removal (cholecystectomy). Her insurer paid the hospital based on a DRG that included two CC diagnoses: E11.65 (type 2 diabetes with hyperglycemia) and I48.91 (atrial fibrillation). Her 20% coinsurance on the insurer's negotiated rate of $24,000 was $4,800.
She reviewed her discharge summary and found no mention of atrial fibrillation. Her medical history included a single episode of palpitations three years earlier that was evaluated and found to be benign. She filed a dispute with her insurer, who initiated a claim review. The atrial fibrillation code was removed, the DRG was reassigned to a lower-severity group, and the negotiated payment dropped to $18,800. Her coinsurance was recalculated at $3,760 — saving her $1,040 out of pocket. She also requested correction of the atrial fibrillation diagnosis in her medical record to prevent it from appearing on future claims.
Pneumonia coded as pneumonia with sepsis—$14,000 reduction after coding review
A 73-year-old Medicare patient was admitted for community-acquired pneumonia. The hospital billed the stay under DRG 871 (septicemia or severe sepsis with MCC), generating a Medicare payment of approximately $22,000. The patient’s family reviewed the discharge summary and found no mention of sepsis, no blood cultures positive for bacteremia, and no documentation that sepsis criteria (SIRS plus confirmed infection source in blood) were met. The medical record supported straightforward pneumonia only.
The family submitted a written dispute requesting a coding review, attaching the discharge summary and lab results showing no sepsis criteria. The hospital’s coding department reassigned the stay to DRG 194 (simple pneumonia without CC/MCC) at approximately $8,000. The $14,000 reduction corrected the Medicare payment and removed the inaccurate sepsis diagnosis from the patient’s medical record.
8. Frequently asked questions
What is an ICD-10 code on my medical bill?
An ICD-10 code is a standardized diagnosis code assigned to every medical condition. These codes determine what your insurer pays, how your claim is processed, and the DRG assignment for inpatient stays. You can request a list of all ICD-10 codes on your bill from the hospital billing department.
What is a DRG and how does it affect my bill?
A DRG is a classification that groups inpatient stays by diagnosis, procedures, and severity. Each DRG has a fixed payment amount. Higher-severity DRGs pay the hospital more and can increase your cost-sharing. A DRG with major complications (MCC) can pay $10,000 to $15,000 more than the same procedure without complications.
What is upcoding and how do I spot it?
Upcoding is assigning a higher-severity code than warranted, inflating payment. Red flags include MCC diagnoses not mentioned in your discharge summary, "acute" versions of your chronic conditions, and conditions that were ruled out but still coded. Compare your ICD-10 codes to your medical record. For automated analysis, upload your bill to BillKarma.
How do complications and comorbidities change my hospital bill?
A CC (complication or comorbidity) increases the DRG payment by $2,000 to $5,000. An MCC (major complication) increases it by $5,000 to $20,000. These amounts flow through to your cost-sharing. If a CC or MCC code on your bill does not match your documented conditions, you may be overpaying.
Can I request my ICD-10 and DRG codes?
Yes. You have the right to an itemized bill showing all ICD-10 codes and, for inpatient stays, the DRG assignment. Call the billing department and specifically request this information. Compare the codes to your discharge summary, which lists your final documented diagnoses.
What should I do if I think my bill has the wrong DRG?
Request your medical record and compare documented diagnoses to the ICD-10 codes. Submit a written dispute identifying the specific codes you believe are incorrect. If the hospital does not correct the error, escalate to your insurer. For suspected intentional upcoding, report to the HHS OIG at 1-800-HHS-TIPS. See our dispute guide for detailed instructions.
Sources
- CMS: MS-DRG Definitions Manual — Version 42.0 (FY 2026)
- HHS Office of Inspector General: Upcoding in Medicare Advantage and Inpatient Hospital Claims
- CMS: ICD-10-CM Official Guidelines for Coding and Reporting (FY 2026)
- MedPAC: Hospital Inpatient and Outpatient Services — March 2025 Report to Congress
- AHIMA: Understanding the MS-DRG Classification System — CC/MCC Impact Analysis
- GAO: Medicare Improper Payments — Coding Accuracy and Overpayment Estimates