The top 7 common medical coding errors are upcoding, downcoding, unbundling, duplicate billing, missing or incorrect modifiers, ICD-10 specificity failures, and inappropriate diagnosis-to-procedure codes. All these medical billing errors result in denial of claims, exposure to auditing, or loss of revenue. Others give rise to compliance inquiries in federal fraud laws.
Based on MGMA data, 13 percent of revenue cycle loss is due to medical coding errors. It is estimated that 49-80 percent of medical bills have at least one error. Billing errors cost U.S. physicians some $125 billion annually. These are not trifling clerical matters but systematic revenue bleeding, which any provider organization should take care of.
By 2026, advanced analytics will be used by payers to identify coding trends. Automated systems review claims prior to a human viewing them. Any mistake is more likely to be denied, audited, or recouped.
Why Common Medical Coding Errors Happen in 2026
There are 3 major factors that will contribute to the rate of coding errors in 2026:
- Staff shortages and coder turnover: The retired coders are being replaced with experienced coders at a slower rate. New coders do not have the depth required to attend to more complicated encounters in a proper manner.
- Rapid code updates: ICD-10 and CPT codes are updated yearly. There are hundreds of additions, deletions, and revisions to be tracked. The ICD series alone commanded 66.88 of the medical coding market share in 2025. Maintaining up-to-date knowledge is an ongoing process of learning and not a single training.
- Complex payer-specific rules: The CMS base is overlaid with each commercial payer, adding its edits, modifiers, and logic of bundling. What is passed by a Medicare claim can be passed by a Blue Cross claim.
Under these forces, even the practiced practices that are experienced turn out to be denied preventably. The section below will subdivide each type of coding error, its causes, and its prevention.
7 Most Common Medical Coding Errors Due to Medical Terminology and Documentation
The errors that arise in medical billing usually start with either ambiguous or wrong documentation and coding decisions. Terminology errors, even minor ones, can result in claims being denied, compliance risks, and lost revenue when they are not detected and corrected at an early stage.
1. Upcoding
Upcoding involves billing a higher level or more complicated procedure or service than was actually recorded or provided. It brings about exaggerated reimbursement and is a violation of compliance with the False Claims Act (FCA).
Common upcoding examples:
- A new patient code is used on an existing patient.
- Charging a 60-minute office visit when the encounter took 15 minutes.
How To Avoid It
Carry out a regular internal documentation audit. Make sure that the chosen level of E/M is the one that is recorded in the medical decision-making (MDM) or total provider time. See the 2026 AMA CPT E/M guidelines to get up-to-date leveling criteria.
2. Downcoding (Undercoding)
“Downcoding” refers to a code assigned that indicates a lower level of service than what was recorded and provided. It is one of the most common coding errors It is the reverse of upcoding, and it silently damages revenue without the compliance flags that upcoding causes.
Audits encourage much downcoding by providers. This is not helpful. It leads to underpayment, lost revenue, and wrong claims data. This, in turn, lowers the amount of money providers get paid over time.
Common downcoding examples:
- A doctor records a detailed exam, which is paid as a limited exam due to vague documentation.
- Assigning CPT 99212 in cases where the documented MDM justifies CPT 99214.
How To Avoid It
Train providers: Train providers should be trained to record in detail. The code chosen must be the maximum service level supported by the documentation and not a safe guess not meant to be questioned.
3. Unbundling
Unbundling is a practice where a coder charges several CPT codes on the separate parts of a procedure rather than a single comprehensive code that is used to charge the complete service. This is regarded as a typical procedural coding error and falls under the Medicare regulations of fraud and abuse.
The National Correct Coding Initiative (NCCI) edits tend to raise red flags about unbundling, being a CMS program that identifies pairs of codes that are not to be billed together.
Examples of unbundling:
- Separate billing of every part of a surgical package rather than one worldwide surgery code.
- Charging an office visit with a procedure that encompasses the visit in the global period of the procedure.
- How To Avoid It
How To Avoid It
Check NCCI edit tables prior to filing claims with multiple procedure codes. Read the guidelines of the National Correct Coding Initiative in CMS.
4. Duplicate Billing
Duplicate billing is when the same service is billed more than once to the same payer or more than once to multiple payers. It is among the simplest types of fraud patterns that can be identified in automated claim review systems.
One such example is when two claims are made on one X-ray done on the same date of service. The payer systems identify the duplications in seconds after they are received. The duplicate claim is refused, and in case of repetition of the pattern, it will generate an audit.
How To Avoid It
Check your practice management system’s duplicate detection option prior to batch submission. Ensure that secondary claims consist of the primary EOB and do not rebill the same service that is already paid by the primary payer.
5. Missing or Incorrect Modifiers
Modifiers have supplementary information about a given service without altering its definition. The most common ICD-10 and CPT coding errors are missing or wrong modifiers since they presuppose certain knowledge of payer regulations.
Billing errors that are associated with common modifiers include:
- Missing modifier 25 (significant, identifiably single E/M service on the same day as a procedure)
- Missing modifier 59 (separate procedural service, to override NCCI bundling edits)
- Incorrect use of modifier 51 (multiple procedures)
- Missing bilateral procedures laterality modifiers (RT/LT).
Payers have certain modifier requirements. What is effective with Medicare might not be effective with a commercial plan.
How To Avoid It
Integrate billing software or a practice management system into your billing system. The use of audit modifiers, quarterlies, and CPT codes with high-volume errors tends to be centered on the use of modifiers.
6. ICD-10 Specificity Failures
Among the most common ICD-10 coding mistakes, there is the use of unspecified codes when there are more specific codes available. The lack of laterality, type of encounter, stage, or clinical information leads to denials or decreased reimbursement and places the provider at risk of dealing with a medical necessity issue.
Common ICD-10 errors:
- Applying unspecified or undetermined codes in situations where the clinical record substantiates a particular code.
- Lacking the 7th character to injury codes (first encounter, follow-up encounter, or follow-up injury).
- Inpatient claims with misdiagnosis of the principal diagnosis.
How To Avoid It
Invest in encoder software that requires specificity. Train coders on clinical documentation improvement (CDI) in terms of getting specificity out of physician notes.
7. Diagnosis-to-Procedure Code Mismatch
Diagnosis-procedure mismatch happens when the ICD-10 diagnosis code submitted does not justify the medical necessity of the CPT procedure code billed. Local Coverage Determinations (LCDs) and National Coverage Determinations (NCDs) help the payers to authenticate these linkages.
When you submit a bill for CPT 93000 (ECG) and your diagnosis is not on the covered diagnosis list of that service, then the bill is rejected. This is also true of diagnostic imaging, lab panels, and durable medical equipment.
How To Avoid It
Compare the cross-reference diagnosis codes with LCD/NCD covered diagnosis lists and submit them beforehand. CMS releases LCDs in the Medicare Coverage Database.
Table: Coding Error Impact Table: Financial Risk by Error Type
| Coding Error | Denial Risk | Compliance Risk | Estimated Revenue Impact |
| Upcoding | High | Critical (FCA exposure) | $28,000+ per claim in civil fines |
| Downcoding | Low | Low | Chronic underpayment |
| Unbundling | High | Critical (fraud/abuse) | Recoupment + exclusion risk |
| Duplicate Billing | High | High | Claim denial + audit trigger |
| Modifier Error | High | Medium | Per-claim denial, pattern = audit |
| ICD-10 Specificity | Medium | Medium | Reduced reimbursement, denials |
| Dx-Procedure Mismatch | High | Medium | Full claim denial |
Is Your Practice Losing Revenue to Coding Errors?
By 2026, automated analytics and payer scrutiny will be greater than at any other time. CMS and the OIG are increasing AI-facilitated audit opportunities. Systematically non-addressing practices will encounter an increase in rates of denials, delays in revenue collection, and audit risk.
The solution is not more staff; it is a better process. The combination of quarterly audits, CDI integration, ongoing coder training, and real-time verification of eligibility helps to maintain the error rate to less than 5%, which constitutes the threshold to initiate payer scrutiny.
In such situations, Maine Billing Services offers specialty medical coding audits and provider revenue cycles.
Our AAPC- and AHIMA-credentialed coders review your claims, identify error patterns, and fix the process, not just the symptoms. We help Maine providers get paid accurately and on time.
Schedule Your Free Coding Audit
Frequently Asked Questions
1. What is the difference between a coding error and coding fraud?
A coding error is an inaccurate code submission resulting from insufficient training, documentation gaps, or software issues. Coding fraud is the intentional misrepresentation of services rendered to receive a higher payment.
2. How often should a practice perform a coding audit?
AHIMA and AAPC recommend coding audits at a minimum of quarterly for high-risk service lines and annually for the full practice.
3. Does using AI coding software eliminate common coding errors?
AI-assisted coding tools reduce certain error categories, particularly ICD-10 specificity prompts and NCCI edit checking, but they do not eliminate errors.