Medical coding turns a patient’s diagnoses, procedures, tests, and treatments into standardized codes that healthcare payers use to process claims. When those codes do not accurately reflect the services documented in the medical record, even a small mistake can affect reimbursement.
So, what are 5 common medical coding errors in healthcare?
The five most common medical coding errors are:
- Upcoding
- Undercoding
- Incorrect or missing modifiers
- Unbundling services
- Using incorrect, outdated, or insufficiently specific codes
These mistakes can contribute to claim denials, underpayments, payment delays, additional administrative work, and compliance concerns. The good news is that many coding problems can be identified before a claim ever reaches the payer.
Understanding why these errors happen is the first step toward building a more accurate coding and billing process.
Why Medical Coding Accuracy Matters in Healthcare
Accurate medical coding creates the connection between clinical care and reimbursement.
A provider may document the appropriate service, but the insurance company largely depends on ICD-10-CM, CPT, HCPCS, modifiers, diagnosis pointers, and other claim information to understand what happened during the encounter.
If those elements do not align, the payer may question whether the service was medically necessary, whether it was reported correctly, or whether the documentation supports the requested reimbursement.
That is why coding accuracy affects more than payment.
Poor coding can contribute to:
- Claim denials and rejections
- Delayed reimbursement
- Underpayments
- Increased accounts receivable
- Additional staff rework
- Compliance exposure
- Inaccurate clinical and financial reporting
The FY 2026 ICD-10-CM Official Guidelines also emphasize the importance of complete and consistent medical-record documentation. Accurate coding cannot be achieved when the necessary clinical information is missing or unclear.
Healthcare organizations therefore need to look at coding as part of the wider revenue cycle rather than treating it as an isolated administrative function.
1. Upcoding
Upcoding occurs when a claim reports a higher level of service or more complex service than the documentation actually supports.
For example, imagine that the clinical record supports a moderate-level evaluation and management service, but the claim is submitted using a code representing a higher level of complexity.
Even when the mistake is unintentional, the code does not accurately represent the documented encounter.
Upcoding can happen because of:
- Incorrect interpretation of documentation
- E/M level selection errors
- Inadequate coder training
- Templates that encourage inappropriate code selection
- Misunderstanding payer or coding guidelines
- Incorrect modifier use
This type of error deserves particular attention because it can become a compliance issue rather than simply a denied-claim issue.
The HHS Office of Inspector General specifically identifies upcoding as reporting a billing code representing a more severe condition or more expensive treatment than was actually provided.
How to Prevent Upcoding
The safest approach is straightforward: the code should follow the documentation, not the desired reimbursement.
Healthcare practices should routinely compare coded encounters with the provider’s clinical documentation and verify that the level of service is fully supported.
Coding audits can also identify patterns. One incorrect claim may be an isolated mistake; the same error appearing repeatedly may indicate a documentation, training, or workflow problem.
Professional medical coding services can provide another layer of review for practices managing complex or specialty-specific coding requirements.
2. Undercoding
Undercoding occurs when a lower-level or less comprehensive code is reported even though the documentation supports a more appropriate higher-level service.
Although upcoding gets more attention because of its compliance implications, undercoding can create a different problem: healthcare providers may not receive the reimbursement supported by the services they actually delivered.
Consider a physician who performs and documents a more complex encounter but routinely selects a lower-level code out of caution.
One claim may represent a small difference.
Repeated across hundreds or thousands of encounters, however, systematic undercoding can create significant revenue leakage.
Why Does Undercoding Happen?
Common causes include uncertainty about coding rules, incomplete documentation, fear of payer audits, insufficient coder education, and a lack of specialty-specific coding knowledge.
Specialties such as cardiology, neurology, orthopedics, radiology, gastroenterology, ophthalmology, and other procedure-heavy fields can have particularly detailed coding requirements.
Using a lower code simply because it appears safer is not necessarily accurate coding.
The goal should always be to report the code that most accurately reflects the documented service.
How to Prevent Undercoding
Regular coding audits can compare medical records with submitted claims and identify whether services are consistently being coded below the level supported by documentation.
Healthcare organizations should also make it easy for coders to clarify unclear clinical documentation instead of guessing.
That feedback loop between providers and coding teams can improve both coding accuracy and future documentation quality.
3. Incorrect or Missing Modifiers
Modifier errors happen when a required modifier is missing, an unnecessary modifier is added, or the wrong modifier is attached to a procedure code.
Modifiers provide additional information about the circumstances surrounding a service without changing the basic meaning of the procedure code.
That makes them small in appearance but important in practice.
For example, modifiers may communicate that services were performed separately, on different anatomical locations, during separate encounters, or under other specific circumstances recognized by coding rules.
CMS identifies several modifiers that may, under appropriate clinical circumstances, be used with National Correct Coding Initiative Procedure-to-Procedure edits. However, CMS also makes clear that these modifiers should only be used when the circumstances actually support them.
Why Modifier Errors Cause Claim Problems
A payer evaluates the relationship between codes on the claim.
If two services normally should not be billed together, the payer may require valid documentation and an appropriate modifier before processing them separately.
An incorrect modifier can therefore lead to:
- A denied procedure
- Reduced reimbursement
- Duplicate-payment concerns
- Requests for additional documentation
- Coding compliance problems
Modifiers such as 25 and 59 receive particular attention because inappropriate use can change how services are reimbursed.
The best rule is not simply to “add a modifier to get the claim paid.”
A modifier should only be reported when the documented circumstances and applicable coding rules justify it.
4. Unbundling Medical Services
Unbundling occurs when services that should be reported together under an appropriate comprehensive code are improperly billed as separate components.
Suppose a procedure includes several steps that are normally considered part of one bundled service.
Reporting each component separately may result in a claim asking for reimbursement multiple times for services that should be included together.
This is one reason healthcare coding teams must understand the National Correct Coding Initiative, or NCCI.
CMS developed NCCI Procedure-to-Procedure edits to prevent inappropriate payment when code combinations that generally should not be reported together appear on the same claim.
Why Unbundling Happens
Unbundling can result from:
- Lack of familiarity with NCCI edits
- Outdated coding software
- Incorrect interpretation of procedural documentation
- Inappropriate modifier use
- Manual claim-entry errors
- Poor communication between coding and billing teams
Because CMS updates coding edits throughout the year, practices also need processes for keeping their coding resources current.
How to Prevent Unbundling
Coding teams should review relevant CPT and HCPCS relationships before claim submission and check applicable NCCI edits.
Claim-scrubbing technology can help identify suspicious code combinations, but technology should not replace professional coding judgment.
The clinical record still needs to support what is being reported.
This is also where coding and medical billing services should work together. Finding an inappropriate code combination before submission is much easier than correcting a preventable denial after the payer has processed the claim.
5. Incorrect, Outdated, or Nonspecific Code Selection
Incorrect code selection occurs when the ICD-10-CM, CPT, or HCPCS code submitted does not accurately represent the documented diagnosis or service.
This broad category includes several very common medical coding mistakes.
A coder might select:
- The wrong diagnosis code
- An outdated or deleted code
- An unspecified code when documentation supports greater specificity
- The wrong laterality
- An incorrect sequence
- The wrong procedure code
- A code that does not match the documented service
ICD-10-CM can require detailed information about a patient’s condition, including factors such as anatomical location, laterality, encounter circumstances, severity, or other clinical details.
The coder cannot accurately assign that specificity when it is absent from the record.
That makes clinical documentation one of the most important pieces of the coding process.
CMS’s official ICD-10-CM guidelines specifically state that the joint effort of the healthcare provider and coder is essential for complete and accurate documentation, code assignment, and reporting.
What Causes Medical Coding Errors?
Medical coding errors rarely come from only one source.
Sometimes the coder selects the wrong code. In other cases, the problem begins earlier in the revenue cycle.
For example, the clinical note may not clearly describe the condition, procedure, laterality, or complexity of the encounter. The coder then has insufficient information for accurate code selection.
Other common causes include outdated coding resources, changing payer policies, manual data entry, high workloads, limited specialty knowledge, communication gaps, inadequate quality control, and insufficient training.
This is why simply telling coders to “be more careful” rarely solves a persistent coding problem.
Practices need to identify where in the workflow the error originates.
How Medical Coding Errors Affect Claim Denials
A coding mistake changes the information the payer receives about the encounter.
Imagine the clinical service itself is completely appropriate.
If the diagnosis does not support the procedure, a required modifier is missing, two procedures conflict under an edit, or the code does not reflect the documentation, the payer may still be unable to process the claim as submitted.
The practice then has to investigate the denial, review the medical record, correct the coding where appropriate, and resubmit or appeal the claim.
That costs staff time while reimbursement remains outstanding.
When the same mistake occurs repeatedly, the impact spreads beyond individual claims and begins affecting the entire revenue cycle management process.
Medical Coding Errors vs. Medical Billing Errors
Medical coding and medical billing are closely connected, but they are not exactly the same thing.
A coding error involves how the diagnosis, procedure, service, or clinical circumstances are represented through medical codes.
A billing error can occur elsewhere on the claim and may involve incorrect patient information, insurance details, provider identifiers, duplicate claim submission, eligibility issues, authorization problems, or filing requirements.
The distinction matters because repeatedly correcting billing errors will not solve a coding problem and vice versa.
Effective denial prevention requires identifying the actual source of the error.
How Healthcare Practices Can Prevent Coding Errors
Preventing medical coding mistakes requires several connected controls rather than a single solution.
Start with documentation.
Providers should document diagnoses, procedures, complexity, anatomical details, and other clinically relevant information clearly enough to support code assignment.
Next, keep coding resources current.
ICD-10-CM, CPT, HCPCS, NCCI edits, payer rules, and other coding requirements can change. Coding teams need access to the versions applicable to the relevant date of service.
Practices should also perform routine coding quality reviews. Rather than waiting for payers to identify mistakes through denials, review a sample of claims before and after submission.
Finally, analyze denial trends.
If a particular modifier, specialty, procedure, provider, or diagnosis repeatedly produces denials, that pattern provides valuable information about where additional training or workflow changes may be needed.
A Simple Pre-Submission Coding Review
Before a claim leaves the practice, the coding and billing workflow should answer several basic questions:
Does the documentation support every diagnosis and procedure being reported?
Is the ICD-10-CM code as specific as the medical record allows?
Does the CPT or HCPCS code accurately represent the documented service?
Are all modifiers necessary and supported?
Have relevant code combinations and NCCI edits been checked?
Are the codes valid for the date of service?
Does the diagnosis-to-procedure relationship make clinical and payer-policy sense?
A short review before submission can prevent considerably more work after a denial.
Why Specialty-Specific Medical Coding Matters
Medical coding becomes more challenging as clinical complexity increases.
A primary care visit, cardiology procedure, neurological test, orthopedic service, radiology study, ophthalmology procedure, or mental health encounter can each involve different coding considerations.
Coders working across these specialties need to understand not only the code set but also the clinical terminology and documentation commonly used within that field.
Specialty knowledge helps coders recognize when information is missing, when documentation requires clarification, and when a code may not accurately reflect the documented encounter.
That combination of clinical understanding and coding knowledge is especially valuable for practices with complex procedure volumes or recurring coding-related denials.
Turning Coding Accuracy Into a Healthier Revenue Cycle
Medical coding accuracy is not about maximizing the amount billed on every claim.
It is about accurately communicating the care documented in the medical record.
When documentation, coding, billing, and payer requirements align, claims have a stronger foundation for accurate processing.
Healthcare practices struggling with repeated coding problems should therefore look beyond individual denied claims.
Review patterns.
Look at which codes are causing problems, where documentation gaps occur, whether modifiers are being applied consistently, and whether coders are working with current guidelines.
Revline Medical Solutions provides medical coding and billing support designed to help healthcare practices strengthen claim accuracy, identify recurring coding issues, and build a more organized revenue cycle.
The goal is simple: code the documented care accurately the first time and reduce preventable problems before the claim reaches the payer.
Frequently Asked Questions
1. What are the 5 most common medical coding errors?
Five common medical coding errors are upcoding, undercoding, incorrect modifier use, unbundling services, and selecting incorrect, outdated, or insufficiently specific ICD-10-CM, CPT, or HCPCS codes.
2. What is the most common cause of medical coding errors?
Incomplete or unclear clinical documentation is one of the major causes of medical coding errors. Other causes include outdated code information, insufficient training, complex payer requirements, manual entry mistakes, and a lack of specialty-specific coding knowledge.
3. How do medical coding errors cause claim denials?
Coding errors may cause a payer to determine that a service is incorrectly reported, unsupported by the diagnosis, improperly bundled, missing required information, or inconsistent with the medical record. The payer may then deny or reduce payment for the claim.
4. What is an example of a coding error in healthcare?
One example is selecting a diagnosis code that does not contain the level of specificity documented in the patient’s medical record. Another is reporting a higher-level service than the clinical documentation supports.
5. What is the difference between upcoding and undercoding?
Upcoding means reporting a higher level or more complex service than the documentation supports. Undercoding means reporting a lower level of service even though the documentation supports a more appropriate higher-level code.
6. What are modifier errors in medical coding?
Modifier errors occur when a coder omits a required modifier, adds an unnecessary modifier, or chooses a modifier that does not accurately describe the circumstances of the service.
7. What is unbundling in medical coding?
Unbundling means improperly reporting separate component codes for services that should be reported together. CMS uses NCCI edits to identify many code combinations that generally should not be paid separately.
8. Can incorrect ICD-10 codes cause claim denials?
Yes. An incorrect, invalid, outdated, or insufficiently specific ICD-10-CM code can contribute to claim denials when the diagnosis information does not meet coding or payer requirements or does not appropriately support the reported service.
9. How can healthcare providers reduce medical coding errors?
Providers can reduce errors through complete clinical documentation, updated coding resources, trained coding staff, routine coding audits, appropriate claim-scrubbing technology, payer-rule monitoring, and regular analysis of coding-related denials.
10. Can outsourcing medical coding help reduce errors?
Outsourcing can help when the coding partner provides trained coders, specialty-specific expertise, quality reviews, current coding knowledge, and clear communication with the practice. However, accurate provider documentation remains essential regardless of whether coding is handled internally or externally.