10 Common Medical Coding Errors and How to Avoid Them

Accurate medical coding plays an essential role in healthcare billing, reimbursement, and compliance. Every diagnosis, procedure, and service must be translated into the correct standardised code to ensure claims are processed accurately. 

However, even minor mistakes can result in payment delays, claim denials, financial losses, and regulatory concerns.

Common Medical Coding Errors and How to Avoid Them explores the most frequent coding mistakes, including unbundling, upcoding, undercoding, incorrect modifier use, and insufficient diagnosis specificity. 

These errors can lead to claim denials, reduced revenue, and compliance risks if not identified and corrected. Preventing them requires accurate clinical documentation, regular coding audits, updated coding knowledge, and continuous training for coding professionals. 

This blog explains why these errors occur and the practical steps healthcare organisations can take to improve coding accuracy and reduce avoidable mistakes.

What Are Medical Coding Errors?

A medical coding error is any mistake made when translating a clinical encounter into standardised diagnosis, procedure, or billing codes. It can be as small as a missing character on an ICD-10-CM code or as significant as billing for a service that was never documented.

It’s worth separating an honest mistake from deliberate fraud. An accidental error usually comes from rushed work, unclear documentation, or a coder misreading a guideline. Fraudulent coding is a deliberate attempt to inflate reimbursement, and payers treat the two very differently during an audit, even though both can result in the same denied or reversed claim.

Errors tend to cluster around four areas:

• ICD-10-CM diagnosis coding – choosing the wrong condition, site, or severity code
• CPT procedure coding – selecting a code that doesn’t match the service performed
• HCPCS coding – misreporting supplies, equipment, or non-physician services
• Modifier use – adding, omitting, or misapplying modifiers that change how a claim is interpreted

10 Most Common Medical Coding Errors

The errors below account for the majority of denials and audit flags across most practices. Each one has a distinct cause and a distinct fix, so it helps to treat them separately rather than as one general “coding mistakes” bucket.

1. Unbundling Services

Unbundling happens when a coder bills separate component codes for a procedure that should be reported under a single comprehensive code. For example, billing individual steps of a surgical procedure separately, rather than the one code that already includes them.

This is one of the most closely monitored errors because it artificially inflates reimbursement, even when unintentional. Payers cross-check submissions against National Correct Coding Initiative (NCCI) edits, and repeated unbundling, even if accidental, can trigger a compliance review.

2. Upcoding

Upcoding is billing a higher-level or more expensive code than the clinical documentation actually supports. A common example is coding a routine, low-complexity office visit as a higher-level evaluation and management (E/M) service.

Upcoding increases short-term revenue but carries serious compliance risk. It’s one of the most heavily scrutinised patterns in payer audits, and repeated instances can result in recoupment, penalties, or exclusion from a payer network.

3. Undercoding

Undercoding is the opposite problem: reporting a lower-level service than what was actually provided and documented. Coders sometimes do this deliberately, believing it’s a “safer” way to avoid audit attention.

It isn’t. Undercoding causes genuine, avoidable revenue loss, and because it still misrepresents the encounter, it doesn’t protect a practice from compliance scrutiny in the way some coders assume it does.

4. Missing or Incorrect Modifiers

CPT modifiers tell a payer that a service was altered in some way, without changing its core definition. Leaving out a required modifier, or applying one the documentation doesn’t support, is one of the most common reasons claims bounce back.

Modifier 25 and Modifier 59 cause a disproportionate share of these errors. Modifier 25 (a significant, separately identifiable E/M service on the same day as another procedure) and Modifier 59 (a distinct procedural service) are both easy to misuse, and payers flag them frequently because they’re also the modifiers most often abused to unbundle claims.

5. Lack of Coding Specificity

ICD-10-CM allows for a high degree of detail, including laterality (left, right, bilateral), anatomical site, severity, and encounter type (initial, subsequent, sequela). Coding to an unspecified code when the documentation supports a more specific one is a persistent, avoidable error.

The rule of thumb is simple: code to the highest level of specificity the documentation actually supports, no higher and no lower.

6. Using Outdated Medical Codes

ICD-10-CM, CPT, and HCPCS code sets are all updated annually, with codes added, revised, or deleted. Submitting a claim with a deleted or replaced code results in an automatic rejection, regardless of how accurate the clinical judgement behind it was.

Keeping reference materials, coding software, and encoder tools current each year is a basic but frequently skipped safeguard.

7. Diagnosis and Procedure Code Mismatches

A payer needs to see medical necessity: the diagnosis code has to justify the procedure billed. When the two don’t align, whether through a typo, a copy-paste error, or a genuine misunderstanding of the encounter, the claim is denied for lack of medical necessity.

8. Coding Without Sufficient Documentation

Coders should never infer a diagnosis or procedure that isn’t explicitly documented in the medical record. Assigning a code based on what “probably happened” rather than what was written down is a documentation error that becomes a coding error the moment it’s submitted.

If documentation is missing or unclear, the correct step is to query the provider, not to guess.

9. Incorrect Diagnosis Code Sequencing

The order of diagnosis codes matters. Choosing the wrong principal or first-listed diagnosis, or sequencing secondary diagnoses incorrectly, can change how a payer processes and reimburses a claim, even when every individual code on the claim is technically correct.

10. Incorrect Procedure Codes

Sometimes the error isn’t the level of the code, but the code itself: selecting a CPT or HCPCS code that describes a similar, but different, procedure. This usually comes down to not checking the full code description and official guidelines closely enough against the documentation.

Common ICD-10-CM Coding Errors

ICD-10-CM errors typically follow a recognisable pattern: missing required characters, wrong laterality, an incorrect encounter character (initial versus subsequent), unnecessary use of unspecified codes, wrong combination codes, poor sequencing, and coding conditions the provider never actually documented.

Because ICD-10-CM codes can run up to seven characters, a single missing digit is enough to invalidate a claim. 

This is one of the most common sources of what practices generally call coding errors or ICD-10 coding errors, and it’s usually caught with a straightforward second-pass review before submission.

Common CPT Coding Errors

CPT errors cluster around wrong code selection, upcoding and undercoding, unbundling, modifier mistakes, coding procedures the documentation doesn’t support, and failing to check for annual CPT updates.

Unlike ICD-10-CM, CPT codes are tied directly to reimbursement rates, so an error here has an immediate financial impact in either direction, whether that’s lost revenue from undercoding or a flagged overpayment from upcoding.

What Is Bundling and Unbundling in Medical Coding?

Bundling means grouping the individual steps of a procedure under one comprehensive CPT code, because that code already accounts for the related components. 

Unbundling means billing those same components separately, which usually results in a higher total charge than the bundled code allows.

For example, a comprehensive surgical code already includes routine pre- and post-operative work, so billing that follow-up work separately would be unbundling. 

Coders should check National Correct Coding Initiative (NCCI) edits before billing components separately, since NCCI defines exactly which code pairs can and can’t be reported together. 

Separate reporting is only appropriate when the services are genuinely distinct, and that distinction usually needs to be supported with the correct modifier and documentation.

Upcoding vs Undercoding: What Is the Difference?

Upcoding reports a higher-level service than the documentation supports, while undercoding reports a lower-level one. 

Both misrepresent the actual encounter, but they carry different risks: upcoding creates serious compliance exposure, while undercoding causes straightforward, avoidable revenue loss.

Upcoding	Undercoding
Reports a higher service level than supported	Reports a lower service level than supported
May increase reimbursement improperly	May reduce legitimate reimbursement
Creates serious compliance risks	Causes revenue loss and can still affect compliance

What Causes Medical Coding Errors?

Most coding errors trace back to a small set of root causes: incomplete clinical documentation, insufficient coder training, the sheer complexity of current coding guidelines, failure to check annual code updates, manual data-entry slips, miscommunication between clinicians and coders, heavy workloads that force rushed coding, and misreading payer-specific requirements.

Documentation gaps are usually the biggest single driver. A coder can only code what’s written down, so if a clinician’s notes are vague or incomplete, even an experienced coder is working with limited information.

How Medical Coding Errors Affect Medical Billing

A coding error rarely stays contained to the code itself. It typically cascades into claim rejection or denial, delayed reimbursement, a growing accounts receivable balance, direct revenue loss, extra rework and administrative cost, incorrect patient bills, compliance audits, and in some cases, an overpayment recovery demand from the payer.

The financial impact compounds the longer an error goes unnoticed, since a single recurring mistake across hundreds of claims can turn a minor coding habit into a significant revenue cycle problem.

Medical Coding Errors vs Medical Billing Errors

Coding errors and billing errors are related but distinct. A coding error involves the wrong diagnosis, procedure, modifier, sequencing, or code selection. A billing error involves problems in claim submission itself, such as incorrect patient information, wrong insurance details, duplicate claims, or payment posting mistakes.

The two are easy to conflate because a coding error becomes a billing problem the moment the incorrect code reaches the submitted claim. In practice, most revenue cycle teams treat coding accuracy as the first line of defence against billing errors downstream.

How to Avoid Common Medical Coding Errors

Reducing coding errors comes down to a handful of consistent habits rather than any single fix. The list below covers the most effective, practical steps a coding team can put in place.

1. Review clinical documentation carefully before assigning any code, rather than coding from memory or assumption.
2. Code to the highest supported specificity the documentation actually allows, no more and no less.
3. Check current coding guidelines rather than relying on how a code was used last year.
4. Keep ICD-10, CPT, and HCPCS codes updated every year, including software and reference tools.
5. Use modifiers correctly, applying them only when the documentation genuinely supports them.
6. Check bundling rules before billing, particularly against NCCI edits for procedure pairs.
7. Verify diagnosis and procedure code compatibility so medical necessity is clearly supported.
8. Conduct regular coding audits to catch patterns before they become habits.
9. Provide ongoing coder training, since guidelines and code sets change every year.
10. Communicate with healthcare providers whenever documentation is unclear, rather than guessing.

Medical Coding Error Example

A physician documents a straightforward, low-complexity office visit that supports a lower-level evaluation and management (E/M) code. The claim is submitted with a higher-level E/M code instead.

Incorrect approach: The coder selects the higher-level code because that’s what was billed for a similar-looking visit previously.

Why it’s wrong: The documentation doesn’t support the complexity, time, or medical decision-making required for the higher-level code. This is a textbook example of upcoding.

Correct approach: The coder matches the code strictly to what’s documented, in this case the lower-level E/M code, and queries the provider if there’s any ambiguity about the complexity of the visit.

How Coding Audits Help Reduce Medical Coding Mistakes

Regular coding audits work by surfacing recurring errors before they reach a payer at scale. They highlight documentation weaknesses, flag modifier misuse, monitor for upcoding and undercoding patterns, and feed directly back into coder training.

Audits also support compliance by demonstrating a good-faith, ongoing effort to catch and correct errors, which matters if a payer ever raises questions about billing patterns. Over time, a consistent audit cycle reduces future claim denials simply because the same mistakes stop repeating.

Best Practices for Accurate Medical Coding

Accurate coding is built on a small set of disciplined habits: follow official coding guidelines rather than informal shortcuts, never code from assumptions, keep documentation complete, verify full code descriptions before selecting them, review payer-specific requirements where relevant, keep coding knowledge updated every year, audit high-risk services more frequently, and correct identified errors promptly rather than letting them recur.

None of these require expensive tools. They require consistency, and a team that treats coding accuracy as an ongoing discipline rather than a one-off training exercise.

FAQs

What are 5 common medical coding errors?

The five most frequent are upcoding, unbundling, modifier errors, lack of specificity, and outdated codes. These account for a large share of claim denials because each one is relatively easy to make and often goes unnoticed until a payer flags it.

What are the most common coding errors?

The most common coding errors span diagnosis coding, procedure coding, modifier use, sequencing, specificity, and documentation. In practice, most errors trace back to either incomplete clinical documentation or a coder not checking a code’s full description and current guidelines.

What are some common coding errors in medical billing?

Common examples include upcoding, undercoding, incorrect or missing modifiers, mismatched diagnosis and procedure codes, unbundling of services, and the use of outdated codes. Each can trigger a denial or delay independently.

Can a felon become a medical coder?

Yes. A felony conviction does not automatically prevent someone from becoming a medical coder. However, employers, certification organisations, and state laws may consider the nature and circumstances of the conviction.

What is unbundling in medical coding?

Unbundling is billing separate codes for the individual components of a procedure that should be reported under one comprehensive code. It typically results in a higher charge than the correct bundled code would allow.

What is an example of upcoding?

A common example is billing a routine, low-complexity office visit under a higher-level evaluation and management code than the documentation actually supports, which improperly increases reimbursement.

What is the difference between upcoding and undercoding?

Upcoding reports a higher-level service than documented, while undercoding reports a lower-level one. Upcoding creates compliance risk; undercoding causes avoidable revenue loss.

What happens when a medical code is incorrect?

An incorrect code typically results in a claim rejection or denial, delayed reimbursement, and in some cases a compliance audit if the pattern repeats across multiple claims.

How can medical coding errors be prevented?

They’re best prevented through careful documentation review, coding to the correct specificity, regular audits, ongoing coder training, and keeping code sets updated every year.

Can medical coding errors cause claim denials?

Yes. Incorrect, mismatched, unspecified, or outdated codes are among the leading causes of claim denials across most payers.

Why is coding specificity important in medical coding?

Specific codes give payers a clearer picture of medical necessity and clinical detail, which reduces the likelihood of a denial and supports more accurate reimbursement.

How often are medical coding codes updated?

ICD-10-CM, CPT, and HCPCS code sets are all updated annually, with new, revised, and deleted codes taking effect each year.

Conclusion

Medical coding errors can have a significant impact on healthcare providers, affecting reimbursement, compliance, and overall revenue cycle performance. 

Mistakes such as incorrect code selection, missing modifiers, outdated codes, and poor documentation can often be avoided through careful review and consistent quality checks. 

By investing in accurate documentation, regular coding audits, updated coding knowledge, and continuous professional training, healthcare teams can improve accuracy, reduce claim denials, and maintain stronger compliance standards. 

Effective medical coding is not just about selecting the right codes; it is about supporting efficient, reliable, and transparent healthcare operations.