What Is Incorrect Coding in Medical Billing?
Contributions by Abid H.
Every time a medical service is provided, it has to be translated into a code. These codes are what help healthcare providers get paid. But when the wrong code is used, whether it’s a mistake or an oversight, that’s called incorrect coding. It’s more common than most people think. One wrong number or a missed update can cause a claim to be rejected or delayed. That means more paperwork, more time lost, and sometimes even missed payments.
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In many cases, these mistakes aren’t intentional. A tired staff member might enter the wrong modifier. A clinic could be using software that hasn’t been updated with the latest code set. Even something as small as a typo in the code can lead to problems with insurance companies. And when claims get bounced back, the patient often ends up confused or frustrated too.
In this blog, we’ll cover what causes incorrect coding, how it affects your billing, and what you can do to avoid it going forward.
Quick Overview
- Even simple incorrect coding mistakes can stop payments
- These errors are usually preventable with checks in place
- Both people and outdated software are common causes
- Training, updates, and code reviews can fix most of it
Different Types of Codes in Medical Billing
Medical billing doesn’t happen without codes. These codes turn a patient’s diagnosis or treatment into a format that insurance companies understand. It’s the system that links care to payment. Without them, claims wouldn’t even get off the ground.
Every type of service—whether it’s a doctor’s visit, a piece of equipment, or a hospital stay—gets a different code. But not all codes are created equal. Each set serves a different purpose, and knowing the difference matters when accuracy means faster payment.
Types of Codes in Medical Billing:
- ICD-10 Codes
- CPT Codes
- HCPCS Codes
- Modifiers
- DRG Codes
ICD-10 Codes (Diagnosis Codes)
These codes explain why the patient came in. They point to the condition, illness, or symptoms the provider treated, and they help prove that the treatment was necessary.
Common ICD-10 Mistakes:
- Picking vague codes when specific ones exist
- Forgetting to update codes every year
- Choosing a diagnosis that doesn’t match the doctor’s note
- Using codes that don’t justify the billed service
Tip: Always double-check the notes and choose the most accurate match.
CPT Codes (Procedure Codes)
CPT codes describe what the doctor or provider actually did. These cover everything from a basic exam to complex surgery. Insurance payouts are often based directly on these.
Common CPT Mistakes:
- Billing for the wrong level of service
- Using outdated or deleted codes
- Charging more or less than what was actually done
- Skipping time-based code details
Tip: Make sure the code lines up with what’s written in the provider’s report.
HCPCS Codes (Medical Supplies and Services)
HCPCS codes are used for things CPT doesn’t cover. This includes medical supplies, wheelchairs, ambulance rides, and other support services or equipment.
Common HCPCS Mistakes:
- Billing a supply that isn’t covered by the plan
- Using the wrong level of HCPCS code
- Forgetting to include a required modifier
- Confusing HCPCS codes with similar CPT ones
Tip: Always check payer rules before using these codes.
Modifiers
Modifiers don’t stand alone. They’re used with other codes to explain special situations, like repeated procedures or services done on both sides of the body.
Common Modifier Mistakes:
- Using modifiers too often or without a reason
- Leaving them off when they’re actually needed
- Putting them on the wrong code line
- Failing to explain the modifier in the notes
Tip: Only use a modifier if it changes the story told by the main code.
DRG Codes (Hospital Billing)
Hospitals use DRG codes for inpatient stays. These group patients by diagnosis, procedures, and other factors to decide how much the hospital gets paid.
Common DRG Mistakes:
- Picking the wrong primary diagnosis
- Leaving out key procedures that change the DRG
- Missing conditions that count as complications
- Not adjusting the DRG after final documentation
Tip: Review the entire hospital stay carefully before assigning a DRG.
Coding only works when the right code is used for the right reason. These systems were built to make billing smoother, but even one error can stop everything. Whether it’s a typo, a skipped update, or the wrong modifier, staying sharp and current is the best way to avoid trouble.
Common Coding Errors and the Problems They Create
Incorrect coding is one of the leading reasons for claim denials and revenue loss in healthcare. From small oversights to deeper process flaws, each error type comes with its own consequences.
1. Upcoding
Upcoding happens when a healthcare provider bills for a more complex or expensive service than what was actually performed. It inflates reimbursement and often lacks proper documentation support.
Example: Billing a 60-minute psychotherapy session (90837) when only 30 minutes (90834) were provided.
Problem It Creates: Upcoding leads to overpayments, audits, and potential fraud allegations. Even when unintentional, it draws payer attention and places providers at compliance risk.
Issues:
- Audit exposure
- Claim reprocessing
- Repayment of funds
- Legal or fraud investigations
Payers aggressively monitor high-level codes. Upcoding not only delays reimbursement but may permanently affect a provider’s risk profile or standing with insurers and government programs.
2. Undercoding
Undercoding means billing less than what was provided. Often done to avoid audits, it results in reduced revenue and an inaccurate record of the patient’s true condition or care complexity.
Example: Billing a basic office visit (99212) when documentation supports a more detailed new patient exam (99204).
Problem It Creates: Undercoding leads to lost income, affects reporting accuracy, and can misrepresent the practice’s service levels in payer data.
Issues:
- Reduced reimbursement
- Skewed patient severity data
- Weak payer contract leverage
- Lower provider performance scores
While intended to play it safe, undercoding limits revenue and can damage long-term reimbursement strategies by making a practice appear less active or skilled than it truly is.
3. Unbundling
Unbundling is when procedures that should be billed under a single, inclusive code are separated and billed individually, usually resulting in an overcharge and duplicate service appearance.
Example:
Billing separately for wound closure and debridement when both are included in a comprehensive procedure code.
Problem It Creates:
Unbundling leads to denials, compliance reviews, and often repayment demands. It can suggest manipulation of billing for financial gain.
Issues:
- Denied claims
- Duplicate charge alerts
- Overbilling suspicions
- Recovery of overpayments
Payers track bundling rules closely. When unbundling occurs, even by mistake, it triggers flags in the system and slows down the reimbursement process while risking compliance penalties.
4. Incorrect or Missing Modifiers
Modifiers are two-character codes that add detail to a claim. When they’re incorrect or missing, the claim often lacks clarity or fails automated edits in payer systems.
Example: Failing to use Modifier -59 for two unrelated procedures on the same day, causing claim bundling.
Problem It Creates: Incorrect or missing modifiers cause payment reductions, denials, and manual reviews. Claims that could have been auto-processed are held back or rejected outright.
Issues:
- Loss of payment
- Editing failures
- Denials for service duplication
- Documentation disputes
Even small modifier errors can block a clean claim. Keeping documentation clear and coding knowledge current helps ensure modifiers support the true scope of the care provided.
5. Diagnosis and Procedure Mismatch
This occurs when the diagnosis code doesn’t support the treatment billed. Insurance payers view it as a lack of medical necessity and often reject the claim immediately.
Example: Billing a respiratory therapy procedure while the listed diagnosis is foot pain.
Problem It Creates: Mismatch errors lead to denials, appeal workloads, and flagged accounts. They usually require clarification or corrected claims before any payment is made.
Issues:
- Medical necessity denials
- Workflow disruptions
- Payer disputes
- Resubmission delays
Proper alignment between diagnoses and procedures is critical. Coding should always reflect medical intent and clinical documentation, or the claim will almost certainly be rejected by payers.
6. Outdated or Deleted Codes
This coding error happens when retired, deleted, or revised codes are used. Every year, medical code sets like ICD-10 and CPT are updated, and old codes become invalid.
Example: Using a CPT code removed in the 2025 revision when the current 2026 code is required.
Problem It Creates: Outdated codes result in instant denials. They reflect poor internal processes and increase administrative costs from reworking claims.
Issues:
- Auto-denials
- Reduced first-pass claim rate
- Billing system errors
- Compliance flagging
Staying up to date on annual code set changes isn’t optional. Using expired codes not only slows revenue but suggests a lack of oversight in billing operations.
7. Duplicate Billing
Duplicate billing means the same service is billed more than once. It can happen from entry errors, overlapping systems, or confusion between provider roles or visits.
Example:
Two departments submitting charges for the same X-ray performed on one patient during a single visit.
Problem It Creates: Even when unintentional, duplicate billing gets flagged as overbilling or potential fraud, delaying payments and increasing audit risk.
Issues:
- Rejected claims
- Overpayment demands
- Audit attention
- Payer trust erosion
Clean, coordinated workflows reduce duplicate claims. Regular cross-checks between departments or software systems are key to preventing these costly and often embarrassing errors.
8. Billing for Services Not Provided
This error involves submitting claims for care never rendered. It often stems from automation defaults or unchecked documentation templates in EHRs and is a serious compliance risk.
Example: Auto-billing for a follow-up blood draw that the patient canceled but was never removed from the billing system.
Problem It Creates: This is one of the most severe coding issues. It can result in audits, legal consequences, payer sanctions, and criminal fraud allegations.
Issues:
- Regulatory penalties
- Patient disputes
- Legal exposure
- Permanent record damage
Billing for unperformed services, even unintentionally, carries heavy consequences. Every claim must reflect actual care delivered, backed by clear, up-to-date clinical documentation.
What Causes Incorrect Coding? Human Error vs Automation
Incorrect coding doesn’t happen in a vacuum. Most errors fall into two main categories—those caused by human error and those resulting from automation or software limitations. Both can disrupt billing, but the root causes are very different.
Human Error: Still the Most Common Source
Even experienced coders make mistakes. Fatigue, time pressure, and unclear documentation all contribute. Coders may select the wrong code, forget a modifier, or misread the provider’s intent.
Typical human-related causes:
- Rushed manual data entry
- Incomplete or vague physician notes
- Lack of ongoing training or certification
- Failure to check updated code guidelines
Example: A coder selects a deleted CPT code from memory instead of verifying it in the latest edition, resulting in an auto-denial.
The best way to reduce human errors is through regular audits, training refreshers, clearer documentation workflows, and peer reviews.
Automation Errors: Fast But Not Always Accurate
Automation tools are designed to speed up billing. But when they’re not properly configured, regularly updated, or supervised, they can produce errors just as harmful as human ones.
Typical automation-related causes:
- Outdated code libraries in billing software
- Auto-populated codes that don’t match documentation
- Missed edits or incorrect flagging
- Overreliance on templates and shortcuts
Example: An EHR auto-selects a code based on a template note, even though the service was canceled, and the code isn’t removed before submission.
To minimize these risks, software must be regularly maintained, with human oversight built into each step.
Human or Automation: Which One’s Worse?
They both carry risk. Human error is more unpredictable but often easier to spot. Automation errors, on the other hand, can go unnoticed for longer—until patterns emerge and trigger audits.
The most effective strategy is to combine human oversight with smart technology. Use tools to catch errors quickly, but never remove people from the loop entirely.
Composition of Coding Errors: A Visual Breakdown
While exact statistics can vary by provider type or payer data, studies and industry reports suggest that some types of coding errors happen far more often than others.
Estimated Composition of Coding Errors:
Error Type | Estimated Share |
Upcoding | 22% |
Undercoding | 18% |
Modifier Errors | 15% |
Diagnosis/Procedure Mismatch | 14% |
Outdated Codes | 10% |
Unbundling | 9% |
Duplicate Billing | 7% |
Billing for Services Not Done | 5% |
Source: Aggregated insights from MGMA, AMA, and payer audit trends.
This chart highlights that upcoding, undercoding, and modifier misuse are the most common forms of incorrect coding. These errors often stem from rushed work, unclear documentation, or improper training.
Recognizing where mistakes are most likely helps organizations target their training, audits, and automation settings more effectively.
Final Thoughts
Not every incorrect coding error is dramatic. Sometimes it’s just someone misreading a chart after a long day or forgetting to check a code update. These small things can grow into bigger problems if no one notices or corrects them early.
The solution usually isn’t fancy software. It’s time, awareness, and clear communication. Professional third-party billers like CHB Medical Billing usually complete claims with much fewer incorrect coding. When billing staff aren’t rushed and understand why accuracy matters, things improve. Claims get paid faster, and everyone, providers and patients, deals with fewer headaches in the long run.
