The most common medical coding errors are unspecified or incomplete diagnosis codes, E/M levels that don’t match the documentation, modifier misuse, unbundling, wrong units, missing laterality or 7th characters, outdated codes and diagnoses that don’t support the procedure. Each one leads to denials, lost revenue or audit risk, and most can be prevented with better documentation, current code sets, claim scrubbing and regular coding audits.
Coding errors look different in every specialty. A therapist’s biggest risk is session time; a wound care clinic’s is debridement depth; an emergency department’s is the E/M level. This guide covers the medical coding mistakes that affect every practice, then the errors that show up most in each specialty, with a link to our detailed guide for each one, and finishes with a practical prevention plan.
Key Takeaways
- Most coding errors fall into a few groups: specificity, E/M level, modifiers, bundling, units and outdated codes.
- Specialty matters. Time-based therapy codes, wound depth, Doppler bundling and injury 7th characters each cause their own denials.
- Undercoding is an error too. It doesn’t create audit risk, but it quietly loses revenue on every visit.
- Prevention works best upstream: documentation templates, current code sets, claim scrubber edits and regular audits.
8 Common Medical Coding Errors in Every Specialty
1. Unspecified diagnosis codes. Using a general code when the record supports a specific one. For example, E11.9 (type 2 diabetes without complications) when the provider documented hyperglycemia, which is E11.65. Unspecified codes can fail medical necessity edits and understate patient complexity.
2. E/M level errors. Choosing office visit levels by habit instead of medical decision making (MDM) or total time on the date of service. Both overcoding and undercoding are common. See how the levels work in our 99214 CPT code guide.
3. Modifier misuse. Adding modifier 25 or 59 to get past an edit without documentation of a significant, separately identifiable E/M service or a distinct procedure, or leaving off a modifier the payer requires.
4. Unbundling. Billing separately for services that are included in another code. CMS publishes these pairs as NCCI edits, and our guide to bundling and unbundling explains how they work.
5. Wrong units. Units that don’t match the time, dose or number of sites documented, especially for drugs, therapy and timed services.
6. Missing laterality or 7th character. Codes such as M25.569 (pain in unspecified knee) when the record says right knee (M25.561), a common miss covered in our knee pain ICD-10 code guide, or injury and external cause codes billed without the required 7th character. W54.0 is not billable on its own; the encounter needs a full code such as W54.0XXA, as our dog bite ICD-10 code guide explains.
7. Outdated or deleted codes. CPT codes change every January 1, and ICD-10-CM every October 1, with smaller April 1 updates. A code that was valid last year can be rejected as invalid for the date of service.
8. Diagnosis doesn’t support the procedure. The diagnosis linked to a service doesn’t meet the payer’s coverage policy, or the wrong diagnosis pointer is attached to the line.
Two related problems sit across all of these. Upcoding and downcoding are the compliance side of E/M and procedure errors.
Common Coding Errors by Specialty
The table shows the error that causes the most trouble in each specialty, and how to prevent it. Each linked specialty opens our detailed coding guide for that area.
| Specialty | Common coding error | How to prevent it |
|---|---|---|
| Primary care | E/M levels chosen by habit; modifier 25 added to every visit with a procedure | Select levels by MDM or time; use modifier 25 only for a significant, separately documented E/M service |
| Behavioral health | Psychotherapy code doesn’t match session time (90832: 16-37 min, 90834: 38-52 min, 90837: 53+ min) | Document start and stop times or total minutes for every session |
| Wound care | Debridement code doesn’t match the depth of tissue removed (11042 for subcutaneous tissue vs 97597 for selective debridement of epidermis or dermis) or the surface area | Document depth, tissue type and wound size in sq cm for each wound |
| Cardiology | Billing Doppler codes 93320, 93321 or 93325 with a complete echo, 93306, which already includes them | Check NCCI edits and the code descriptor before adding Doppler |
| Ophthalmology | Billing an eye exam code (92002-92014) and an E/M code for the same visit | Choose one code family per visit, based on the documentation |
| Physical therapy | Timed units that don’t follow Medicare’s 8-minute rule (commercial payers may count differently); missing GP or KX modifiers | Total timed minutes per day and apply Medicare’s unit chart |
| Orthopedics and injuries | Missing laterality or wrong 7th character (such as A, D or S) on injury codes | Code the side and encounter type every time |
| Radiology | Billing the global service when only the interpretation was performed | Use modifier 26 for the professional component and TC for the technical component |
| Emergency medicine | ED visit levels (99281-99285) chosen by time, which doesn’t apply to ED codes | Level ED visits by MDM only |
Example: One Visit, Three Coding Errors
A patient with type 2 diabetes comes in for a diabetes follow-up and also complains of right knee pain. The provider manages the diabetes, documents hyperglycemia, examines the knee and gives a joint injection (20610). The claim goes out with three errors:
- E11.9 is billed instead of E11.65, so the diabetes code understates what was documented.
- M25.569 (unspecified knee) is billed instead of M25.561 (right knee).
- The office visit is billed without modifier 25, so the payer bundles it into the injection and pays only the procedure.
Corrected, the claim carries E11.65 and M25.561, and the significant, separately documented diabetes visit is billed with modifier 25 alongside 20610. One claim, three small fixes, and the difference between a partial payment and full, defensible reimbursement.
How Coding Errors Show Up as Denials
Medical billing and coding errors usually come back on the remittance as a claim adjustment reason code (CARC). Learning the common ones makes it faster to trace a denial back to the coding problem behind it.
| Coding error | Typical denial code | What it means |
|---|---|---|
| Missing or wrong modifier | CO-4 | The procedure code is inconsistent with the modifier used |
| Diagnosis doesn’t support the service | CO-11 | The diagnosis is inconsistent with the procedure |
| Unbundled service | CO-97 | The service is included in the payment for another service |
| Deleted or invalid code | CO-181 (procedure) or CO-146 (diagnosis) | The code was invalid on the date of service |
| Level not supported | CO-150 | The information submitted doesn’t support this level of service |
Our guides to the CO-11 denial code and the CO-97 denial code show how to correct and prevent two of the most common coding-related denials.
Undercoding: The Error Nobody Reports
Most coding error lists focus on overcoding because it creates compliance risk. Undercoding gets less attention because payers rarely complain about being billed too little, yet it is one of the most common medical coding mistakes in busy practices. It happens when providers pick a lower E/M level to feel safe, forget add-on codes, skip a separately billable procedure, or leave chronic conditions out of the diagnosis list.
The cost adds up quietly. A practice that consistently bills one E/M level lower than its documentation supports loses the difference on every visit, every day, and no denial ever flags it. The fix is the same as for overcoding: code to the documentation, not to habit, and use audits to measure both directions.
What Coding Errors Cost a Practice
Coding errors cost money in four ways:
- Denials and rework. Each denied line has to be researched, corrected and resubmitted or appealed.
- Lost revenue. Undercoded visits and missed add-on codes are paid at a lower rate, and the difference is rarely recovered.
- Repayments. Overcoding found in a payer audit leads to recoupment, and for Medicare and Medicaid, identified overpayments must be reported and returned within 60 days of identification.
- Audit exposure. Repeated patterns, such as unusually high E/M levels or heavy modifier 59 use, increase the chance of payer reviews.
How to Prevent Medical Coding Errors: 8 Steps
- Build documentation templates. Prompt providers for MDM elements, time, laterality, wound depth and other details each specialty needs.
- Update code sets on schedule. Load new CPT and HCPCS codes each January and ICD-10-CM each October, and remove deleted codes from charge masters and favorites lists.
- Use a claim scrubber. Run NCCI, medically unlikely edits, payer-specific and medical necessity checks before every submission.
- Code to the highest specificity. Avoid unspecified codes when the record supports a more specific one, and query the provider when it doesn’t.
- Set modifier rules. Write down when modifiers 25, 59 and the X modifiers may be used, and review every claim that uses them.
- Track denials by reason. Group coding denials by CARC, provider and code to find patterns quickly.
- Audit regularly. Review a sample of claims per provider each quarter, and run focused audits on high-risk codes.
- Train with real examples. Share audit findings with providers and coders, using their own claims, and repeat after code-set changes.
Practices without in-house certified coders can use our medical coding services, and an independent review from our medical coding audit services shows exactly where errors are happening.
Want to know which coding errors are costing your practice? Steady Medical Billing will review a sample of your claims and show you the errors, the dollar impact and the fixes, at no cost. Request your free RCM audit.
Frequently Asked Questions
What are the most common medical coding errors?
The most common errors are unspecified diagnosis codes, E/M levels that don’t match the documentation, modifier misuse, unbundling, wrong units, missing laterality or 7th characters, outdated codes, and diagnoses that don’t support the procedure.
What is the difference between upcoding and undercoding?
Upcoding is billing a higher level or more expensive service than the documentation supports, which creates overpayment and audit risk. Undercoding is billing a lower level than the documentation supports, which loses revenue the practice earned. Downcoding usually describes a payer lowering the level it pays.
How do coding errors affect reimbursement?
They cause denials and rework, lower payment when services are undercoded, repayment when overcoding is found, and a higher chance of payer audits when error patterns repeat.
Why are unspecified ICD-10 codes a problem?
Unspecified codes can fail payer medical necessity edits and don’t show how complex the patient is. If the record supports a more specific code, such as E11.65 instead of E11.9 for type 2 diabetes with hyperglycemia, use it.
How can a practice prevent medical coding errors?
Use documentation templates, update code sets each January and October, run a claim scrubber with NCCI and payer edits, set modifier rules, track denials by reason, audit claims each quarter and train providers and coders on the findings.
How often should coding be audited?
A common cadence is auditing a sample of claims per provider each quarter, with focused audits when denials rise, a new provider or service starts, or code sets change.
