Top 10 ICD-10-CM Coding Errors and How to Prevent Them

The Top 10 ICD-10-CM Coding Errors costing healthcare organizations the most revenue in 2026 are unspecified code overuse, missing 7th-character extensions, upcoding/undercoding, sequencing mistakes, mismatched laterality, outdated code use after annual updates, missing Excludes1/Excludes2 checks, incomplete documentation-to-code linkage, modifier-coding conflicts, and failure to apply new payer-specific edits. Each one triggers denials, audit exposure, or silent underpayment, and most are preventable with the right coding infrastructure in place.

Content verified and updated as of July 2026, reflecting the FY 2026 ICD-10-CM code set effective October 1, 2025, and the April 1, 2026 mid-year instructional note update.

If your denial rate has crept up since October, you are not imagining it. CMS added 487 new diagnosis codes, revised 38, and deleted 28 for FY 2026, and coders still working from muscle memory are the ones absorbing the impact (CMS FY 2026 ICD-10-CM release). Below are the ten mistakes we see most often when auditing incoming coding work, and what actually fixes them.

We built this list from real denial patterns rather than a generic industry checklist, because the Top 10 ICD-10-CM Coding Errors we see repeated in live audits rarely match what most coding guides describe.

The Top 10 ICD-10-CM Coding Errors Draining Revenue Right Now

1. Defaulting to unspecified codes.

When documentation supports a specific diagnosis but the coder selects the unspecified variant anyway, payers increasingly flag the claim for medical necessity review. This habit is now easier to break: FY 2026 added 16 new R-codes just for abdominal, pelvic, and perineal pain specificity, closing the exact gap coders used to default around (AAPC, FY 2026 update summary).

2. Missing 7th-character extensions.

Fracture, injury, and encounter-type codes require a 7th character (initial, subsequent, sequela). Dropping it or defaulting to “A” for a follow-up visit generates an automatic edit rejection at the clearinghouse level.

3. Upcoding or undercoding severity.

Coding a condition more or less severe than what the chart supports is the single fastest path to an OIG audit letter. The OIG’s own toolkit for identifying high-risk diagnosis codes exists specifically because this pattern recurs across Medicare Advantage submissions (OIG high-risk diagnosis code toolkit).

4. Sequencing errors.

Listing a manifestation code before its underlying etiology code, or reversing the required order on combination codes, is a documentation-to-code mapping failure that clean-claim scrubbing should catch before submission.

5. Laterality mismatches.

Right/left/bilateral errors are one of the more embarrassing denial reasons because they are entirely avoidable, yet they remain common in orthopedic, ophthalmology, and dermatology charts where operative notes and coding software fall out of sync.

6. Coding from last year’s book.

FY 2026 converted several parent codes into more granular families, including thyroid orbitopathy, eyelid inflammation, and non-pressure chronic ulcers by severity and site. Coders still billing the old parent code are generating automatic rejections the moment the payer’s edit engine updates (FY 2026 ICD-10-CM Official Guidelines, CMS).

7. Ignoring Excludes1 and Excludes2 notes.

This is where staying current matters most, and it’s the one place the code count doesn’t tell the whole story.

The April 1, 2026 mid-year update added zero new diagnosis codes, but it converted 16 Excludes1 notes to Excludes2 across chapters like circulatory disease, neurodevelopmental disorders, and vitamin deficiency anemia, meaning code pairs that were previously mutually exclusive can now be reported together (AAPC, April 2026 ICD-10-CM update). A coder relying on the October tabular list alone will still apply a restriction that no longer exists as of April 1.

8. Documentation that doesn’t support the code.

This remains the largest driver of improper payments industry-wide. Of the FY 2025 Medicaid improper payments nationally, more than three-quarters were traced to insufficient documentation rather than fraud (CMS FY 2025 Improper Payments Fact Sheet).

9. Modifier-code conflicts.

Pairing a diagnosis code with a procedure modifier that contradicts it (for example, a laterality modifier that doesn’t match the ICD-10-CM laterality already coded) creates a red flag that many payer systems now catch automatically before a human ever reviews the claim.

10. Missing new SDoH and risk-adjustment codes.

FY 2026 expanded social determinants of health codes significantly, including new financial insecurity and war-exposure categories. Practices not capturing these are leaving risk-adjustment and quality-reporting revenue on the table entirely.

Key FY 2026 ICD-10-CM Codes Coders Need to Know

Several of the errors above trace directly back to specific code-family changes CMS made effective October 1, 2025. This is not an exhaustive list of all 487 additions, but it covers the families most likely to trip up a coding team still working from the FY 2025 book (CMS FY 2026 Official Guidelines; AAPC FY 2026 update summary).

Always confirm the full child-code list in the CMS FY 2026 Tabular List before finalizing a claim. Several of the codes above are parent codes only and cannot be billed without a valid child extension.

Why This Is Costing More in 2026 Than in Past Years

Every one of the Top 10 ICD-10-CM Coding Errors above compounds when left unchecked, and the math backs this up at the national level.

The FY 2025 Medicare Fee-for-Service improper payment rate came in at 6.55%, or $28.83 billion, down from the prior year but still substantial, and coding and documentation failures remain a primary driver across nearly every service category CMS tracks (CMS CERT Program report).

Lab tests alone accounted for over $1.3 billion in projected improper payments in the same reporting period, with roughly 80% of that tied to documentation gaps rather than intentional fraud (HHS/AAPC coverage of the 2025 CERT supplemental data). Add 487 new codes and 38 revisions into that mix, and a coding team without a structured update process is coding blind for months at a time.

How to Prevent These Errors Going Forward

Fixing the Top 10 ICD-10-CM Coding Errors one claim at a time does not scale. What works is pairing certified coders with a revenue integrity partner that treats every CMS code cycle as a required system update, not an optional training session.

That means dual-layer review on high-risk diagnosis families, real-time edit checks before a claim ever leaves the building, and a documentation feedback loop back to providers so the chart supports the code the first time.

This is the core of what proper medical billing services and Revenue Cycle Management should deliver: not just claim submission, but active prevention of the ten errors above before they ever reach a payer.

Organizations that treat medical billing and coding services as a compliance function, not an afterthought, consistently see fewer denials and faster reimbursement across every specialty.

Our full RCM Services and Medical Coding Services offerings are built around exactly this kind of update-cycle discipline, applied to every ICD 10 Codes change CMS releases.

If you want a clear view of what dedicated coding oversight costs against what these errors are already costing you, our medical billing pricing plans break down exactly where that investment goes.

Ready to stop losing revenue to preventable coding errors?

Call us at 888-357-3226 or email info@medicalbillersandcoders.com to schedule a coding accuracy review for your organization.

FAQs on the Top 10 ICD-10-CM Coding Errors

Alex Peter

A Medical Coding Subject Matter Expert with over 16 years of experience in ICD-10 and CPT coding, clinical documentation, and revenue cycle management. Shares actionable insights to improve billing accuracy and support compliance-driven healthcare practices.