Top 10 Most Commonly Miscoded HCCs

Introduction: Why HCC Coding Errors Matter

HCC coding errors are not just academic concerns — they carry real financial and regulatory consequences. When conditions are miscoded, health plans either receive too much or too little reimbursement for a patient's care. Overcoding triggers Risk Adjustment Data Validation audit findings that can result in extrapolated payment recoveries across an entire plan's membership. Undercoding means the plan is not funded adequately to care for its sickest patients.

CMS has intensified its Risk Adjustment Data Validation audit program, and the coding accuracy bar continues to rise. For medical coders working in risk adjustment, understanding the most common HCC coding mistakes is essential for audit defensibility, accurate risk scores, and career longevity in the field.

This guide covers the 10 most frequently miscoded HCC categories based on patterns observed in risk adjustment audits and quality reviews. For each, we explain what goes wrong, why it happens, and how to code it correctly. If you are new to HCC coding, start with our beginner's guide to HCC coding before diving in.

1. Unspecified Diabetes Coded as Complicated (HCC 37)

The error: Coding E11.65 (Type 2 diabetes mellitus with hyperglycemia) when the documentation only states "uncontrolled diabetes" or "poorly controlled diabetes" without explicitly documenting hyperglycemia as a clinical finding.

Why it happens: Many coders assume that "uncontrolled" is synonymous with "hyperglycemia." While uncontrolled diabetes often involves hyperglycemia, the ICD-10-CM coding guidelines require that the specific complication — hyperglycemia, in this case — be explicitly documented by the provider. The word "uncontrolled" alone, without a specified complication, defaults to E11.65 per Coding Clinic guidance, but auditors increasingly scrutinize whether the provider actually assessed and documented hyperglycemia versus simply writing "uncontrolled."

How to code it correctly: Look for explicit documentation of hyperglycemia, including lab values (blood glucose above normal range), provider assessment of hyperglycemic state, or treatment adjustments specifically targeting hyperglycemia. If documentation says only "uncontrolled diabetes" without further specification, consider a query to the provider to clarify the nature of the lack of control. Search E11.65 in HCC Buddy to see the full HCC mapping and coding guidance.

Audit impact: Overcoding HCC 37 is one of the most common Risk Adjustment Data Validation audit findings. The financial impact is significant because HCC 37 carries a substantial Risk Adjustment Factor weight.

2. Heart Failure Without Acuity Specification (HCC 221)

The error: Defaulting to I50.9 (heart failure, unspecified) when the clinical documentation contains enough detail to support a more specific code. In V28, I50.9 maps to a lower-weighted HCC than the specified codes for systolic, diastolic, or combined heart failure with acuity.

Why it happens: Provider documentation frequently states "heart failure" or "congestive heart failure" without specifying whether it is systolic (heart failure with reduced ejection fraction), diastolic (heart failure with preserved ejection fraction), or combined. Without the type and acuity (acute, chronic, acute-on-chronic), the coder is forced to use the unspecified code.

How to code it correctly: Review echocardiogram reports for ejection fraction data. Query the provider for heart failure type and acuity when documentation is incomplete. Codes like I50.22 (chronic systolic heart failure), I50.32 (chronic diastolic heart failure), and I50.42 (chronic combined systolic and diastolic heart failure) map to HCC 221 and carry significantly more specificity for audit purposes.

Audit impact: Using I50.9 when more specific documentation exists represents a missed HCC capture opportunity. Auditors also flag cases where I50.9 is used but the chart contains echocardiogram data that would support a specific code.

3. Chronic Kidney Disease Stage Miscoding (HCC 326/329)

The error: Coding the wrong stage of Chronic Kidney Disease, or coding Chronic Kidney Disease without a stage designation. In V28, Chronic Kidney Disease Stage 3 maps to HCC 329, while Stages 4, 5, and end-stage renal disease map to HCC 326 — different categories with different weights.

Why it happens: Lab values (specifically the glomerular filtration rate) fluctuate between visits. A patient whose glomerular filtration rate was 28 (Stage 4) at the last visit might be 32 (Stage 3b) at the current visit. Coders sometimes use the staging from a previous encounter rather than the staging documented at the current encounter.

How to code it correctly: Always code the stage documented by the provider at the current encounter. If the provider documents Chronic Kidney Disease without staging, check the lab values in the chart and query the provider to confirm the stage. The ICD-10-CM codes are: N18.1 (Stage 1), N18.2 (Stage 2), N18.30-N18.32 (Stage 3), N18.4 (Stage 4), N18.5 (Stage 5), N18.6 (end-stage renal disease). Use HCC Buddy's encoder to verify which stages map to which HCCs.

Audit impact: Coding a higher stage than what is documented at the current encounter is an overcoding finding. Coding a lower stage or no stage is a missed HCC opportunity.

4. Body Mass Index Without Morbid Obesity Diagnosis (HCC 328)

The error: Coding only the Body Mass Index Z code (Z68.41 through Z68.45 for Body Mass Index 40 and above) without the E66.01 (morbid obesity due to excess calories) diagnosis code. The Body Mass Index code alone does not map to an HCC.

Why it happens: The Body Mass Index is almost always documented because it is calculated automatically from height and weight measurements. However, a Body Mass Index of 40 or higher does not automatically establish a diagnosis of morbid obesity. The provider must explicitly diagnose morbid obesity — the Body Mass Index code is a supplementary code that provides additional data.

How to code it correctly: E66.01 must be documented by the provider as a diagnosis, not assumed from the Body Mass Index value. If the Body Mass Index is 40+ but the provider has not documented morbid obesity, a clinical documentation improvement query is appropriate. When E66.01 is supported, code it alongside the appropriate Z68.4x code for the specific Body Mass Index value.

Audit impact: Coding E66.01 based solely on an elevated Body Mass Index without a provider diagnosis is an overcoding finding. Failing to query when the Body Mass Index clearly supports the diagnosis is a missed opportunity.

5. History-of Cancer Coded as Active (HCC 17-24)

The error: Coding an active malignancy C-code when the patient is actually in remission, under surveillance, or has completed treatment. Active cancer codes map to cancer HCCs (categories 17 through 24 in V28) with high Risk Adjustment Factor weights, while "personal history of" Z85.x codes do not map to any HCC.

Why it happens: Problem lists in electronic health records frequently carry forward active cancer diagnoses from prior encounters. A patient diagnosed with colon cancer in 2023 who completed treatment and is now in surveillance may still have "C18.9 — Malignant neoplasm of colon, unspecified" on their active problem list in 2026. Coders who code from the problem list without reviewing the current encounter context will overcapture.

How to code it correctly: Determine whether the cancer is currently being actively treated (surgery, chemotherapy, radiation, immunotherapy) or whether the patient is in remission or surveillance. If the patient is in remission, the appropriate code is the "in remission" code if available, or the Z85.x personal history code. Only code active cancer if the provider documents ongoing disease or active treatment at the current encounter. Refer to our MEAT criteria guide for documentation requirements.

Audit impact: Active cancer overcoding is a high-dollar audit finding because cancer HCCs carry some of the highest Risk Adjustment Factor weights in the model. Risk Adjustment Data Validation auditors specifically target cancer codes.

6. Major Depression Single Episode vs. Recurrent (HCC 155)

The error: Coding major depressive disorder as single episode (F32.x) when the patient's history clearly indicates recurrent episodes (F33.x), or vice versa. While both map to HCC 155, the specificity matters for audit accuracy and clinical integrity.

Why it happens: Provider documentation often says simply "major depression" or "major depressive disorder" without specifying whether it is a single episode or recurrent. Coders must then make a judgment call or query the provider. Many coders default to single episode when uncertain, but most patients presenting with major depression in an ongoing treatment context have recurrent disease.

How to code it correctly: Review the patient's medication history and treatment timeline. If the patient has been treated for depression previously and is presenting with a new or ongoing episode, F33.x (recurrent) is almost certainly the appropriate code. If this is genuinely the first lifetime episode, F32.x (single episode) applies. Severity (mild, moderate, severe, with or without psychotic features) must also be specified. Query the provider if documentation is ambiguous.

Audit impact: While the HCC mapping is the same for both, auditors review the clinical accuracy of the episode designation. Consistent miscoding of episode type raises concerns about overall coding quality.

7. Vascular Disease Specificity Errors (HCC 237/238)

The error: Coding I25.10 (atherosclerotic heart disease of native coronary artery without angina pectoris) when documentation supports a more specific code — such as a code specifying the type of vessel (native vs. bypass graft), presence or absence of angina, or specific artery involvement.

Why it happens: Cardiologists and primary care providers often document "coronary artery disease" or "atherosclerotic heart disease" without specifying native vs. graft vessel, angina status, or specific artery. Coders default to I25.10 as the most common general atherosclerotic heart disease code.

How to code it correctly: Review prior cardiac catheterization reports, surgical history (bypass surgery indicates graft vessels), and current documentation for chest pain or angina mentions. If the patient has a history of coronary artery bypass grafting, the code should reflect bypass graft vessel disease (I25.7xx series), not native vessel disease. Use the ICD-10-CM code search at hccbuddy.com/encoder to explore the full range of I25 codes and their HCC mappings.

Audit impact: Specificity determines audit defensibility. I25.10 is technically correct when documentation is truly unspecified, but auditors may question why further specificity was not pursued through queries or chart review.

8. Chronic Obstructive Pulmonary Disease Without Exacerbation Status (HCC 280)

The error: Coding J44.9 (Chronic Obstructive Pulmonary Disease, unspecified) instead of J44.1 (Chronic Obstructive Pulmonary Disease with acute exacerbation) when an exacerbation is clearly documented in the encounter note.

Why it happens: Coders may not recognize clinical language that describes an exacerbation. Phrases like "worsening shortness of breath," "increased sputum production," "flare of Chronic Obstructive Pulmonary Disease," or "stepped up bronchodilator therapy" may all indicate an exacerbation, but coders who are looking only for the literal word "exacerbation" will miss these.

How to code it correctly: Both J44.1 and J44.9 map to the same HCC category, but audit accuracy requires using the most specific code supported by documentation. Look for clinical indicators of exacerbation: change in baseline symptoms, increased use of rescue medications, emergency department visits, or provider statements about worsening disease. J44.0 applies when the exacerbation is accompanied by a lower respiratory infection.

Audit impact: While the HCC mapping does not change, consistent use of unspecified codes when specificity is available erodes auditor confidence in overall coding quality and may trigger expanded reviews.

9. Protein-Calorie Malnutrition Severity (HCC 21)

The error: Coding unspecified protein-calorie malnutrition (E46) when documentation supports severe (E43) or moderate (E44.0) malnutrition. Severity directly determines the HCC category and Risk Adjustment Factor weight.

Why it happens: Malnutrition assessments are frequently performed by dietitians, and their documentation may use clinical terminology (such as Subjective Global Assessment ratings) that does not directly align with ICD-10-CM severity classifications. Coders who are unfamiliar with the crosswalk between nutritional assessment tools and ICD-10 severity designations may default to the unspecified code.

How to code it correctly: Severe malnutrition (E43) corresponds to conditions like kwashiorkor, nutritional marasmus, or severe protein-calorie malnutrition as documented by the treating provider. Moderate malnutrition (E44.0) corresponds to moderate protein-calorie malnutrition. The dietitian's assessment can support the diagnosis, but the treating physician or qualified provider must document the diagnosis for it to be coded. If the dietitian documents "severe malnutrition" but the physician only documents "malnutrition," query for severity clarification.

Audit impact: Severity determines the HCC tier. Coding severe malnutrition without documentation supporting that severity level is an overcoding finding. Coding unspecified when severity is documented is a missed opportunity.

10. Drug and Alcohol Use vs. Dependence vs. Remission

The error: Coding substance "use" (F1x.10) when documentation supports "dependence" (F1x.20), or coding active dependence when the patient is actually in remission (F1x.21). In V28, only dependence codes typically map to the substance use disorder HCC categories.

Why it happens: Clinical documentation frequently uses "use," "abuse," and "dependence" interchangeably, but ICD-10-CM draws clear distinctions. The term "abuse" was deprecated in ICD-10-CM. "Use" implies a pattern of use without meeting dependence criteria. "Dependence" implies physiological or psychological dependence with defined clinical criteria. Coders who code the term used by the provider without evaluating the clinical context may select the wrong category.

How to code it correctly: Review the clinical evidence for dependence criteria: tolerance, withdrawal symptoms, inability to cut down, continued use despite harm. If these are documented, "dependence" is the appropriate code even if the provider writes "use" or "abuse." If the patient is in a maintenance program (methadone, buprenorphine) or has completed treatment and is in sustained remission, the "in remission" codes apply. Confirm with the provider when documentation is ambiguous. See our V28 changes guide for details on the new substance use disorder HCCs.

Audit impact: Coding dependence when documentation only supports use is overcoding. Coding use when dependence is documented is undercoding and a missed HCC capture under V28.

How to Prevent These Errors

Preventing HCC coding errors requires a combination of better tools, better processes, and continuous education:

Conclusion

The 10 coding errors above represent the most common patterns found in risk adjustment audits. The through-line across all of them is the same: defaulting to unspecified or less specific codes when documentation supports — or could support with a query — a more accurate, more specific code.

Accurate HCC coding protects your organization from Risk Adjustment Data Validation audit risk, ensures patients receive appropriate care funding, and demonstrates professional coding competency. Every code you assign should be defensible on audit with the documentation in the chart.

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Free resource: Download the HCC Coding Cheat Sheet — a printable V28 quick reference with top HCC categories and documentation tips.