C4A.9 ICD-10-CM Code: Merkel cell carcinoma, unspecified
C4A.9 maps to CMS-HCC V28 21. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · free HCC coding tools
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FY 2026 Apr update / Neoplasms (C00-D49) / Melanoma and other malignant neoplasms of skin (C43-C44)
C4A.9
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMerkel cell carcinoma, unspecified
A rare aggressive skin cancer (Merkel cell carcinoma) where the specific location on the body is not documented or specified.
Buddy Insight
Merkel cell carcinoma, unspecified indicates this aggressive neuroendocrine skin malignancy without documentation of the anatomical site.
CMS-HCC V28
MappedHCC 21
Coefficient HCC 21: 0.671 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 012
Code-level coefficient reference
ESRD/PACE
MappedHCC 12
Code-level coefficient reference
RXHCC
MappedHCC 22
Code-level coefficient reference
Code Book Path
Inclusion Terms
Official- Merkel cell carcinoma of unspecified site
- Merkel cell carcinoma NOS
Excludes 2
OfficialNo Excludes 2 notes are included in this display for C4A.9. Check the code and parent instructions in the Code Book.
Related Codes
Includes
OfficialNo Includes notes are included in this display for C4A.9. Check the code and parent instructions in the Code Book.
Excludes 1
OfficialNo Excludes 1 notes are included in this display for C4A.9. Check the code and parent instructions in the Code Book.
Code First
OfficialNo Code First sequencing instructions are included in this display for C4A.9. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for C4A.9. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for C4A.9. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →
Is C4A.9 an HCC code?
Yes. C4A.9 (Merkel cell carcinoma, unspecified) maps to HCC 21, Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.671. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: C4A.9 is billable and maps to V28 HCC 21, Lymphoma and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- C4A.9
- Description
- Merkel cell carcinoma, unspecified
- HCC (V28)
- HCC 21 — Lymphoma and Other Cancers
- RAF reference coefficient
- 0.671
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work C4A.9 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for C4A.9
For C4A.9, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Coder workflow notes
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What This Code Means
C4A.9 is the ICD-10-CM diagnosis code for merkel cell carcinoma, unspecified. A rare aggressive skin cancer (Merkel cell carcinoma) where the specific location on the body is not documented or specified. C4A.9 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering melanoma and other malignant neoplasms of skin (c43-c44).
Under the CMS-HCC V28 risk adjustment model, C4A.9 maps to Lymphoma and Other Cancers (HCC 21) with a source-labeled community, non-dual, aged reference coefficient of 0.671. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
This is a default code used only when the anatomical site cannot be determined from documentation. For C4A.9, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, source-labeled coefficient references, and MEAT documentation criteria for C4A.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
Clinical Significance
Merkel cell carcinoma, unspecified indicates this aggressive neuroendocrine skin malignancy without documentation of the anatomical site. This is a default code that should prompt a query to the provider for site-specific information. Merkel cell carcinoma requires aggressive treatment regardless of site, making accurate documentation essential for treatment planning and cancer registry reporting.
Documentation Requirements
- ✓Histopathologic confirmation of Merkel cell carcinoma
- ✓Provider query for specific anatomical site — this code should be used only as a last resort
- ✓If site cannot be determined, document why (e.g., unknown primary, records unavailable)
- ✓Treatment plan documentation
- ✓Staging information if available
- ✓Any immunosuppression history (organ transplant, HIV, chronic lymphocytic leukemia)
Commonly Confused Codes
- •C4A.0-C4A.7: Site-specific Merkel cell carcinoma codes; always preferred over unspecified
- •C4A.8: Merkel cell carcinoma of overlapping sites; use when multiple contiguous sites are known
- •C80.1: Malignant (primary) neoplasm, unspecified; even more nonspecific: at least C4A.9 identifies the histology
- •C7A.8: Other malignant neuroendocrine tumors; for non-cutaneous neuroendocrine tumors, not Merkel cell carcinoma of skin