C4A.9
BillableMerkel cell carcinoma, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.9 an HCC code?
Yes. C4A.9 maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Breast, Prostate, and Other Cancers and Tumors under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for C4A.9
For C4A.9to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically — it has to be re-documented and supported each calendar year.
- 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
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed C4A.9 during that encounter — not just copy-forwarded from a problem list.
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 community, non-dual, aged base RAF weight of 0.671. Under the older CMS-HCC V24 model, C4A.9 maps to Breast, Prostate, and Other Cancers and Tumors (HCC 12) with a community, non-dual, aged base RAF weight of 0.150. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
This is a default code used only when the anatomical site cannot be determined from documentation. Because C4A.9 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. 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, RAF weights, 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
- •This is a default code used only when the anatomical site cannot be determined from documentation
- •Query the provider to obtain site-specific information, as more specific codes (C4A.0-C4A.8) are preferred
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