C4A.31
BillableMerkel cell carcinoma of nose
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.31 an HCC code?
Yes. C4A.31 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors 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.31
For C4A.31 to 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.31 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C4A.31 is the ICD-10-CM diagnosis code for merkel cell carcinoma of nose. A rare aggressive skin cancer (Merkel cell carcinoma) that develops on the nose. C4A.31 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.31 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors (HCC 21) with a community, non-dual, aged base RAF weight of 0.545. Under the older CMS-HCC V24 model, C4A.31 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 code specifically indicates nasal location; ensure documentation confirms the tumor is on the nose rather than another facial site. Because C4A.31 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.31 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code specifically indicates nasal location; ensure documentation confirms the tumor is on the nose rather than another facial site
- •This is a more specific code than C4A.30 and should be used when nasal involvement is documented
Clinical Significance
Merkel cell carcinoma of the nose is a rare but aggressive neuroendocrine skin cancer in a highly sun-exposed, cosmetically sensitive location. Nasal Merkel cell carcinoma requires careful surgical planning due to functional and aesthetic considerations, and its central facial location provides access to bilateral lymphatic drainage, increasing the risk of nodal metastasis.
Documentation Requirements
- ✓Histopathologic confirmation of Merkel cell carcinoma with immunohistochemistry panels
- ✓Specific nasal subsite (dorsum, tip, ala, bridge, sidewall)
- ✓Tumor dimensions and depth of invasion
- ✓Surgical margin status — Mohs micrographic surgery is often preferred for nasal lesions
- ✓Regional lymph node evaluation (cervical, submandibular)
- ✓Merkel cell polyomavirus testing results if obtained
- ✓Current treatment phase (active treatment, adjuvant radiation, surveillance)
Commonly Confused Codes
- •C4A.30 — Merkel cell carcinoma of unspecified face; less specific — use C4A.31 when nose is documented
- •C4A.39 — Merkel cell carcinoma of other parts of face; for non-nasal facial sites like cheek or forehead
- •C44.301 — Basal cell carcinoma of nose skin; different histology, not Merkel cell
- •C43.31 — Melanoma of nose; entirely different tumor type
- •C30.0 — Malignant neoplasm of nasal cavity; internal nasal malignancy, not skin surface