2027 ICD-10-CM Diagnosis Code C4A.31Merkel cell carcinoma of nose
ICD-10-CM Codes›C00–D49›C43-C44›C4A
- Billable — Valid for Submission
- Risk Adjusts — HCC 21
- Chronic Condition
C4A.31 is a billable ICD-10-CM diagnosis code for merkel cell carcinoma of nose. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 595 through 596. Coders also document this condition as primary malignant neoplasm of skin of nose. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Skin cancers - all other types.
For Medicare Advantage risk adjustment, C4A.31 maps to CMS-HCC Category 21 (Lymphoma and Other Cancers) under the V28 model, adding a risk factor of about 0.671 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
C4A.31 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.
Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Primary malignant neoplasm of skin of nose
- Primary Merkel cell carcinoma of skin of nose
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to C4A.31: its own notes plus those printed at Chapter 2. A note printed at a category, block or chapter applies to every code under it.
Notes
General instructions on how these codes are used.
- Functional activity
- All neoplasms are classified in this chapter, whether they are functionally active or not. An additional code from Chapter 4 may be used, to identify functional activity associated with any neoplasm.
- Morphology [Histology]
- Chapter 2 classifies neoplasms primarily by site (topography), with broad groupings for behavior, malignant, in situ, benign, etc. The Table of Neoplasms should be used to identify the correct topography code. In a few cases, such as for malignant melanoma and certain neuroendocrine tumors, the morphology (histologic type) is included in the category and codes.
- Primary malignant neoplasms overlapping site boundaries
- A primary malignant neoplasm that overlaps two or more contiguous (next to each other) sites should be classified to the subcategory/code .8 ('overlapping lesion'), unless the combination is specifically indexed elsewhere. For multiple neoplasms of the same site that are not contiguous, such as tumors in different quadrants of the same breast, codes for each site should be assigned.
- Malignant neoplasm of ectopic tissue
- Malignant neoplasms of ectopic tissue are to be coded to the site mentioned, e.g., ectopic pancreatic malignant neoplasms are coded to pancreas, unspecified (C25.9).
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name C4A.31, its category, or a range that includes it.
Applicable To 1
These notes mention C4A.31.
Excludes1 1
These codes carry an Excludes1 note naming C4A.31: they are not reported together with it, unless the two conditions are unrelated.
Excludes2 2
These codes carry an Excludes2 note naming C4A.31: its condition is not part of those codes, and both may be reported when the patient has both.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Carcinoma (malignant) See Also: Neoplasm, by site, malignant;
nose C4A.31
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Carcinoma, Merkel Cell
a carcinoma arising from merkel cells located in the basal layer of the epidermis and occurring most commonly as a primary neuroendocrine carcinoma of the skin. merkel cells are tactile cells of neuroectodermal origin and histologically show neurosecretory granules. the skin of the head and neck are a common site of merkel cell carcinoma, occurring generally in elderly patients. (holland et al., cancer medicine, 3d ed, p1245)
Patient EducationClinical
Skin Cancer
Skin cancer is the most common form of cancer in the United States. The two most common types are basal cell cancer and squamous cell cancer. They usually form on the head, face, neck, hands, and arms. Another type of skin cancer, melanoma, is more dangerous but less common.
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert C4A.31 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About C4A.31Overview
What is the ICD-10 code for merkel cell carcinoma of nose?
The ICD-10-CM code for merkel cell carcinoma of nose is C4A.31 (sometimes written as C4A31). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is C4A.31 (Merkel cell carcinoma of other and unspecified parts of face) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report merkel cell carcinoma of nose on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does C4A.31 group to?
When merkel cell carcinoma of nose is the principal diagnosis on an inpatient stay, it groups to MS-DRG 596, 595, with relative weights from 1.1067 to 2.0230 depending on complications. Higher weights mean higher Medicare reimbursement.
What is the ICD-9 equivalent of C4A.31?
Under the General Equivalence Mappings, merkel cell carcinoma of nose converts to ICD-9-CM 209.31 (merkel cell ca-face). The mapping is approximate, so confirm the match fits the documentation.
What HCC is C4A.31?
C4A.31 (merkel cell carcinoma of nose) maps to CMS-HCC Category 21 (Lymphoma and Other Cancers), commonly written as HCC 21, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 12 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 22.
Does C4A.31 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, C4A.31 adds a risk adjustment factor of about 0.671 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.493 to 0.739 depending on the payment segment). A more severe related category (HCC 17, HCC 18, HCC 19, and HCC 20) supersedes it when both are reported. See the full factor table on the HCC 21 category page.
