2026 ICD-10-CM Diagnosis Code C4A.51Merkel cell carcinoma of anal skin

ICD-10-CM Codes›C00–D49›C43-C44›C4A

ICD-10-CM C4A.51
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

C4A.51 is a billable ICD-10-CM diagnosis code for merkel cell carcinoma of anal skin. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 595 through 596. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Skin cancers - all other types.

Code Identity

ICD-10-CM Code
C4A.51
Billable Status
Yes — Valid for Submission
Code Describes
Merkel cell carcinoma of anal skin
Short Description
Merkel cell carcinoma of anal skin
Same as the full description in the CMS dataset.
Parent Code
C4A.5
Merkel cell carcinoma of trunk

Code Classification

ChapterC00–D49Neoplasms
SectionC43-C44Melanoma and other malignant neoplasms of skin
CategoryC4AMerkel cell carcinoma
This CodeC4A.51Merkel cell carcinoma of anal skin

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Merkel cell carcinoma of anal margin
  • Merkel cell carcinoma of perianal skin

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Carcinoma(malignant)
      • Merkel cell
        • anal margin
    • Carcinoma(malignant)
      • Merkel cell
        • anal skin
    • Carcinoma(malignant)
      • Merkel cell
        • perianal skin

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NEO028
Skin cancers - all other types
Default principal diagnosis: inpatient Yes · outpatient Yes

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.51 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
209.35 Merkel cell ca-trunk
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About C4A.51Overview

Is C4A.51 (Merkel cell carcinoma of trunk) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report merkel cell carcinoma of anal skin on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does C4A.51 group to?

When merkel cell carcinoma of anal skin is the principal diagnosis on an inpatient stay, it groups to MS-DRG 595, 596, with relative weights from 1.0825 to 2.1207 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of C4A.51?

Under the General Equivalence Mappings, merkel cell carcinoma of anal skin converts to ICD-9-CM 209.35 (merkel cell ca-trunk). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.