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C4A.9 ICD-10-CM Code: Merkel cell carcinoma, unspecified

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Coding at a Glance

Tabular directives
2 inclusion terms
Risk adjustment
CMS-HCC V28: 1 category · CMS-HCC V22 category 12 · RxHCC V08 category 22

Inpatient Payment Groups (MS-DRG)

Potential MS-DRG participation — not a DRG assignment.

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v44, Appendix B.

  • MS-DRG 595 — MAJOR SKIN DISORDERS WITH MCC (MDC 09)
  • MS-DRG 596 — MAJOR SKIN DISORDERS WITHOUT MCC (MDC 09)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Risk Adjustment (CMS-HCC)

Payment categories this diagnosis maps to under CMS-HCC V28, payment year 2026.

  • HCC 21 — Lymphoma and Other Cancers (supersedes HCC 22, HCC 23)

Other models: CMS-HCC V22 HCC 12 · RxHCC V08 HCC 22

Risk scores depend on the enrollee's full accepted diagnosis set and segment; a category mapping alone does not determine payment.

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for C4A.9 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2027Effective: October 1, 2026

Trace:FY2027 changesChange historyRelease, file and checksum

Inclusion Terms

Alternative terms the tabular list files under this code.

  • Merkel cell carcinoma of unspecified site
  • Merkel cell carcinoma NOS

Coder workflow for C4A.9

MedCoder structured workflow — derived from this code’s own official record

Before you code C4A.9

  1. Unspecified does not mean incorrect. When the record gives no greater specificity, C4A.9 may be the appropriate code. Check the record for detail that supports a more specific sibling. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

    ReviewC4A.0, C4A.1, C4A.2, C4A.4, C4A.5, C4A.6, C4A.7, C4A.8

    See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

  2. Confirm the documented site and behavior (this code’s block classifies malignant neoplasms), whether the site is primary or secondary, and whether the malignancy is current or a personal history: a previously excised malignancy with no further treatment and no evidence of disease is history, not current disease. Verify the site against the Table of Neoplasms. Neoplasm chapter guidelines (Guidelines I.C.2.a, I.C.2.d, I.C.2.m).

    See the Table of Neoplasms entries · Guide: Active cancer vs history of cancer →

Choose the right path

  1. Does the record document the detail a more specific sibling code needs?
    Yes → Review the specific siblings in this subcategory.
    No → Continue — C4A.9 is appropriate when the documentation goes no further.

    ReviewC4A.0, C4A.1, C4A.2, C4A.4, C4A.5, C4A.6, C4A.7, C4A.8

Consider C4A.9. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement
Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).
Anatomical site
At the most specific level the record states; a site the classification separates cannot be assumed from a procedure or a measurement.
Behavior and current-versus-history status
Malignant, in situ, benign or uncertain; primary or secondary; current disease or personal history (Guidelines I.C.2.m).
Any detail beyond this code’s title
What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Documentation: The provider documents the condition in the terms of this code’s title and records no further detail.

Coding question: Is a more specific sibling code supportable?

Path: Review the specific siblings in this subcategory and what each requires the record to state.

Reason: A more specific code needs documentation of the distinguishing element; without it the unspecified code is appropriate, and a provider query is the route to specificity (Guidelines I.A.9.b, I.B.18).

ReviewC4A.0, C4A.1, C4A.2, C4A.4, C4A.5, C4A.6, C4A.7, C4A.8

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Code Overview

Merkel cell carcinoma, unspecified is a billable ICD-10-CM diagnosis code (C4A.9).

MedCoder summary Summary composed by MedCoder from this code's official ICD-10-CM record. The tabular instructional notes themselves appear verbatim below.

Indexed Clinical Terms (1)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

Verify Before Coding

  • No Medicare Code Editor or MS-DRG Definitions Manual restrictions apply to this code.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name C4A.9 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes1 note: C44 — Other and unspecified malignant neoplasm of skin (via C4A.-).

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

Referenced by 2 Excludes2 notes: C43 — Malignant melanoma of skin (via C4A.-), C7A — Malignant neuroendocrine tumors (via C4A.-).

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

MS-DRG Grouper Relationships (FY2027)

Potential MS-DRG participation — not a DRG assignment.

FY2027 MS-DRG: not on the CMS CC/MCC list — as a secondary diagnosis this code does not change MS-DRG severity for that release.

Named in the grouper logic of 2 MS-DRGs: DRG 595 (MDC 09), DRG 596 (MDC 09).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):NEO028 — Skin cancers - all other types (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same CMS-HCC risk category (V28)

CMS maps these diagnoses to the same Hierarchical Condition Category (Lymphoma and Other Cancers) for risk-adjusted payment.

C4A.51 — Merkel cell carcinoma of anal skin, C4A.52 — Merkel cell carcinoma of skin of breast, C4A.59 — Merkel cell carcinoma of other part of trunk, C4A.60 — Merkel cell carcinoma of unspecified upper limb, including shoulder, C4A.61 — Merkel cell carcinoma of right upper limb, including shoulder, C4A.62 — Merkel cell carcinoma of left upper limb, including shoulder, C4A.70 — Merkel cell carcinoma of unspecified lower limb, including hip, C4A.71 — Merkel cell carcinoma of right lower limb, including hip, C4A.72 — Merkel cell carcinoma of left lower limb, including hip, C4A.8 — Merkel cell carcinoma of overlapping sites, C7A.00 — Malignant carcinoid tumor of unspecified site, C7A.010 — Malignant carcinoid tumor of the duodenum, C7A.011 — Malignant carcinoid tumor of the jejunum, C7A.012 — Malignant carcinoid tumor of the ileum, C7A.019 — Malignant carcinoid tumor of the small intestine, unspecified portion, C7A.020 — Malignant carcinoid tumor of the appendix, C7A.021 — Malignant carcinoid tumor of the cecum, C7A.022 — Malignant carcinoid tumor of the ascending colon, C7A.023 — Malignant carcinoid tumor of the transverse colon, C7A.024 — Malignant carcinoid tumor of the descending colon, +442 more

Related risk categories

These categories interact through CMS's HCC hierarchy — one can suppress the other's risk-adjustment weight when both are present on a claim.

Cancer Metastatic to Lung, Liver, Brain, and Other Organs; Acute Myeloid Leukemia Except Promyelocytic, Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid, Myelodysplastic Syndromes, Multiple Myeloma, and Other Cancers, Lung and Other Severe Cancers, Bladder, Colorectal, and Other Cancers, Prostate, Breast, and Other Cancers and Tumors

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Skin cancers - all other types).

C4A.51 — Merkel cell carcinoma of anal skin, C4A.52 — Merkel cell carcinoma of skin of breast, C4A.59 — Merkel cell carcinoma of other part of trunk, C4A.60 — Merkel cell carcinoma of unspecified upper limb, including shoulder, C4A.61 — Merkel cell carcinoma of right upper limb, including shoulder, C4A.62 — Merkel cell carcinoma of left upper limb, including shoulder, C4A.70 — Merkel cell carcinoma of unspecified lower limb, including hip, C4A.71 — Merkel cell carcinoma of right lower limb, including hip, C4A.72 — Merkel cell carcinoma of left lower limb, including hip, C4A.8 — Merkel cell carcinoma of overlapping sites, D04.0 — Carcinoma in situ of skin of lip, D04.10 — Carcinoma in situ of skin of unspecified eyelid, including canthus, D04.11 — Carcinoma in situ of skin of right eyelid, including canthus, D04.111 — Carcinoma in situ of skin of right upper eyelid, including canthus, D04.112 — Carcinoma in situ of skin of right lower eyelid, including canthus, D04.12 — Carcinoma in situ of skin of left eyelid, including canthus, D04.121 — Carcinoma in situ of skin of left upper eyelid, including canthus, D04.122 — Carcinoma in situ of skin of left lower eyelid, including canthus, D04.20 — Carcinoma in situ of skin of unspecified ear and external auricular canal, D04.21 — Carcinoma in situ of skin of right ear and external auricular canal, +85 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main term “Carcinoma”; these codes share that main term but sit in a different category of the Tabular List.

C25.4 — Malignant neoplasm of endocrine pancreas (islet cell, pancreas), C25.9 — Malignant neoplasm of pancreas, unspecified (islet cell, with exocrine, mixed, unspecified site), C30.0 — Malignant neoplasm of nasal cavity (Schneiderian, unspecified site), C34.90 — Malignant neoplasm of unspecified part of unspecified bronchus or lung (oat cell, unspecified site), C37 — Malignant neoplasm of thymus (thymic), C41.0 — Malignant neoplasm of bones of skull and face (odontogenic, upper jaw), C41.1 — Malignant neoplasm of mandible (odontogenic), C44.29 — Other specified malignant neoplasm of skin of ear and external auricular canal (ceruminous), C44.91 — Basal cell carcinoma of skin, unspecified (basal cell), C44.99 — Other specified malignant neoplasm of skin, unspecified (apocrine, unspecified site), C50.91 — Malignant neoplasm of breast of unspecified site, female (lipid-rich), C50.92 — Malignant neoplasm of breast of unspecified site, male (lipid-rich, male), C53.9 — Malignant neoplasm of cervix uteri, unspecified (solid, microinvasive, unspecified site), C56 — Malignant neoplasm of ovary (granulosa cell), C56.9 — Malignant neoplasm of unspecified ovary (papillocystic, unspecified site), C58 — Malignant neoplasm of placenta (chorionic, unspecified site, female), C61 — Malignant neoplasm of prostate (endometrioid, unspecified site, male), C62.90 — Malignant neoplasm of unspecified testis, unspecified whether descended or undescended (Sertoli cell, unspecified site), C64 — Malignant neoplasm of kidney, except renal pelvis (renal cell), C73 — Malignant neoplasm of thyroid gland (Hurthle cell), +20 more

Lab tests where this diagnosis supports Medicare coverage (NCD)

Medicare's National Coverage Determination (NCD) program lists this diagnosis as medical justification for these lab tests.

Iron Panel

Contextual Map

Every relationship of C4A.9 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run C4A.9 with these 3 related codes in Claim Check

Hierarchy

Referenced by Excludes1 notes

Referenced by Excludes2 notes

Clinical classification (CCSR)

Risk adjustment (CMS-HCC)

  • HCC 21 — Lymphoma and Other Cancers [CMS-HCC]: “Lymphoma and Other Cancers — supersedes HCC 22 (Bladder, Colorectal, and Other Cancers), HCC 23 (Prostate, Breast, and Other Cancers and Tumors)”— CMS-HCC V28 · 2026

MS-DRG Grouper

MDC crossing

  • MDC 09 — Diseases and Disorders of the Skin, Subcutaneous Tissue and Breast[MDC crossing]: “Diseases and Disorders of the Skin, Subcutaneous Tissue and Breast — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 7,378 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2027

Index entries

  • Carcinoma (malignant), Merkel cell[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (32)

Change history

  • FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data
CMS/CDC ICD-10-CM FY2027 tabular list, index and tables, effective October 1, 2026 Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v44.0 (October 2026) Release, file and checksum · Publisher’s page
Inpatient payment groups Official source data
CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v44 Release, file and checksum · Publisher’s page
Risk adjustment Official source data
2026 Mid-Year Final ICD-10 Mappings + Model Software (cms.gov/files/zip/2026-midyear-final-icd-10-mappings.zip, 2026-midyear-final-model-software.zip) — CMS-HCC V28, PY2026 mid-year final release Release, file and checksum · Publisher’s page
Change history and date-of-service validity Official source data
CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page
Comparisons, relationships and the contextual map MedCoder structured relationship
Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears
Summary and FAQ answers MedCoder explanation
Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Cite this page

Reference this page in a research guide, syllabus or article. The release is included because code content changes each year.

MedCoder.ai. "C4A.9 — Merkel cell carcinoma, unspecified." ICD-10-CM FY2027. https://medcoder.ai/icd10/code/c4a.9-merkel-cell-carcinoma-unspecified

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Merkel cell carcinoma, unspecified

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027, and none are recorded for this code. Note changes are tracked from FY2027 only.

Nearest Codes in This Family

Official ICD-10-CM classifications closest to C4A.9 in its code family, with their registry titles.

View all codes in the C4A family