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C18.1 ICD-10-CM Code: Malignant neoplasm of appendix

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

Coding at a Glance

Tabular directives
2 Excludes1 · 1 Excludes2
Risk adjustment
CMS-HCC V28: 1 category · CMS-HCC V22 category 11

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 v43, Appendix B.

  • MS-DRG 374 — DIGESTIVE MALIGNANCY WITH MCC (MDC 06)
  • MS-DRG 375 — DIGESTIVE MALIGNANCY WITH CC (MDC 06)
  • MS-DRG 376 — DIGESTIVE MALIGNANCY WITHOUT CC/MCC (MDC 06)

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 22 — Bladder, Colorectal, and Other Cancers (supersedes HCC 23)

Other models: CMS-HCC V22 HCC 11

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 C18.1 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on C18.1 itself; “inherited from” names the category or block whose note applies here.

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Excludes2 — Not Included Here

Conditions not covered by this code, but which may be reported alongside it when both are present.

Source: inherited from C15-C26

Coder workflow for C18.1

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

Before you code C18.1

  1. 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 →

  2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with C18.1. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

    See the official tabular notes

Choose the right path

  1. Does the documentation support a condition named in C18.1’s Excludes1 note?
    Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
    No → Continue.

    ReviewC7A.02, C46.4

Consider C18.1. 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).

Official instructions as workflow

  • Excludes1 — check before selecting C18.1(2 notes)

    Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with C18.1: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

    CompareC7A.02, C46.4

    See the official tabular notes · Guidelines I.A.12.a

  • Excludes2 — not part of C18.1(1 note)

    Coding workflow: The conditions named in this note are not included in C18.1. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

    CompareC49.A

    See the official tabular notes · Guidelines I.A.12.b

Coding decision scenarios

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

Documentation: Both the condition C18.1 describes and a condition named in its Excludes1 note are documented for the same encounter.

Coding question: Can both codes be reported?

Path: Review the Excludes1 note and the excluded code, and look for a provider statement on whether the two conditions are related.

Reason: Excludes1 means the two are not coded together; the exception is when the record shows the conditions are unrelated to each other (Guidelines I.A.12.a).

ReviewC7A.02, C46.4

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

Malignant neoplasm of appendix is a billable ICD-10-CM diagnosis code (C18.1).

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.

Neoplasm Table Sites (2)

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

Anatomical sites from the official ICD-10-CM Table of Neoplasms that map to this code, with the matching behavior column.

  • Neoplasm, neoplastic, appendix — Malignant, Primary
  • Neoplasm, neoplastic, intestine, intestinal, large, appendix — Malignant, Primary

Verify Before Coding

  • CC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

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 C18.1 or its code family, from the CMS ICD-10-CM tabular instructional notes, the CMS Table of Neoplasms and the CMS Table of Neoplasms (site match derived from the code title). Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Code Also instruction: D13.91 — Familial adenomatous polyposis (via C18.-).

These codes suggest coding this condition alongside when both are present.

Referenced by 5 Same-site neoplasm codes: C78.5 — Secondary malignant neoplasm of large intestine and rectumCompare C18.1 vs C78.5 →, D01.0 — Carcinoma in situ of colonCompare C18.1 vs D01.0 →, D12.1 — Benign neoplasm of appendixCompare C18.1 vs D12.1 →, D37.3 — Neoplasm of uncertain behavior of appendixCompare C18.1 vs D37.3 →, D49.0 — Neoplasm of unspecified behavior of digestive systemCompare C18.1 vs D49.0 →.

These codes represent related neoplasm behaviors affecting the same anatomic site.

Referenced by 7 Screening & history codes: Z12.13 — Encounter for screening for malignant neoplasm of small intestineCompare C18.1 vs Z12.13 →, Z85.03 — Personal history of malignant neoplasm of large intestineCompare C18.1 vs Z85.03 →, Z85.030 — Personal history of malignant carcinoid tumor of large intestineCompare C18.1 vs Z85.030 →, Z85.038 — Personal history of other malignant neoplasm of large intestineCompare C18.1 vs Z85.038 →, Z85.06 — Personal history of malignant neoplasm of small intestineCompare C18.1 vs Z85.06 →, Z85.060 — Personal history of malignant carcinoid tumor of small intestineCompare C18.1 vs Z85.060 →, Z85.068 — Personal history of other malignant neoplasm of small intestineCompare C18.1 vs Z85.068 →.

These codes represent screening tests and history codes that refer to this active disease condition.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: CC — Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 23 clinically related codes on its CMS exclusion list.

Named in the grouper logic of 3 MS-DRGs: DRG 374 (MDC 06), DRG 375 (MDC 06), DRG 376 (MDC 06).

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

Clinical classification (AHRQ CCSR):NEO015 — Gastrointestinal cancers - colorectal (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 clinical process (MS-DRG)

Acts as CC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

C18.8 — Malignant neoplasm of overlapping sites of colon, C18.9 — Malignant neoplasm of colon, unspecified, C26.0 — Malignant neoplasm of intestinal tract, part unspecified, C26.9 — Malignant neoplasm of ill-defined sites within the digestive system, C45.7 — Mesothelioma of other sites, C45.9 — Mesothelioma, unspecified, C46.4 — Kaposi's sarcoma of gastrointestinal sites, C76.8 — Malignant neoplasm of other specified ill-defined sites, C7A.00 — Malignant carcinoid tumor of unspecified site, C7A.020 — Malignant carcinoid tumor of the appendix, C7A.029 — Malignant carcinoid tumor of the large intestine, unspecified portion, C7A.098 — Malignant carcinoid tumors of other sites, C7A.1 — Malignant poorly differentiated neuroendocrine tumors, C7A.8 — Other malignant neuroendocrine tumors, C7B.00 — Secondary carcinoid tumors, unspecified site, C7B.1 — Secondary Merkel cell carcinoma, C80.0 — Disseminated malignant neoplasm, unspecified, C80.1 — Malignant (primary) neoplasm, unspecified, D49.0 — Neoplasm of unspecified behavior of digestive system, D49.81 — Neoplasm of unspecified behavior of retina and choroid, +2 more

Same CMS-HCC risk category (V28)

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

C18.0 — Malignant neoplasm of cecum, C18.2 — Malignant neoplasm of ascending colon, C18.3 — Malignant neoplasm of hepatic flexure, C18.4 — Malignant neoplasm of transverse colon, C18.5 — Malignant neoplasm of splenic flexure, C18.6 — Malignant neoplasm of descending colon, C18.7 — Malignant neoplasm of sigmoid colon, C18.8 — Malignant neoplasm of overlapping sites of colon, C18.9 — Malignant neoplasm of colon, unspecified, C19 — Malignant neoplasm of rectosigmoid junction, C20 — Malignant neoplasm of rectum, C21.0 — Malignant neoplasm of anus, unspecified, C21.1 — Malignant neoplasm of anal canal, C21.2 — Malignant neoplasm of cloacogenic zone, C21.8 — Malignant neoplasm of overlapping sites of rectum, anus and anal canal, C26.0 — Malignant neoplasm of intestinal tract, part unspecified, C26.1 — Malignant neoplasm of spleen, C26.9 — Malignant neoplasm of ill-defined sites within the digestive system, C51.0 — Malignant neoplasm of labium majus, C51.1 — Malignant neoplasm of labium minus, +105 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, Lymphoma 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 (Gastrointestinal cancers - colorectal).

C18.0 — Malignant neoplasm of cecum, C18.2 — Malignant neoplasm of ascending colon, C18.3 — Malignant neoplasm of hepatic flexure, C18.4 — Malignant neoplasm of transverse colon, C18.5 — Malignant neoplasm of splenic flexure, C18.6 — Malignant neoplasm of descending colon, C18.7 — Malignant neoplasm of sigmoid colon, C18.8 — Malignant neoplasm of overlapping sites of colon, C18.9 — Malignant neoplasm of colon, unspecified, C19 — Malignant neoplasm of rectosigmoid junction, C20 — Malignant neoplasm of rectum, C26.0 — Malignant neoplasm of intestinal tract, part unspecified, C49.A4 — Gastrointestinal stromal tumor of large intestine, C49.A5 — Gastrointestinal stromal tumor of rectum, D01.0 — Carcinoma in situ of colon, D01.1 — Carcinoma in situ of rectosigmoid junction, D01.2 — Carcinoma in situ of rectum, D01.40 — Carcinoma in situ of unspecified part of intestine

Same Index main term, other category

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

A52.17 — General paresis (syphilitic, brain), C22.4 — Other sarcomas of liver (mesodermal, mixed, liver), C22.7 — Other specified carcinomas of liver (embryonal, liver), C24.0 — Malignant neoplasm of extrahepatic bile duct (Klatskin's), C25.4 — Malignant neoplasm of endocrine pancreas (G cell, malignant, pancreas), C30.0 — Malignant neoplasm of nasal cavity (neurogenic olfactory), C41.0 — Malignant neoplasm of bones of skull and face (odontogenic, malignant, upper jaw), C41.1 — Malignant neoplasm of mandible (odontogenic, malignant), C49.A — Gastrointestinal stromal tumor (stromal, gastrointestinal), C49.A0 — Gastrointestinal stromal tumor, unspecified site (stromal, gastrointestinal, malignant), C49.A1 — Gastrointestinal stromal tumor of esophagus (stromal, gastrointestinal, esophagus), C49.A2 — Gastrointestinal stromal tumor of stomach (stromal, stomach), C49.A3 — Gastrointestinal stromal tumor of small intestine (stromal, intestine, malignant, small), C49.A4 — Gastrointestinal stromal tumor of large intestine (stromal, intestine, malignant, large), C49.A5 — Gastrointestinal stromal tumor of rectum (stromal, gastrointestinal, rectum), C49.A9 — Gastrointestinal stromal tumor of other sites (stromal, gastrointestinal, malignant, omentum), C54.9 — Malignant neoplasm of corpus uteri, unspecified (Müllerian, mixed, unspecified site), C56 — Malignant neoplasm of ovary (Brenner, malignant), C56.9 — Malignant neoplasm of unspecified ovary (yolk sac, unspecified site, female), C62.90 — Malignant neoplasm of unspecified testis, unspecified whether descended or undescended (yolk sac, unspecified site, male), +97 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.

CEA Test, Fecal Occult Blood Test (FOBT), Gamma-glutamyl Transferase (GGT) Test, Iron Panel

Contextual Map

Every relationship of C18.1 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 C18.1 with this related code in Claim Check

Hierarchy

Same site

Referenced by Code Also instructions

Referenced by Same-site neoplasm codes

Referenced by Screening & history codes MedCoder-derived

Clinical classification (CCSR)

Risk adjustment (CMS-HCC)

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

MS-DRG Grouper

  • CC — Complication or Comorbidity [MS-DRG severity]: “As a secondary diagnosis this code can raise the stay's MS-DRG severity tier (CC).”— CMS MS-DRG Definitions Manual (Appendix C) · FY2026
  • DRG 374 — DIGESTIVE MALIGNANCY WITH MCC[MS-DRG]: “DIGESTIVE MALIGNANCY WITH MCC (MDC 06)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
  • DRG 375 — DIGESTIVE MALIGNANCY WITH CC[MS-DRG]: “DIGESTIVE MALIGNANCY WITH CC (MDC 06)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
  • DRG 376 — DIGESTIVE MALIGNANCY WITHOUT CC/MCC[MS-DRG]: “DIGESTIVE MALIGNANCY WITHOUT CC/MCC (MDC 06)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

  • MDC 06 — Diseases and Disorders of the Digestive System[MDC crossing]: “Diseases and Disorders of the Digestive System — 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. 5,744 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Neoplasm sites

  • Neoplasm, neoplastic, appendix — Neoplasm, neoplastic, appendix — Malignant, Primary [Neoplasm table]: “Neoplasm, neoplastic, appendix (Malignant, Primary)”— CMS Table of Neoplasms · icd10cm-fy2026
  • Neoplasm, neoplastic, intestine, intestinal, large, appendix — Neoplasm, neoplastic, intestine, intestinal, large, appendix — Malignant, Primary [Neoplasm table]: “Neoplasm, neoplastic, intestine, intestinal, large, appendix (Malignant, Primary)”— CMS Table of Neoplasms · icd10cm-fy2026

Index entries

  • Tumor, mucocarcinoid, unspecified site[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (10)

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 FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v43.1 (April 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 — v43 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. "C18.1 — Malignant neoplasm of appendix." ICD-10-CM FY2026. https://medcoder.ai/icd10/code/c18.1-malignant-neoplasm-of-appendix

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Malignant neoplasm of appendix

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027 (effective October 1, 2026), 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 C18.1 in its code family, with their registry titles.

View all codes in the C18 family