2027 ICD-10-CM Diagnosis Code C18.6Malignant neoplasm of descending colon
ICD-10-CM Codes›C00–D49›C15-C26›C18
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 22
- Chronic Condition
C18.6 is a billable ICD-10-CM diagnosis code for malignant neoplasm of descending colon. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 374 through 376. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 23 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Gastrointestinal cancers - colorectal.
For Medicare Advantage risk adjustment, C18.6 maps to CMS-HCC Category 22 (Bladder, Colorectal, and Other Cancers) under the V28 model, adding a risk factor of about 0.363 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
C18.6 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.
- Carcinoma of descending colon
- Malignant neoplasm of descending colon
- Malignant neoplasm of splenic flexure
- Neoplasm of splenic flexure of colon
- Primary adenocarcinoma of colon
- Primary adenocarcinoma of descending colon
- Primary adenocarcinoma of descending colon and splenic flexure
- Primary adenocarcinoma of transverse colon
- Primary carcinoma of colon
- Primary carcinoma of descending colon
- Primary malignant neoplasm of descending colon
- Primary malignant neoplasm of splenic flexure of colon
- Primary malignant neoplasm of transverse colon
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to C18.6: its own notes plus those printed at C18, block C15-C26, and Chapter 2. A note printed at a category, block or chapter applies to every code under it.
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
- malignant carcinoid tumors of the colon (C7A.02-)
- Kaposi's sarcoma of gastrointestinal sites (C46.4)
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
- gastrointestinal stromal tumors (C49.A-)
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 C18.6, its category, or a range that includes it.
Applicable To 2
These notes mention C18.6.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Malignant Colon Neoplasm
a primary or metastatic malignant neoplasm that affects the colon. representative examples include carcinoma, lymphoma, and sarcoma.
Table of NeoplasmsClinical
Anatomical sites in the Table of Neoplasms that reference this code family.
Patient EducationClinical
Colorectal Cancer
Colorectal cancer is cancer that develops in the tissues of the colon or rectum. Your colon and rectum are part of your digestive system:
The full article covers:
- What is colorectal cancer?
- What causes colorectal cancer?
- Who is more likely to develop colorectal cancer?
- What are the symptoms of colorectal cancer?
- What is screening for colorectal cancer and who needs it?
- How is colorectal cancer diagnosed?
- What are the treatments for colorectal cancer?
- Can colorectal cancer be prevented?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert C18.6 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About C18.6Overview
What is the ICD-10 code for malignant neoplasm of descending colon?
The ICD-10-CM code for malignant neoplasm of descending colon is C18.6 (sometimes written as C186). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is C18.6 (Malignant neoplasm of colon) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report malignant neoplasm of descending colon on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does C18.6 group to?
When malignant neoplasm of descending colon is the principal diagnosis on an inpatient stay, it groups to MS-DRG 375, 374, 376, with relative weights from 0.8925 to 2.1101 depending on complications. Higher weights mean higher Medicare reimbursement.
Is C18.6 a CC or MCC?
CMS lists C18.6 as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 23 closely related codes in its exclusion list.
Can C18.6 be reported with a code from C7A.02?
Not as a rule. The Excludes1 note at C18, which applies to C18.6, lists "malignant carcinoid tumors of the colon (C7A.02-)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
What HCC is C18.6?
C18.6 (malignant neoplasm of descending colon) maps to CMS-HCC Category 22 (Bladder, Colorectal, and Other Cancers), commonly written as HCC 22, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 11 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. It does not map to any RxHCC in the Part D prescription drug model.
Does C18.6 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, C18.6 adds a risk adjustment factor of about 0.363 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.314 to 0.410 depending on the payment segment). A more severe related category (HCC 17, HCC 18, HCC 19, HCC 20, and HCC 21) supersedes it when both are reported. See the full factor table on the HCC 22 category page.
