C21.0 — Malignant neoplasm of anus, unspecified
Is C21.0 billable?
Yes, but C21.0 is an unspecified code. C21.0 is a valid, billable ICD-10-CM code for encounters from October 1, 2026 through September 30, 2027. However, C21.0 is an unspecified or catch-all code. Payers apply additional scrutiny to unspecified codes and many deny them when the medical record supports a more specific option, so check the documentation before selecting C21.0 — the more specific alternatives are listed below.
C21.0 at a glance
| Code | C21.0 |
|---|---|
| Description | Malignant neoplasm of anus, unspecified |
| Billable | Yes |
| Code set | ICD-10-CM FY2027 |
| Valid for encounters | October 1, 2026 – September 30, 2027 |
Which MS-DRGs does C21.0 group to?
C21.0 sits in MDC 06 and helps define the logic of 3 MS-DRGs (version 44, FY2027). Which one a stay actually groups to also depends on procedures and secondary diagnoses.
- 374Digestive Malignancy with MCCmedical
- 375Digestive Malignancy with CCmedical
- 376Digestive Malignancy without CC/MCCmedical
As a secondary diagnosis, C21.0 is a complication or comorbidity (CC), which can move a stay into a higher-paying DRG.
How is C21.0 listed in the Alphabetic Index?
These are the routes through the Alphabetic Index that lead to C21.0. They show the wording a clinician may have documented, which often differs from the Tabular description.
- Paget's disease › extramammary › anus
Excludes1 — never code together — inherited from C15-C26
The conditions below can never be reported together with C21.0 on the same claim. An Excludes1 note means the two conditions cannot occur in the same patient, so reporting both is a coding error.
- Kaposi's sarcoma of gastrointestinal sites (C46.4)
Excludes2 — not included here — inherited from C15-C26
The conditions below are not part of C21.0, but a patient may have both at the same time. When documentation supports it, C21.0 and the excluded code may both be reported.
- gastrointestinal stromal tumors (C49.A-)
Excludes2 — not included here — inherited from C21
The conditions below are not part of C21.0, but a patient may have both at the same time. When documentation supports it, C21.0 and the excluded code may both be reported.
- malignant carcinoid tumors of the colon (C7A.02-)
- malignant melanoma of anal margin (C43.51)
- malignant melanoma of anal skin (C43.51)
- malignant melanoma of perianal skin (C43.51)
- other and unspecified malignant neoplasm of anal margin (C44.500, C44.510, C44.520, C44.590)
- other and unspecified malignant neoplasm of anal skin (C44.500, C44.510, C44.520, C44.590)
- other and unspecified malignant neoplasm of perianal skin (C44.500, C44.510, C44.520, C44.590)
How long has C21.0 existed?
C21.0 has been in ICD-10-CM since at least FY2016, the earliest fiscal year on record here.
Derived from the official order files for FY2016–FY2026. FY2017 and FY2020 publish no order file, so those years are not covered.
Which codes are confused with C21.0?
ICD-10-CM declares these codes mutually exclusive with C21.0 — exactly one of each pair can be correct for a given encounter.