Is C21 a billable ICD-10-CM code?

No. C21 is a non-billable header code. Code to a higher level of specificity using one of its child codes.

Where does C21 sit in the tabular list?

Neoplasms (C00-D49), in the block Malignant neoplasms of digestive organs (C15-C26).

C21

C21 — Malignant neoplasm of anus and anal canal

Non-billable headerICD-10-CM

C21 is a non-billable header ICD-10-CM code for malignant neoplasm of anus and anal canal. It cannot be billed on its own: choose one of the more specific child codes below.

Excludes2 — not included here, but may coexist

The excluded condition is not part of this code, but a patient can have both. You may code both.

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)

Other codes that can never be billed with C21

These codes carry an Excludes1 note pointing at C21. The conflict binds both ways, but ICD-10-CM only writes it down on one side — so you would not find this by reading C21 alone.

C44.5Other and unspecified malignant neoplasm of skin of trunk

Risk adjustment

C21 does not map to an HCC and does not risk-adjust under CMS-HCC V28. That is normal — most ICD-10 codes do not. It still needs to be coded correctly; it just will not move a RAF score.

Code to one of these instead C21

4 child codes. Green means billable.

Questions about C21

Is C21 a billable ICD-10-CM code?: No. C21 is a non-billable header code. Code to a higher level of specificity using one of its child codes.
Where does C21 sit in the tabular list?: Neoplasms (C00-D49), in the block Malignant neoplasms of digestive organs (C15-C26).