Is M21 a billable ICD-10-CM code?

No. M21 is a non-billable header code. Code to a higher level of specificity using one of its child codes.

Where does M21 sit in the tabular list?

Diseases of the musculoskeletal system and connective tissue (M00-M99), in the block Other joint disorders (M20-M25).

What cannot be coded together with M21?

M21 carries an Excludes1 note, which means the conditions it lists can never be coded together with it on the same encounter.

M21

M21 — Other acquired deformities of limbs

Non-billable headerICD-10-CM

M21 is a non-billable header ICD-10-CM code for other acquired deformities of limbs. It cannot be billed on its own: choose one of the more specific child codes below.

Excludes1 — never code together

These conditions cannot occur together. Billing both on the same encounter is a denial.

acquired absence of limb (Z89.-)
congenital absence of limbs (Q71-Q73)
congenital deformities and malformations of limbs (Q65-Q66, Q68-Q74)

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.

acquired deformities of fingers or toes (M20.-)
coxa plana (M91.2)

Other codes that can never be billed with M21

These codes carry an Excludes1 note pointing at M21. The conflict binds both ways, but ICD-10-CM only writes it down on one side — so you would not find this by reading M21 alone.

M23Internal derangement of knee
Q66Congenital deformities of feet
Q66.5Congenital pes planus
Z89Acquired absence of limb

Risk adjustment

M21 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 M21

10 child codes. Green means billable.

Questions about M21

Is M21 a billable ICD-10-CM code?: No. M21 is a non-billable header code. Code to a higher level of specificity using one of its child codes.
Where does M21 sit in the tabular list?: Diseases of the musculoskeletal system and connective tissue (M00-M99), in the block Other joint disorders (M20-M25).
What cannot be coded together with M21?: M21 carries an Excludes1 note, which means the conditions it lists can never be coded together with it on the same encounter.