Is S21 a billable ICD-10-CM code?

No. S21 is a non-billable header code. Code to a higher level of specificity using one of its child codes.

Where does S21 sit in the tabular list?

Injury, poisoning and certain other consequences of external causes (S00-T88), in the block Injuries to the thorax (S20-S29).

What cannot be coded together with S21?

S21 carries an Excludes1 note, which means the conditions it lists can never be coded together with it on the same encounter.

S21

S21 — Open wound of thorax

Non-billable headerICD-10-CM

S21 is a non-billable header ICD-10-CM code for open wound of thorax. It cannot be billed on its own: choose one of the more specific child codes below. This code also requires a 7th character before it is complete.

This code requires a 7th character

The appropriate 7th character is to be added to each code from category S21

A: initial encounter
D: subsequent encounter
S: sequela

Excludes1 — never code together

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

traumatic amputation (partial) of thorax (S28.1)

Code also

Both codes may be needed to describe the encounter fully. Sequencing depends on the reason for the visit.

any associated injury, such as:
injury of heart (S26.-)
injury of intrathoracic organs (S27.-)
rib fracture (S22.3-, S22.4-)
spinal cord injury (S24.0-, S24.1-)
traumatic hemopneumothorax (S27.3)
traumatic hemothorax (S27.1)
traumatic pneumothorax (S27.0)
wound infection

Other codes that can never be billed with S21

These codes carry an Excludes1 note pointing at S21. The conflict binds both ways, but ICD-10-CM only writes it down on one side — so you would not find this by reading S21 alone.

S20.17Other superficial bite of breast
S20.37Other superficial bite of front wall of thorax
S20.47Other superficial bite of back wall of thorax
S20.97Other superficial bite of unspecified parts of thorax
S20.97XAOther superficial bite of unspecified parts of thorax, initial encounter
S20.97XDOther superficial bite of unspecified parts of thorax, subsequent encounter
S20.97XSOther superficial bite of unspecified parts of thorax, sequela

Risk adjustment

S21 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 S21

6 child codes. Green means billable.

Questions about S21

Is S21 a billable ICD-10-CM code?: No. S21 is a non-billable header code. Code to a higher level of specificity using one of its child codes.
Where does S21 sit in the tabular list?: Injury, poisoning and certain other consequences of external causes (S00-T88), in the block Injuries to the thorax (S20-S29).
What cannot be coded together with S21?: S21 carries an Excludes1 note, which means the conditions it lists can never be coded together with it on the same encounter.