2026 ICD-10-CM Diagnosis Code S21.159AOpen bite of unspecified front wall of thorax without penetration into thoracic cavity, initial encounter

ICD-10-CM Codes›S00–T88›S20-S29›S21

ICD-10-CM S21.159A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

S21.159A is a billable ICD-10-CM diagnosis code for open bite of unspecified front wall of thorax without penetration into thoracic cavity, initial encounter. The 7th character A marks it as an initial encounter code, used while the patient is receiving active treatment. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 604 through 605, 963 through 965. The code is flagged as an unspecified code, since codes identifying laterality exist in the same family. Coders also document this condition as animal bite of chest. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Open wounds of trunk, initial encounter.

Code Identity

ICD-10-CM Code
S21.159A
Billable Status
Yes — Valid for Submission
Code Describes
Open bite of unspecified front wall of thorax without penetration into thoracic cavity, initial encounter
Short Description
Open bite of unsp frnt wall of thrx w/o penet thor cav, init
Parent Code
S21.159
Open bite of unspecified front wall of thorax without penetration into thoracic cavity

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionS20-S29Injuries to the thorax
CategoryS21Open wound of thorax
This CodeS21.159AOpen bite of unspecified front wall of thorax without penetration into thoracic cavity, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for S21.159A.

Unspecified codes
Unspecified codes exist in the ICD-10-CM classification for circumstances when documentation in the medical record does not provide the level of detail needed to support reporting a more specific code. However, in the inpatient setting, there should generally be very limited and rare circumstances for which the laterality (right, left, bilateral) of a condition is unable to be documented and reported. The following pages contain the list of unspecified ICD-10-CM diagnosis codes for which there is a more specific code to identify laterality (right, left, bilateral) within that code family.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Animal bite of chest
  • Bite wound of anterior chest wall
  • Dog bite of chest
  • Human bite of chest

Coding GuidelinesGuidance

The appropriate 7th character is to be added to each code from block Open wound of thorax (S21). Use the following options for the applicable episode of care:

  • A - initial encounter
  • D - subsequent encounter
  • S - sequela

Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026, published by CMS and the National Center for Health Statistics.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR INJ013
Open wounds of trunk, initial encounter
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Animal Bites

Wild animals usually avoid people. They might attack, however, if they feel threatened, are sick, or are protecting their young or territory. Attacks by pets are more common. Animal bites rarely are life-threatening, but if they become infected, you can develop serious medical problems.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert S21.159A to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
875.0 Open wound of chest
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About S21.159AOverview

Is S21.159A a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report open bite of unspecified front wall of thorax without penetration into thoracic cavity, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in S21.159A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for open bite of unspecified front wall of thorax without penetration into thoracic cavity, such as an emergency visit or first evaluation.

What MS-DRG does S21.159A group to?

When open bite of unspecified front wall of thorax without penetration into thoracic cavity, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 604, 605, 963, 964, 965, with relative weights from 0.9160 to 2.7338 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of S21.159A?

Under the General Equivalence Mappings, open bite of unspecified front wall of thorax without penetration into thoracic cavity, initial encounter converts to ICD-9-CM 875.0 (open wound of chest). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.