2026 ICD-10-CM Diagnosis Code S21.402AUnspecified open wound of left back wall of thorax with penetration into thoracic cavity, initial encounter
S21.402A is a billable ICD-10-CM diagnosis code for unspecified open wound of left back wall of thorax with 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 913 through 914, 963 through 965. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Open wounds of trunk, initial encounter.
Code Identity
Code Classification
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.
Patient EducationClinical
Back Injuries
Your back is made of bones, muscles, and other tissues extending from your neck to your pelvis. Back injuries can result from sports injuries, work around the house or in the garden, or a sudden jolt such as a car accident. The lower back is the most common site of back injuries and back pain. Common back injuries include :
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Convert S21.402A to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About S21.402AOverview
Is S21.402A a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report unspecified open wound of left back wall of thorax with 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 S21.402A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for unspecified open wound of left back wall of thorax with penetration into thoracic cavity, such as an emergency visit or first evaluation.
What MS-DRG does S21.402A group to?
When unspecified open wound of left back wall of thorax with penetration into thoracic cavity, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 913, 914, 963, 964, 965, with relative weights from 0.8855 to 2.7338 depending on complications. Higher weights mean higher Medicare reimbursement.
What is the ICD-9 equivalent of S21.402A?
Under the General Equivalence Mappings, unspecified open wound of left back wall of thorax with penetration into thoracic cavity, initial encounter converts to ICD-9-CM 862.9 (intrathorac inj NOS-open). 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.
