2026 ICD-10-CM Diagnosis Code S20.422ABlister (nonthermal) of left back wall of thorax, initial encounter

ICD-10-CM Codes›S00–T88›S20-S29›S20

ICD-10-CM S20.422A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

S20.422A is a billable ICD-10-CM diagnosis code for blister (nonthermal) of left back wall of thorax, 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 606 through 607, 963 through 965. Coders also document this condition as traumatic blister of back. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Superficial injury; contusion, initial encounter.

Code Identity

ICD-10-CM Code
S20.422A
Billable Status
Yes — Valid for Submission
Code Describes
Blister (nonthermal) of left back wall of thorax, initial encounter
Short Description
Blister (nonthermal) of left back wall of thorax, init
Parent Code
S20.422
Blister (nonthermal) of left back wall of thorax

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionS20-S29Injuries to the thorax
CategoryS20Superficial injury of thorax
This CodeS20.422ABlister (nonthermal) of left back wall of thorax, initial encounter

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Traumatic blister of back
  • Traumatic blister of left posterior thorax
  • Traumatic blister of posterior thorax

Coding GuidelinesGuidance

The appropriate 7th character is to be added to each code from block Superficial injury of thorax (S20). 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 INJ017
Superficial injury; contusion, initial encounter
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Blisters

Blisters are fluid-filled sacs on the outer layer of your skin. They form because of rubbing, heat, or diseases of the skin. They are most common on your hands and feet.

The full article covers:

  • What are blisters?
  • What causes blisters?
  • What are the treatments for blisters?
  • Can blisters be prevented?

Read the full article at MedlinePlus

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

Convert S20.422A to ICD-9-CMHistory

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

ICD-9-CM
911.2 Blister trunk
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 S20.422AOverview

Is S20.422A (Blister (nonthermal) of left back wall of thorax) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report blister (nonthermal) of left back wall of thorax, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in S20.422A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for blister (nonthermal) of left back wall of thorax, such as an emergency visit or first evaluation.

What MS-DRG does S20.422A group to?

When blister (nonthermal) of left back wall of thorax, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 606, 607, 963, 964, 965, with relative weights from 0.9064 to 2.7338 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of S20.422A?

Under the General Equivalence Mappings, blister (nonthermal) of left back wall of thorax, initial encounter converts to ICD-9-CM 911.2 (blister trunk). 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.