2026 ICD-10-CM Diagnosis Code T58.91XAToxic effect of carbon monoxide from unspecified source, accidental (unintentional), initial encounter

ICD-10-CM Codes›S00–T88›T51-T65›T58

ICD-10-CM T58.91XA
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T58.91XA is a billable ICD-10-CM diagnosis code for toxic effect of carbon monoxide from unspecified source, accidental (unintentional), 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 917 through 918. Coders also document this condition as accidental poisoning by carbon monoxide. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under External cause codes: intent of injury, accidental/unintentional; External cause codes: poisoning by non-drug; and Toxic effects, initial encounter.

Code Identity

ICD-10-CM Code
T58.91XA
Billable Status
Yes — Valid for Submission
Code Describes
Toxic effect of carbon monoxide from unspecified source, accidental (unintentional), initial encounter
Short Description
Toxic effect of carb monx from unsp source, acc, init
Parent Code
T58.91
Toxic effect of carbon monoxide from unspecified source, accidental (unintentional)

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionT51-T65Toxic effects of substances chiefly nonmedicinal as to source
CategoryT58Toxic effect of carbon monoxide
This CodeT58.91XAToxic effect of carbon monoxide from unspecified source, accidental (unintentional), initial encounter

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Accidental poisoning by carbon monoxide
  • Carbon monoxide encephalopathy
  • Carboxyhemoglobinemia
  • Chronic carbon monoxide poisoning
  • Dementia due to carbon monoxide poisoning
  • Headache caused by carbon monoxide

Coding GuidelinesGuidance

The appropriate 7th character is to be added to each code from block Toxic effect of carbon monoxide (T58). 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 EXT020
External cause codes: intent of injury, accidental/unintentional
Default principal diagnosis: inpatient No · outpatient No
CCSR EXT015
External cause codes: poisoning by non-drug
Default principal diagnosis: inpatient No · outpatient No
CCSR INJ023
Toxic effects, initial encounter
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Carbon Monoxide Poisoning

Carbon monoxide (CO) is a gas that has no odor or color. But it is very dangerous. It can cause sudden illness and death. CO is found in combustion fumes, such as those made by cars and trucks, lanterns, stoves, gas ranges and heating systems. CO from these fumes can build up in places that don't have a good flow of fresh air.

Read the full article at MedlinePlus

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

Convert T58.91XA to ICD-9-CMHistory

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

ICD-9-CM
986 Tox eff carbon monoxide
ApproximateCombination The match is approximate, and more than one code can be needed to describe the source diagnosis. Confirm with contextual judgment.
ICD-9-CM
E868.9 Acc pois-carbn monox NOS
ApproximateCombination The match is approximate, and more than one code can be needed to describe the source diagnosis. Confirm with contextual judgment.

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 T58.91XAOverview

Is T58.91XA a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report toxic effect of carbon monoxide from unspecified source, accidental (unintentional), initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T58.91XA mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for toxic effect of carbon monoxide from unspecified source, accidental (unintentional), such as an emergency visit or first evaluation.

What MS-DRG does T58.91XA group to?

When toxic effect of carbon monoxide from unspecified source, accidental (unintentional), initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 917, 918, with relative weights from 0.8571 to 1.5684 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of T58.91XA?

Under the General Equivalence Mappings, toxic effect of carbon monoxide from unspecified source, accidental (unintentional), initial encounter converts to ICD-9-CM 986 (tox eff carbon monoxide) and E868.9 (acc pois-carbn monox NOS). 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.