2026 ICD-10-CM Diagnosis Code T45.AX5AAdverse effect of immune checkpoint inhibitors and immunostimulant drugs, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T45

ICD-10-CM T45.AX5A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T45.AX5A is a billable ICD-10-CM diagnosis code for adverse effect of immune checkpoint inhibitors and immunostimulant drugs, 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 immune checkpoint inhibitor therapy induced enterocolitis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Adverse effects of drugs and medicaments, initial encounter.

Code Identity

ICD-10-CM Code
T45.AX5A
Billable Status
Yes — Valid for Submission
Code Describes
Adverse effect of immune checkpoint inhibitors and immunostimulant drugs, initial encounter
Short Description
Advrs effect of immune chkpt inhibtr/immunostim drugs, init
Parent Code
T45.AX5
Adverse effect of immune checkpoint inhibitors and immunostimulant drugs

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionT36-T50Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances
CategoryT45Poisoning by, adverse effect of and underdosing of primarily systemic and hematological agents, not elsewhere classified
This CodeT45.AX5AAdverse effect of immune checkpoint inhibitors and immunostimulant drugs, initial encounter

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Immune checkpoint inhibitor therapy induced enterocolitis

Coding GuidelinesGuidance

When coding an adverse effect of a drug that has been correctly prescribed and properly administered, assign the appropriate code for the nature of the adverse effect followed by the appropriate code for the adverse effect of the drug.

The appropriate 7th character is to be added to each code from block Poisoning by, adverse effect of and underdosing of primarily systemic and hematological agents, not elsewhere classified (T45). 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 INJ028
Adverse effects of drugs and medicaments, initial encounter
Default principal diagnosis: inpatient No · outpatient Yes

Table of Drugs and ChemicalsClinical

Substances in the Table of Drugs and Chemicals that reference this code family. Always confirm in the Tabular List before coding.

SubstanceAccidentalSelf-harmAssaultUndeter­minedAdverse
Effect		Under­dosing
ImmuneT45.AX1T45.AX2T45.AX3T45.AX4T45.AX5T45.AX6
Immune::checkpoint inhibitorsT45.AX1T45.AX2T45.AX3T45.AX4T45.AX5T45.AX6
Immune::globulinT45.AX1T45.AX2T45.AX3T45.AX4T45.AX5T45.AX6
Immune::serum globulinT45.AX1T45.AX2T45.AX3T45.AX4T45.AX5T45.AX6
Immunostimulant drugT45.AX1T45.AX2T45.AX3T45.AX4T45.AX5T45.AX6

Code History & ChangesHistory

Replacement T45.AX5A replaces the following previously assigned code(s):

  • T45.1X5A - Adverse effect of antineoplastic and immunosup drugs, init
FY 2025AddedAdded to the ICD-10-CM code setEffective October 1, 2024.
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About T45.AX5AOverview

Is T45.AX5A a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report adverse effect of immune checkpoint inhibitors and immunostimulant drugs, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T45.AX5A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for adverse effect of immune checkpoint inhibitors and immunostimulant drugs, such as an emergency visit or first evaluation.

What MS-DRG does T45.AX5A group to?

When adverse effect of immune checkpoint inhibitors and immunostimulant drugs, 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.

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.