2026 ICD-10-CM Diagnosis Code T45.AX4APoisoning by immune checkpoint inhibitors and immunostimulant drugs, undetermined, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T45

ICD-10-CM T45.AX4A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T45.AX4A is a billable ICD-10-CM diagnosis code for poisoning by immune checkpoint inhibitors and immunostimulant drugs, undetermined, 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under External cause codes: intent of injury, undetermined; External cause codes: poisoning by drug; and Poisoning by drugs, initial encounter.

Code Identity

ICD-10-CM Code
T45.AX4A
Billable Status
Yes — Valid for Submission
Code Describes
Poisoning by immune checkpoint inhibitors and immunostimulant drugs, undetermined, initial encounter
Short Description
Poisn by immune chkpt inhibtr/immunostim drugs, undet, init
Parent Code
T45.AX4
Poisoning by immune checkpoint inhibitors and immunostimulant drugs, undetermined

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.AX4APoisoning by immune checkpoint inhibitors and immunostimulant drugs, undetermined, initial encounter

Coding GuidelinesGuidance

When coding a poisoning or reaction to the improper use of a medication (e.g., overdose, wrong substance given or taken in error, wrong route of administration), first assign the appropriate code from categories T36-T50. The poisoning codes have an associated intent as their 5th or 6th character (accidental, intentional self-harm, assault and undetermined). If the intent of the poisoning is unknown or unspecified, code the intent as accidental intent. The undetermined intent is only for use if the documentation in the record specifies that the intent cannot be determined. Use additional code(s) for all manifestations of poisonings.

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 EXT023
External cause codes: intent of injury, undetermined
Default principal diagnosis: inpatient No · outpatient No
CCSR EXT014
External cause codes: poisoning by drug
Default principal diagnosis: inpatient No · outpatient No
CCSR INJ022
Poisoning by drugs, initial encounter
Default principal diagnosis: inpatient Yes · 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.AX4A replaces the following previously assigned code(s):

  • T45.1X4A - Poisoning by antineopl and immunosup drugs, undet, 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.AX4AOverview

Is T45.AX4A a billable code?

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

What does the 7th character A in T45.AX4A mean?

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

What MS-DRG does T45.AX4A group to?

When poisoning by immune checkpoint inhibitors and immunostimulant drugs, undetermined, 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.