2026 ICD-10-CM Diagnosis Code T45.AX6AUnderdosing of immune checkpoint inhibitors and immunostimulant drugs, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T45

ICD-10-CM T45.AX6A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T45.AX6A is a billable ICD-10-CM diagnosis code for underdosing 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). In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Underdosing of drugs and medicaments, initial encounter.

Code Identity

ICD-10-CM Code
T45.AX6A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of immune checkpoint inhibitors and immunostimulant drugs, initial encounter
Short Description
Undrdose of immune checkpoint inhibtr/immunostim drugs, init
Parent Code
T45.AX6
Underdosing 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.AX6AUnderdosing of immune checkpoint inhibitors and immunostimulant drugs, initial encounter

Coding GuidelinesGuidance

Underdosing refers to taking less of a medication than is prescribed by a provider or a manufacturer's instruction. Codes for underdosing should never be assigned as principal or first-listed codes. If a patient has a relapse or exacerbation of the medical condition for which the drug is prescribed because of the reduction in dose, then the medical condition itself should be coded.

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 INJ029
Underdosing of drugs and medicaments, initial encounter
Default principal diagnosis: inpatient No · outpatient No

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.AX6A replaces the following previously assigned code(s):

  • T45.1X6A - Underdosing 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.AX6AOverview

Is T45.AX6A a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report underdosing 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.AX6A mean?

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

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.