2027 ICD-10-CM Diagnosis Code T45.AX6AUnderdosing of immune checkpoint inhibitors and immunostimulant drugs, initial encounter
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 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 951. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Underdosing of drugs and medicaments, initial encounter.
Code Identity
Code Classification
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 2027, published by CMS and the National Center for Health Statistics.
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to T45.AX6A: its own notes plus those printed at T45.A, T45, block T36-T50, and Chapter 19. A note printed at a category, block or chapter applies to every code under it.
Includes
Further defines, or gives examples of, the content of the category.
- adverse effect of correct substance properly administered
- poisoning by overdose of substance
- poisoning by wrong substance given or taken in error
- underdosing by (inadvertently) (deliberately) taking less substance than prescribed or instructed
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
- poisoning by, adverse effect of and underdosing of antineoplastic and immunosuppressive drug (T45.1)
- toxic reaction to local anesthesia in pregnancy (O29.3-)
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
Code First
The underlying condition named in the note is sequenced before this code.
Use Additional Code
Add a second code, after this one, to fully describe the condition.
- code to identify any retained foreign body, if applicable (Z18.-)
Notes
General instructions on how these codes are used.
- Use secondary code(s) from Chapter 20, External causes of morbidity, to indicate cause of injury. Codes within the T section that include the external cause do not require an additional external cause code
- The chapter uses the S-section for coding different types of injuries related to single body regions and the T-section to cover injuries to unspecified body regions as well as poisoning and certain other consequences of external causes.
7th Character
Every code in the category takes a 7th character from the listed values, with the placeholder X filling any empty positions before it.
- The appropriate 7th character is to be added to each code from category T45
- Ainitial encounter this code
- Dsubsequent encounter
- Ssequela
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name T45.AX6A, its category, or a range that includes it.
Excludes1 1
These codes carry an Excludes1 note naming T45.AX6A: they are not reported together with it, unless the two conditions are unrelated.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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.
Code History & ChangesHistory
Replacement T45.AX6A replaces the following previously assigned code(s):
- T45.1X6A - Underdosing of antineoplastic and immunosup drugs, init
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, 2026 through September 30, 2027.
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.
What MS-DRG does T45.AX6A group to?
When underdosing of immune checkpoint inhibitors and immunostimulant drugs, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 951 (other Factors Influencing Health Status), which carries a relative weight of 0.5574. Higher weights mean higher Medicare reimbursement.
Can T45.AX6A be reported with T45.1?
Not as a rule. The Excludes1 note at T45.A, which applies to T45.AX6A, lists "poisoning by, adverse effect of and underdosing of antineoplastic and immunosuppressive drug (T45.1)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
