2026 ICD-10-CM Diagnosis Code T36.AX6AUnderdosing of fluoroquinolone antibiotics, initial encounter
T36.AX6A is a billable ICD-10-CM diagnosis code for underdosing of fluoroquinolone antibiotics, 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). The code is not accepted as a principal diagnosis by the Medicare Code Editor.
Code Identity
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T36.AX6A.
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 systemic antibiotics (T36). 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.
Table of Drugs and ChemicalsClinical
Code History & ChangesHistory
New Code T36.AX6A was added to the ICD-10-CM code set for FY 2026, effective October 1, 2025.
Replacement T36.AX6A replaces the following previously assigned code(s):
- T36.8X6A - Underdosing of other systemic antibiotics, initial encounter
Questions About T36.AX6AOverview
Is T36.AX6A (Underdosing of fluoroquinolone antibiotics) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of fluoroquinolone antibiotics, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T36.AX6A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of fluoroquinolone antibiotics, such as an emergency visit or first evaluation.
Can T36.AX6A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of fluoroquinolone antibiotics, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.
