2026 ICD-10-CM Diagnosis Code T39.4X6AUnderdosing of antirheumatics, not elsewhere classified, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T39

ICD-10-CM T39.4X6A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T39.4X6A is a billable ICD-10-CM diagnosis code for underdosing of antirheumatics, not elsewhere classified, 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Underdosing of drugs and medicaments, initial encounter.

Code Identity

ICD-10-CM Code
T39.4X6A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of antirheumatics, not elsewhere classified, initial encounter
Short Description
Underdosing of antirheumatics, NEC, init
Parent Code
T39.4X6
Underdosing of antirheumatics, not elsewhere classified

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
CategoryT39Poisoning by, adverse effect of and underdosing of nonopioid analgesics, antipyretics and antirheumatics
This CodeT39.4X6AUnderdosing of antirheumatics, not elsewhere classified, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T39.4X6A.

Unacceptable principal diagnosis
There are selected codes that describe a circumstance which influences an individual's health status but not a current illness or injury, or codes that are not specific manifestations but may be due to an underlying cause. These codes are considered unacceptable as a principal diagnosis.

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 nonopioid analgesics, antipyretics and antirheumatics (T39). 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

Clinical InformationClinical

  • Auranofin

    an oral chrysotherapeutic agent for the treatment of rheumatoid arthritis. its exact mechanism of action is unknown, but it is believed to act via immunological mechanisms and alteration of lysosomal enzyme activity. its efficacy is slightly less than that of injected gold salts, but it is better tolerated, and side effects which occur are potentially less serious.

  • Aurothioglucose

    a thioglucose derivative used as an antirheumatic and experimentally to produce obesity in animals.

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
Antiphlogistic NECT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6
Antirheumatic NECT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6
AuranofinT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6
AurothioglucoseT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6
AurothioglycanideT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6
Aurothiomalate sodiumT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6
AurotioprolT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6
Glucosamine sulfateT39.4X1T39.4X2T39.4X3T39.4X4T39.4X5T39.4X6

Patient EducationClinical

Medication Errors

Medicines treat infectious diseases, prevent problems from chronic diseases, and ease pain. But medicines can also cause harmful reactions if not used correctly. Errors can happen in the hospital, at the health care provider's office, at the pharmacy, or at home. You can help prevent errors by:

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert T39.4X6A to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
No Map There is no ICD-9 equivalent for this code.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About T39.4X6AOverview

Is T39.4X6A (Underdosing of antirheumatics, not elsewhere classified) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of antirheumatics, not elsewhere classified, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T39.4X6A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of antirheumatics, not elsewhere classified, such as an emergency visit or first evaluation.

Can T39.4X6A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of antirheumatics, not elsewhere classified, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.

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.