2026 ICD-10-CM Diagnosis Code T36.6X6AUnderdosing of rifampicins, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T36

ICD-10-CM T36.6X6A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T36.6X6A is a billable ICD-10-CM diagnosis code for underdosing of rifampicins, 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
T36.6X6A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of rifampicins, initial encounter
Short Description
Underdosing of rifampicins, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T36.6X6
Underdosing of rifampicins

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
CategoryT36Poisoning by, adverse effect of and underdosing of systemic antibiotics
This CodeT36.6X6AUnderdosing of rifampicins, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T36.6X6A.

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 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.

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

  • Rifabutin

    a broad-spectrum antibiotic that is being used as prophylaxis against disseminated mycobacterium avium complex infection in hiv-positive patients.

  • Rifampin

    a semisynthetic antibiotic produced from streptomyces mediterranei. it has a broad antibacterial spectrum, including activity against several forms of mycobacterium. in susceptible organisms it inhibits dna-dependent rna polymerase activity by forming a stable complex with the enzyme. it thus suppresses the initiation of rna synthesis. rifampin is bactericidal, and acts on both intracellular and extracellular organisms. (from gilman et al., goodman and gilman's the pharmacological basis of therapeutics, 9th ed, p1160)

  • Rifaximin

    a synthetic rifamycin derivative and anti-bacterial agent that is used for the treatment of gastroenteritis caused by escherichia coli infections. it may also be used in the treatment of hepatic encephalopathy.

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
AnsamycinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifabutinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifamideT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifampicinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
Rifampicin::with isoniazidT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifampinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifamycinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifaximinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6

Patient EducationClinical

Antibiotics

Antibiotics are medicines that fight bacterial infections in people and animals. They work by killing the bacteria or by making it hard for the bacteria to grow and multiply.

The full article covers:

  • What are antibiotics?
  • What do antibiotics treat?
  • Do antibiotics treat viral infections?
  • What are the side effects of antibiotics?
  • Why is it important to take antibiotics only when they're needed?
  • How do I use antibiotics correctly?

Read the full article at MedlinePlus

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

Convert T36.6X6A 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 T36.6X6AOverview

Is T36.6X6A (Underdosing of rifampicins) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of rifampicins, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T36.6X6A mean?

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

Can T36.6X6A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of rifampicins, 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.