2027 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 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

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 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 951. 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 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 T36.6X6A: its own notes plus those printed at T36, 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.

From Block T36-T50 Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances applies to 3,186 codes
  • 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.

From T36 Poisoning by, adverse effect of and underdosing of systemic antibiotics applies to 198 codes
  • antineoplastic antibiotics (T45.1-)
  • locally applied antibiotic NEC (T49.0)
  • topically used antibiotic for ear, nose and throat (T49.6)
  • topically used antibiotic for eye (T49.5)
From Block T36-T50 Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances applies to 3,186 codes
  • 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.

From Block T36-T50 Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances applies to 3,186 codes
  • abuse and dependence of psychoactive substances (F10-F19)
  • abuse of non-dependence-producing substances (F55.-)
  • immunodeficiency due to drugs (D84.821)
  • drug reaction and poisoning affecting newborn (P00-P96)
  • pathological drug intoxication (inebriation) (F10-F19)
From Chapter 19 (S00-T88) Injury, poisoning and certain other consequences of external causes applies to 41,187 codes

Code First

The underlying condition named in the note is sequenced before this code.

From Block T36-T50 Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances applies to 3,186 codes
  • for adverse effects, the nature of the adverse effect, such as:
  • adverse effect NOS (T88.7)
  • aspirin gastritis (K29.-)
  • blood disorders (D56-D76)
  • contact dermatitis (L23-L25)
  • dermatitis due to substances taken internally (L27.-)
  • nephropathy (N14.0-N14.2)

Use Additional Code

Add a second code, after this one, to fully describe the condition.

From Block T36-T50 Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances applies to 3,186 codes
  • code(s) to specify:
  • manifestations of poisoning
  • underdosing or failure in dosage during medical and surgical care (Y63.6, Y63.8-Y63.9)
  • underdosing of medication regimen (Z91.12-, Z91.13-)
From Chapter 19 (S00-T88) Injury, poisoning and certain other consequences of external causes applies to 41,187 codes
  • code to identify any retained foreign body, if applicable (Z18.-)

Notes

General instructions on how these codes are used.

From Block T36-T50 Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances applies to 3,186 codes
  • The drug giving rise to the adverse effect should be identified by use of codes from categories T36-T50 with fifth or sixth character 5.
From Chapter 19 (S00-T88) Injury, poisoning and certain other consequences of external causes applies to 41,187 codes
  • 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.

From T36 Poisoning by, adverse effect of and underdosing of systemic antibiotics applies to 198 codes
  • The appropriate 7th character is to be added to each code from category T36
Values for category T36 Encounter: Initial, Subsequent, Sequela
  • 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 T36.6X6A, its category, or a range that includes it.

Excludes1 1

These codes carry an Excludes1 note naming T36.6X6A: they are not reported together with it, unless the two conditions are unrelated.

  • T37.1 Poisoning by, adverse effect of and underdosing of antimycobacterial drugs
    rifampicins (T36.6-) names T36.6-, which includes this code

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
Isoniazid::with::rifampicinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifabutinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifamideT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
RifampicinT36.6X1T36.6X2T36.6X3T36.6X4T36.6X5T36.6X6
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 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About T36.6X6AOverview

What is the ICD-10 code for underdosing of rifampicins, initial encounter?

The ICD-10-CM code for underdosing of rifampicins, initial encounter is T36.6X6A (sometimes written as T366X6A). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

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, 2026 through September 30, 2027.

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.

What MS-DRG does T36.6X6A group to?

On inpatient claims, underdosing of rifampicins, initial encounter maps to MS-DRG 951 (other Factors Influencing Health Status), which carries a relative weight of 0.5574. Higher weights mean higher Medicare reimbursement.

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.