2026 ICD-10-CM Diagnosis Code T36.4X6AUnderdosing of tetracyclines, initial encounter
T36.4X6A is a billable ICD-10-CM diagnosis code for underdosing of tetracyclines, 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
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T36.4X6A.
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.
Clinical InformationClinical
Chlortetracycline
a tetracycline with a 7-chloro substitution.Demeclocycline
a tetracycline analog having a 7-chloro and a 6-methyl. because it is excreted more slowly than tetracycline, it maintains effective blood levels for longer periods of time.Doxycycline
a synthetic tetracycline derivative with similar antimicrobial activity.Lymecycline
a semisynthetic antibiotic related to tetracycline. it is more readily absorbed than tetracycline and can be used in lower doses.Methacycline
a broad-spectrum semisynthetic antibiotic related to tetracycline but excreted more slowly and maintaining effective blood levels for a more extended period.Minocycline
a tetracycline analog, having a 7-dimethylamino and lacking the 5 methyl and hydroxyl groups, which is effective against tetracycline-resistant staphylococcus infections.Oxytetracycline
a tetracycline analog isolated from the actinomycete streptomyces rimosus and used in a wide variety of clinical conditions.Rolitetracycline
a pyrrolidinylmethyl tetracycline.Tetracycline
a naphthacene antibiotic that inhibits amino acyl trna binding during protein synthesis.Tetracycline Resistance
nonsusceptibility of bacteria to the action of tetracycline which inhibits aminoacyl-trna binding to the 30s ribosomal subunit during protein synthesis.Tetracyclines
closely congeneric derivatives of the polycyclic naphthacenecarboxamide. (gilman et al., goodman and gilman's the pharmacological basis of therapeutics, 8th ed, p1117)
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.
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 T36.4X6A to ICD-9-CMHistory
Code HistoryHistory
Questions About T36.4X6AOverview
Is T36.4X6A (Underdosing of tetracyclines) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of tetracyclines, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T36.4X6A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of tetracyclines, such as an emergency visit or first evaluation.
Can T36.4X6A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of tetracyclines, 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.
