2026 ICD-10-CM Diagnosis Code T43.021APoisoning by tetracyclic antidepressants, accidental (unintentional), initial encounter
T43.021A is a billable ICD-10-CM diagnosis code for poisoning by tetracyclic antidepressants, accidental (unintentional), 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) and groups to MS-DRG 917 through 918. Coders also document this condition as accidental maprotiline overdose. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under External cause codes: intent of injury, accidental/unintentional; External cause codes: poisoning by drug; and Poisoning by drugs, initial encounter.
Code Identity
Code Classification
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Accidental maprotiline overdose
- Accidental maprotiline poisoning
- Accidental mianserin overdose
- Accidental mianserin poisoning
- Maprotiline overdose
- Maprotiline poisoning
- Mianserin overdose
- Mianserin poisoning
- Tetracyclic antidepressant drug overdose
Coding GuidelinesGuidance
When coding a poisoning or reaction to the improper use of a medication (e.g., overdose, wrong substance given or taken in error, wrong route of administration), first assign the appropriate code from categories T36-T50. The poisoning codes have an associated intent as their 5th or 6th character (accidental, intentional self-harm, assault and undetermined). If the intent of the poisoning is unknown or unspecified, code the intent as accidental intent. The undetermined intent is only for use if the documentation in the record specifies that the intent cannot be determined. Use additional code(s) for all manifestations of poisonings.
The appropriate 7th character is to be added to each code from block Poisoning by, adverse effect of and underdosing of psychotropic drugs, not elsewhere classified (T43). 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
Maprotiline
a bridged-ring tetracyclic antidepressant that is both mechanistically and functionally similar to the tricyclic antidepressants, including side effects associated with its use.Mianserin
a tetracyclic compound with antidepressant effects. it may cause drowsiness and hematological problems. its mechanism of therapeutic action is not well understood, although it apparently blocks alpha-adrenergic, histamine h1, and some types of serotonin receptors.Mirtazapine
a piperazinoazepine tetracyclic compound that enhances the release of norepinephrine and serotonin through blockage of presynaptic alpha-2 adrenergic receptors. it also blocks both 5-ht2 and 5-ht3 serotonin receptors and is a potent histamine h1 receptor antagonist. it is used for the treatment of depression, and may also be useful for the treatment of anxiety disorders.
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
Antidepressants
Antidepressants are prescription medicines to treat depression. Depression is more than feeling a little sad or "blue" for a few days. It's a very common, serious medical illness that affects your mood and general mental health It can make you feel tired, hopeless, worried, or fearful. It can change your thinking, sleeping, and eating.
The full article covers:
- What are antidepressants?
- Are antidepressants used for other conditions?
- What are the different types of antidepressants?
- Which type of antidepressant is right for me?
- How long do antidepressants take to work?
- How long will I need to take an antidepressant?
- What are the side effects of antidepressants?
- What can I do to take antidepressants safely?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T43.021A to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T43.021AOverview
Is T43.021A a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report poisoning by tetracyclic antidepressants, accidental (unintentional), initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T43.021A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for poisoning by tetracyclic antidepressants, accidental (unintentional), such as an emergency visit or first evaluation.
What MS-DRG does T43.021A group to?
When poisoning by tetracyclic antidepressants, accidental (unintentional), initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 917, 918, with relative weights from 0.8571 to 1.5684 depending on complications. Higher weights mean higher Medicare reimbursement.
What is the ICD-9 equivalent of T43.021A?
Under the General Equivalence Mappings, poisoning by tetracyclic antidepressants, accidental (unintentional), initial encounter converts to ICD-9-CM 969.04 (pois tetracyclc andepres) and E854.0 (acc poison-antidepressnt). The mapping is approximate, so confirm the match fits the documentation.
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.
