2027 ICD-10-CM Diagnosis Code T42.0X6AUnderdosing of hydantoin derivatives, initial encounter

ICD-10-CM T42.0X6A

CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

T42.0X6A is a billable ICD-10-CM diagnosis code for underdosing of hydantoin derivatives, 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. Coders also document this condition as phenytoin taken at lower dose than instructed. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Underdosing of drugs and medicaments, initial encounter.

Code Identity

ICD-10-CM Code

T42.0X6A

Billable Status

Yes — Valid for Submission

Code Describes

Underdosing of hydantoin derivatives, initial encounter

Short Description

Underdosing of hydantoin derivatives, initial encounter

Same as the full description in the CMS dataset.

Parent Code

T42.0X6

Underdosing of hydantoin derivatives

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

CategoryT42Poisoning by, adverse effect of and underdosing of antiepileptic, sedative- hypnotic and antiparkinsonism drugs

This CodeT42.0X6AUnderdosing of hydantoin derivatives, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T42.0X6A.

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.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

Phenytoin taken at lower dose than instructed

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 antiepileptic, sedative- hypnotic and antiparkinsonism drugs (T42). 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 T42.0X6A: its own notes plus those printed at T42, block T36-T50, and Chapter 19. A note printed at a category, block or chapter applies to every code under it.