2026 ICD-10-CM Diagnosis Code T42.0X4SPoisoning by hydantoin derivatives, undetermined, sequela

ICD-10-CM Codes›S00–T88›T36-T50›T42

ICD-10-CM T42.0X4S
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T42.0X4S is a billable ICD-10-CM diagnosis code for poisoning by hydantoin derivatives, undetermined, sequela. The 7th character S marks it as a sequela code, which reports a problem that remains after the original condition has resolved. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 922 through 923. The code is exempt from POA reporting. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Poisoning/toxic effect/adverse effects/underdosing, sequela.

Code Identity

ICD-10-CM Code
T42.0X4S
Billable Status
Yes — Valid for Submission
Code Describes
Poisoning by hydantoin derivatives, undetermined, sequela
Short Description
Poisoning by hydantoin derivatives, undetermined, sequela
Same as the full description in the CMS dataset.
Parent Code
T42.0X4
Poisoning by hydantoin derivatives, undetermined

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 CodeT42.0X4SPoisoning by hydantoin derivatives, undetermined, sequela

Present on Admission (POA)Billing

T42.0X4S is exempt from POA reporting on inpatient claims to general acute care hospitals. Review other POA exempt codes.

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 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 2026, published by CMS and the National Center for Health Statistics.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR INJ075
Poisoning/toxic effect/adverse effects/underdosing, sequela
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Mephenytoin

    an anticonvulsant effective in tonic-clonic epilepsy (epilepsy, tonic-clonic). it may cause blood dyscrasias.

  • Phenytoin

    an anticonvulsant that is used to treat a wide variety of seizures. it is also an anti-arrhythmic and a muscle relaxant. the mechanism of therapeutic action is not clear, although several cellular actions have been described including effects on ion channels, active transport, and general membrane stabilization. the mechanism of its muscle relaxant effect appears to involve a reduction in the sensitivity of muscle spindles to stretch. phenytoin has been proposed for several other therapeutic uses, but its use has been limited by its many adverse effects and interactions with other drugs.

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
AlbutoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
DilantinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
DiphenylhydantoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
EpanutinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
EthotoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
Hydantoin derivative NECT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MephenytoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
Mephenytoin::with phenobarbitalT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MetetoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MethetoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MethoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
PhenantoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
PhenytoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
Phenytoin::with PhenobarbitalT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6

Patient EducationClinical

Poisoning

A poison is any substance that is harmful to your body. You might swallow it, inhale it, inject it, or absorb it through your skin. Any substance can be poisonous if too much is taken. Poisons can include:

Read the full article at MedlinePlus

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

Convert T42.0X4S to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
909.0 Late eff drug poisoning
ApproximateCombination The match is approximate, and more than one code can be needed to describe the source diagnosis. Confirm with contextual judgment.
ICD-9-CM
E989 Late eff inj-undet circ
ApproximateCombination The match is approximate, and more than one code can be needed to describe the source diagnosis. Confirm with contextual judgment.

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 T42.0X4SOverview

Is T42.0X4S (Poisoning by hydantoin derivatives, undetermined) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report poisoning by hydantoin derivatives, undetermined, sequela on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character S in T42.0X4S mean?

The final character S makes this a sequela code: it reports a lingering problem that remains after the poisoning by hydantoin derivatives, undetermined itself has resolved, not the original event.

What MS-DRG does T42.0X4S group to?

When poisoning by hydantoin derivatives, undetermined, sequela is the principal diagnosis on an inpatient stay, it groups to MS-DRG 922, 923, with relative weights from 1.0177 to 1.7494 depending on complications. Higher weights mean higher Medicare reimbursement.

Is T42.0X4S exempt from POA reporting?

Yes. CMS lists this code among those exempt from present on admission reporting, so hospitals do not assign a POA indicator for poisoning by hydantoin derivatives, undetermined, sequela on inpatient claims.

What is the ICD-9 equivalent of T42.0X4S?

Under the General Equivalence Mappings, poisoning by hydantoin derivatives, undetermined, sequela converts to ICD-9-CM 909.0 (late eff drug poisoning) and E989 (late eff inj-undet circ). 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.