2026 ICD-10-CM Diagnosis Code T65.821AToxic effect of harmful algae and algae toxins, accidental (unintentional), initial encounter

ICD-10-CM Codes›S00–T88›T51-T65›T65

ICD-10-CM T65.821A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T65.821A is a billable ICD-10-CM diagnosis code for toxic effect of harmful algae and algae toxins, 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 azaspiracid poisoning. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under External cause codes: intent of injury, accidental/unintentional; External cause codes: natural/environment; initial encounter; and Toxic effects, initial encounter.

Code Identity

ICD-10-CM Code
T65.821A
Billable Status
Yes — Valid for Submission
Code Describes
Toxic effect of harmful algae and algae toxins, accidental (unintentional), initial encounter
Short Description
Toxic effect of harmful algae and algae toxins, acc, init
Parent Code
T65.821
Toxic effect of harmful algae and algae toxins, accidental (unintentional)

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionT51-T65Toxic effects of substances chiefly nonmedicinal as to source
CategoryT65Toxic effect of other and unspecified substances
This CodeT65.821AToxic effect of harmful algae and algae toxins, accidental (unintentional), initial encounter

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Azaspiracid poisoning
  • Blue-green algae toxin poisoning
  • Microcystis flos-aquae poisoning
  • Microcystis poisoning
  • Toxic effect from eating shellfish

Coding GuidelinesGuidance

The appropriate 7th character is to be added to each code from block Toxic effect of other and unspecified substances (T65). 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 EXT020
External cause codes: intent of injury, accidental/unintentional
Default principal diagnosis: inpatient No · outpatient No
CCSR EXT011
External cause codes: natural/environment; initial encounter
Default principal diagnosis: inpatient No · outpatient No
CCSR INJ023
Toxic effects, initial encounter
Default principal diagnosis: inpatient Yes · outpatient Yes

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
Algae (harmful) (toxin)T65.821T65.822T65.823T65.824––

Convert T65.821A to ICD-9-CMHistory

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

ICD-9-CM
989.89 Tox eff nonmed subst NEC
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
E928.6 Envir expose algae/toxin
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 T65.821AOverview

Is T65.821A a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report toxic effect of harmful algae and algae toxins, accidental (unintentional), initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T65.821A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for toxic effect of harmful algae and algae toxins, accidental (unintentional), such as an emergency visit or first evaluation.

What MS-DRG does T65.821A group to?

When toxic effect of harmful algae and algae toxins, 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 T65.821A?

Under the General Equivalence Mappings, toxic effect of harmful algae and algae toxins, accidental (unintentional), initial encounter converts to ICD-9-CM 989.89 (tox eff nonmed subst NEC) and E928.6 (envir expose algae/toxin). 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.