2026 ICD-10-CM Diagnosis Code D89.42Idiopathic mast cell activation syndrome

ICD-10-CM Codes›D50–D89›D80-D89›D89

ICD-10-CM D89.42
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

D89.42 is a billable ICD-10-CM diagnosis code for idiopathic mast cell activation syndrome. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 814 through 816. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Immunity disorders.

Code Identity

ICD-10-CM Code
D89.42
Billable Status
Yes — Valid for Submission
Code Describes
Idiopathic mast cell activation syndrome
Short Description
Idiopathic mast cell activation syndrome
Same as the full description in the CMS dataset.
Parent Code
D89.4
Mast cell activation syndrome and related disorders

Code Classification

ChapterD50–D89Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism
SectionD80-D89Certain disorders involving the immune mechanism
CategoryD89Other disorders involving the immune mechanism, not elsewhere classified
This CodeD89.42Idiopathic mast cell activation syndrome

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Idiopathic mast cell activation syndrome

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Activation
      • mast cell (disorder) (syndrome)
        • idiopathic

Clinical ClassificationClinical

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

CCSR BLD008
Immunity disorders
Default principal diagnosis: inpatient Yes · outpatient Yes

Convert D89.42 to ICD-9-CMHistory

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

ICD-9-CM
279.8 Immune mechanism dis NEC
Approximate The match is approximate rather than exact.

Code History & ChangesHistory

Replacement D89.42 replaces the following previously assigned code(s):

  • D89.89 - Oth disrd involving the immune mechanism, NEC
FY 2018AddedAdded to the ICD-10-CM code setEffective October 1, 2017.
FY 2019–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About D89.42Overview

Is D89.42 (Mast cell activation syndrome and related disorders) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report idiopathic mast cell activation syndrome on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does D89.42 group to?

When idiopathic mast cell activation syndrome is the principal diagnosis on an inpatient stay, it groups to MS-DRG 814, 815, 816, with relative weights from 0.6320 to 2.1267 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of D89.42?

Under the General Equivalence Mappings, idiopathic mast cell activation syndrome converts to ICD-9-CM 279.8 (immune mechanism dis NEC). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.