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.

D89.42 no longer risk-adjusts for Medicare Advantage: it mapped to HCC 47 under the retired CMS-HCC V24 model through payment year 2025 but maps to no category in the live V28 model. It still risk-adjusts in the PACE (CMS-HCC V22) category 47, ESRD (V21) category 47, ESRD (V24) category 47, and RxHCC Part D (V08) category 99 for payment year 2026.

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

Medicare Risk Adjustment (HCC)Billing

D89.42 no longer risk-adjusts for Medicare Advantage: it maps to no payment category in the live CMS-HCC V28 model, although it still risk-adjusts in the other CMS models shown below.

CMS-HCC V28 (Medicare Advantage Payment Model)
Not mapped
Dropped in V28 see all codes that no longer risk-adjust
Prior Model (CMS-HCC V24)
HCC 47
V24 retired last contributed to a Medicare Advantage risk score in payment year 2025
Other CMS Models
PACE (CMS-HCC V22): HCC 47 · ESRD (V21): HCC 47 · ESRD (V24): HCC 47
ESRD V21 weights: 0.097 dialysis, 0.549–0.688 functioning graft · ESRD V24 weights: 0.078 dialysis, 0.523–0.803 functioning graft
Part D (RxHCC)
RxHCC 99 — Immune Disorders
also risk-adjusts in the Part D prescription drug model (V08)

Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.

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.

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 2019AddedAdded to the ICD-10-CM code setEffective October 1, 2018.
FY 2020–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About D89.42Overview

What is the ICD-10 code for idiopathic mast cell activation syndrome?

The ICD-10-CM code for idiopathic mast cell activation syndrome is D89.42 (sometimes written as D8942). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

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.

Does D89.42 risk-adjust for Medicare Advantage payment?

Not for Medicare Advantage. D89.42 mapped to HCC 47 in the retired CMS-HCC V24 model, which last determined payment in 2025, but it maps to no category in the live V28 model; see all codes that no longer risk-adjust. It still risk-adjusts in the PACE (CMS-HCC V22) category 47 (Disorders of Immunity), ESRD (V21) category 47 (Disorders of Immunity), ESRD (V24) category 47 (Disorders of Immunity), and RxHCC Part D (V08) category 99 (Immune Disorders).