What is ICD-10 code D89.49?

D89.49 is the ICD-10-CM diagnosis code for Other mast cell activation disorder. Other types of mast cell activation disorders not specifically classified elsewhere, including rare or atypical presentations of mast cell dysfunction. D89.49 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. D89.49 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. D89.49 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. D89.49 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for D89.49?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. D89.49 was a payment HCC under V24 (Disorders of Immunity) but the V28 recalibration removed it from the risk adjustment model. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting D89.49?

Use this code only when the specific type of mast cell activation disorder does not fit other D89.4x codes Include detailed documentation of the specific disorder in the medical record Because D89.49 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

2026 ICD-10-CM D89.49: Other mast cell activation

ICD-10-CM Code View

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Code lookupD89.49Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism (D50-D89) / Certain disorders involving the immune mechanism (D80-D89)

D89.49

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Other mast cell activation disorder

Other types of mast cell activation disorders not specifically classified elsewhere, including rare or atypical presentations of mast cell dysfunction.

Buddy Insight

This code captures other specified mast cell activation disorders that do not fit into the monoclonal, idiopathic, secondary, or hereditary alpha tryptasemia categories.

CMS-HCC V28

N/A

—

Not risk-adjusted for this model/period.

Not mapped

CMS-HCC V24

Mapped

HCC 47

Disorders of Immunity

RAF 0.665

ACA/HHS

Mapped

HCC 74

HHS-HCC 74

Varies by metal level

ESRD/PACE

Mapped

HCC 47

Disorders of Immunity

RAF 0.078

RXHCC

Mapped

HCC 99

Immunodeficiencies

RAF 0.943

Code Book Path

Official

D89Other disorders involving the immune mechanism, not elsewhere classified

D89.4Mast cell activation syndrome and related disorders

D89.49Other mast cell activation disorder

Inclusion Terms

Official

Other mast cell activation syndrome

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for D89.49 in this effective period.

Related Child Codes

Official

D89.40Mast cell activation, unspecified

D89.41Monoclonal mast cell activation syndrome

D89.42Idiopathic mast cell activation syndrome

D89.43Secondary mast cell activation

D89.44Hereditary alpha tryptasemia

Includes

Official

ICD-10-CM does not list Includes notes for D89.49 in this effective period.

Excludes 1

Official

aggressive systemic mastocytosis (C96.21)
congenital cutaneous mastocytosis (Q82.2)
(non-congenital) cutaneous mastocytosis (D47.01)
(indolent) systemic mastocytosis (D47.02)
malignant mast cell neoplasm (C96.2-)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for D89.49 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for D89.49 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for D89.49 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation should describe the specific type of mast cell activation disorder that does not fit the established categories, including clinical presentation, laboratory findings (tryptase levels, other mediator measurements), and the rationale for classifying it outside the standard subtypes.

Workup results that exclude monoclonal, idiopathic, and secondary forms should be documented.

MEAT Support

HCC Buddy guidance

Workup results that exclude monoclonal, idiopathic, and secondary forms should be documented.

Audit Caution

HCC Buddy guidance

This code should not be used as a default when workup is incomplete

use D89.40 (unspecified) instead if the type has not been investigated. Ensure documentation supports why the condition does not fit into the monoclonal, idiopathic, or secondary categories. Avoid using this code interchangeably with unspecified mast cell activation.

Common Mistakes

HCC Buddy guidance

D89.40 (Mast cell activation, unspecified) is used when the type has not been determined at all, whereas D89.49 implies a specific type has been identified but does not fit standard categories.

D89.41-D89.43 capture the three main classified subtypes.

D89.44 (Hereditary alpha tryptasemia) captures the genetic form specifically.

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is D89.49 an HCC code?

Yes. D89.49 maps to Disorders of Immunity under the V24 model but is not retained in V28.

Code: D89.49
Description: Other mast cell activation disorder
HCC (V28): No CMS-HCC V28 mapping
RAF: —
Billable: Yes
Payment year: 2026

HCC Category Mapping

V24HCC 47, Disorders of Immunity

0.665

ESRDHCC 47, Disorders of Immunity

0.078

RxHCCHCC 99, Immunodeficiencies

0.943

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.

Work D89.49 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for D89.49

For D89.49 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed D89.49 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

D89.49 is the ICD-10-CM diagnosis code for other mast cell activation disorder. Other types of mast cell activation disorders not specifically classified elsewhere, including rare or atypical presentations of mast cell dysfunction. D89.49 sits in the ICD-10-CM chapter for diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism (d50-d89), within the section covering certain disorders involving the immune mechanism (d80-d89).

Under the older CMS-HCC V24 model, D89.49 maps to Disorders of Immunity (HCC 47) with a community, non-dual, aged base RAF weight of 0.665. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.

Use this code only when the specific type of mast cell activation disorder does not fit other D89.4x codes. Because D89.49 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for D89.49 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use this code only when the specific type of mast cell activation disorder does not fit other D89.4x codes
•Include detailed documentation of the specific disorder in the medical record

Clinical Significance

This code captures other specified mast cell activation disorders that do not fit into the monoclonal, idiopathic, secondary, or hereditary alpha tryptasemia categories. It serves as a residual category for atypical or evolving presentations of mast cell activation where the clinician has identified a specific type that lacks its own unique ICD-10-CM code. These cases may require specialized immunology or allergy management.

Documentation Requirements

✓Documentation should describe the specific type of mast cell activation disorder that does not fit the established categories, including clinical presentation, laboratory findings (tryptase levels, other mediator measurements), and the rationale for classifying it outside the standard subtypes.
✓Workup results that exclude monoclonal, idiopathic, and secondary forms should be documented.

Commonly Confused Codes

•D89.40 (Mast cell activation, unspecified) is used when the type has not been determined at all, whereas D89.49 implies a specific type has been identified but does not fit standard categories.
•D89.41-D89.43 capture the three main classified subtypes.
•D89.44 (Hereditary alpha tryptasemia) captures the genetic form specifically.