What is ICD-10 code D89.40?

D89.40 is the ICD-10-CM diagnosis code for Mast cell activation, unspecified. It belongs to the ICD-10-CM chapter for diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism (d50-d89). D89.40 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. D89.40 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. D89.40 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. D89.40 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.

Which HCC does D89.40 map to under V28?

D89.40 has no payable CMS-HCC V28 mapping in the final FY2026 CMS file.

What RAF reference is published for D89.40?

D89.40 has no V28 reference coefficient because the final FY2026 source verifies no payable V28 mapping.

What is the V28 vs V24 difference for D89.40?

D89.40 has verified no payable V28 mapping in the final FY2026 source. Select a historical effective period before making any V24 comparison.

What documentation supports D89.40?

Use D89.40 only when the documentation does not specify the type of mast cell activation disorder (such as systemic mastocytosis or mast cell activation syndrome); if a specific type is documented, use the more specific code Review clinical documentation carefully for associated symptoms or triggers, as this unspecified code may need to be queried to the provider for clarification and more precise coding D89.40 is not a CMS-HCC V28 payment code in the verified current source, so MEAT is not the payment hook. Documentation still has to support the diagnosis if you report it.

2026 ICD-10-CM D89.40: Mast cell activation, unspecified

D89.40 is not a CMS-HCC payment code. MEAT criteria · RAF Calculator · HCC Buddy coding tools

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupD89.40Effective 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)

D89.40

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

Mast cell activation, unspecified

A condition where mast cells (immune system cells) become overly active and release chemicals that cause allergic-like symptoms throughout the body, but the specific type or cause is not identified. This can result in symptoms like flushing, itching, swelling, and gastrointestinal problems.

Buddy Insight

CMS-HCC V28

N/A

—

Not risk-adjusted for this model/period.

Not mapped

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 074

HHS-HCC 074 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 47

Disorders of Immunity

Code-level coefficient reference

RXHCC

Mapped

HCC 99

Immune Disorders

Code-level coefficient reference

Code Book Path

Official

D89Other disorders involving the immune mechanism, not elsewhere classified

D89.4Mast cell activation syndrome and related disorders

D89.40Mast cell activation, unspecified

Inclusion Terms

Official

Mast cell activation disorder, unspecified
Mast cell activation syndrome, NOS

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for D89.40 in this effective period.

Related Child Codes

Official

D89.41Monoclonal mast cell activation syndrome

D89.42Idiopathic mast cell activation syndrome

D89.43Secondary mast cell activation

D89.44Hereditary alpha tryptasemia

D89.49Other mast cell activation disorder

Includes

Official

ICD-10-CM does not list Includes notes for D89.40 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 D89.40 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for D89.40 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for D89.40 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation should include clinical symptoms consistent with mast cell activation, elevated tryptase levels (baseline and during episodes), response to mast cell-directed therapy, and any workup to classify the type of mast cell activation disorder.

The provider should document the frequency and severity of episodes, triggering factors, and current medication regimen including antihistamines and mast cell stabilizers.

MEAT Support

HCC Buddy guidance

The provider should document the frequency and severity of episodes, triggering factors, and current medication regimen including antihistamines and mast cell stabilizers.

Audit Caution

HCC Buddy guidance

This unspecified code should prompt further workup to determine the specific type of mast cell activation disorder (D89.41-D89.49).

Do not use this code long-term if a more specific diagnosis has been established.

Avoid confusing mast cell activation with simple allergic reactions (T78.40) or idiopathic urticaria (L50.1), which are more common and clinically distinct.

Common Mistakes

HCC Buddy guidance

D89.41 (Monoclonal mast cell activation syndrome) requires documented clonal mast cell markers.

D89.42 (Idiopathic mast cell activation syndrome) is used when clonal disease is excluded.

D89.43 (Secondary mast cell activation) is for activation secondary to another known condition.

D47.01-D47.09 (Mastocytosis) involves neoplastic mast cell proliferation, a distinct entity from activation syndromes.

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 D89.40 an HCC code?

D89.40 is not in the CMS-HCC V28 or V24 community payment model. D89.40 has a separate mapping under the CMS-HCC ESRD model (HCC 47 (Disorders of Immunity)) and the Part D RxHCC model (HCC 99 (Immune Disorders)); the applicable result needs member context. D89.40 also appears in the HHS-HCC commercial risk model (HCC 074 (HHS-HCC 074 adult, RAF varies by metal level)), which is a commercial market model rather than a Medicare Advantage payment mapping.

Code: D89.40
Description: Mast cell activation, unspecified
HCC (V28): No CMS-HCC V28 mapping
RAF reference coefficient: —
Billable: Yes
Payment year: 2026

HCC Category Mapping

ESRDHCC 47, Disorders of Immunity

Not separately weighted

RxHCCHCC 99, Immune Disorders

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. 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 context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work D89.40 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT Criteria for D89.40

For D89.40 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 D89.40 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

Get the V28 mapping + MEAT cheat sheet

One printable reference: check representative V28 mappings and the documentation reminders your note needs. Free, no card.

Free PDF. No card. Unsubscribe anytime.

What This Code Means

D89.40 is the ICD-10-CM diagnosis code for mast cell activation, unspecified. A condition where mast cells (immune system cells) become overly active and release chemicals that cause allergic-like symptoms throughout the body, but the specific type or cause is not identified. This can result in symptoms like flushing, itching, swelling, and gastrointestinal problems. D89.40 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).

D89.40 has no mapping under the CMS-HCC V28 or V24 community payment models. D89.40 has a separate mapping under the CMS-HCC ESRD model (HCC 47 (Disorders of Immunity)) and the Part D RxHCC model (HCC 99 (Immune Disorders)); the applicable result needs member context. D89.40 also appears in the HHS-HCC commercial risk model (HCC 074 (HHS-HCC 074 adult, RAF varies by metal level)), which is a commercial market model rather than a Medicare Advantage payment mapping. Do not assign V28 risk adjustment value from this page; verify the applicable model and payment year before using this code for risk adjustment.

Use D89.40 only when the documentation does not specify the type of mast cell activation disorder (such as systemic mastocytosis or mast cell activation syndrome); if a specific type is documented, use the more specific code.

HCC Buddy maintains structured V28 and V24 mapping, source-labeled coefficient references, and MEAT documentation criteria for D89.40 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use D89.40 only when the documentation does not specify the type of mast cell activation disorder (such as systemic mastocytosis or mast cell activation syndrome); if a specific type is documented, use the more specific code
•Review clinical documentation carefully for associated symptoms or triggers, as this unspecified code may need to be queried to the provider for clarification and more precise coding

Clinical Significance

Mast cell activation, unspecified, represents a condition where mast cells release excessive mediators (histamine, tryptase, prostaglandins, leukotrienes) causing multisystem symptoms including flushing, urticaria, hypotension, gastrointestinal distress, and anaphylaxis. This unspecified code is used when the specific type of mast cell activation disorder has not yet been determined. The condition significantly impacts quality of life and can pose acute anaphylactic risk.

Documentation Requirements

✓Documentation should include clinical symptoms consistent with mast cell activation, elevated tryptase levels (baseline and during episodes), response to mast cell-directed therapy, and any workup to classify the type of mast cell activation disorder.
✓The provider should document the frequency and severity of episodes, triggering factors, and current medication regimen including antihistamines and mast cell stabilizers.

Commonly Confused Codes

•D89.41 (Monoclonal mast cell activation syndrome) requires documented clonal mast cell markers.
•D89.42 (Idiopathic mast cell activation syndrome) is used when clonal disease is excluded.
•D89.43 (Secondary mast cell activation) is for activation secondary to another known condition.
•D47.01-D47.09 (Mastocytosis) involves neoplastic mast cell proliferation, a distinct entity from activation syndromes.