D89.42
BillableIdiopathic mast cell activation syndrome
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is D89.42 an HCC code?
Yes. D89.42 maps to Disorders of Immunity under the V24 model but is not retained in V28.
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. 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.
MEAT Criteria for D89.42
For D89.42 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 D89.42 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
D89.42 is the ICD-10-CM diagnosis code for idiopathic mast cell activation syndrome. A condition where mast cells become abnormally activated causing symptoms like flushing, itching, and gastrointestinal issues, but without an identifiable cause or genetic mutation. D89.42 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, D89.42 maps to Disorders of Immunity (HCC 47) with a community, non-dual, aged base RAF weight of 0.472. 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.
This is a diagnosis of exclusion; ensure secondary causes have been ruled out before coding. Because D89.42 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 D89.42 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a diagnosis of exclusion; ensure secondary causes have been ruled out before coding
- •Document specific symptoms and triggers to support medical necessity
Clinical Significance
Idiopathic mast cell activation syndrome is diagnosed when patients have episodic mast cell mediator release symptoms meeting diagnostic criteria but lack evidence of clonal mast cell disease or an identifiable secondary cause. It is the most common form of mast cell activation syndrome and can cause significant morbidity with recurrent anaphylaxis, flushing, gastrointestinal symptoms, and cardiovascular instability. Management focuses on anti-mediator therapy and trigger avoidance.
Documentation Requirements
- ✓Documentation must demonstrate that the condition meets consensus diagnostic criteria: episodic symptoms consistent with mast cell activation affecting two or more organ systems, laboratory evidence of mast cell activation (elevated tryptase, urinary histamine metabolites), and response to anti-mediator therapy.
- ✓Documentation should confirm exclusion of both clonal mast cell disease (bone marrow biopsy negative for KIT mutations) and identifiable secondary causes.
Commonly Confused Codes
- •D89.41 (Monoclonal mast cell activation syndrome) requires clonal mast cell markers.
- •D89.43 (Secondary mast cell activation) is used when a causative condition is identified.
- •T78.2 (Anaphylactic shock, unspecified) captures acute anaphylactic episodes but not the underlying chronic syndrome.
- •L50.1 (Idiopathic urticaria) may overlap symptomatically but is a distinct and narrower diagnosis.