D89.49
BillableOther mast cell activation disorder
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is D89.49 an HCC code?
Yes. D89.49 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 D89.49
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.
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 0.472. 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.
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.