C96.21
BillableAggressive systemic mastocytosis
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C96.21 an HCC code?
Yes. C96.21 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors under the CMS-HCC V28 risk adjustment model (and Lymphoma and Other Cancers under V24).
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 C96.21
For C96.21 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 C96.21 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C96.21 is the ICD-10-CM diagnosis code for aggressive systemic mastocytosis. An aggressive form of mast cell cancer that spreads throughout the body and affects multiple organ systems. C96.21 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of lymphoid, hematopoietic and related tissue (c81-c96).
Under the CMS-HCC V28 risk adjustment model, C96.21 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors (HCC 21) with a community, non-dual, aged base RAF weight of 0.545. Under the older CMS-HCC V24 model, C96.21 maps to Lymphoma and Other Cancers (HCC 10) with a community, non-dual, aged base RAF weight of 0.675. 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 code indicates systemic involvement; ensure documentation describes aggressive behavior and multi-organ involvement. Because C96.21 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 C96.21 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code indicates systemic involvement; ensure documentation describes aggressive behavior and multi-organ involvement
- •Aggressive systemic mastocytosis has a worse prognosis than indolent forms
Clinical Significance
Aggressive systemic mastocytosis is a rare, life-threatening form of systemic mastocytosis characterized by progressive organ damage from mast cell infiltration. Unlike indolent mastocytosis, this form causes organ dysfunction manifesting as cytopenias from bone marrow failure, ascites from liver involvement, malabsorption from gastrointestinal infiltration, or pathologic fractures from skeletal involvement.
Documentation Requirements
- ✓Documentation must confirm systemic mastocytosis criteria (elevated tryptase, KIT D816V mutation, bone marrow mast cell aggregates) plus at least one C-finding indicating organ damage (cytopenias, hepatomegaly with impaired function, skeletal lesions with large osteolyses, or malabsorption).
- ✓Treatment plan and response should be documented.
Commonly Confused Codes
- •D47.02 (systemic mastocytosis) is the indolent form without organ damage.
- •C94.30 (mast cell leukemia) requires at least 20% mast cells in bone marrow and circulating mast cells.
- •C96.22 (mast cell sarcoma) is a localized solid tumor, not systemic disease.
- •C96.20 is the unspecified malignant mast cell code.