ICD-10 Code C96.21 – Aggressive systemic mastocytosis (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C96.21 – Aggressive systemic mastocytosis
What it is
C96.21 identifies aggressive systemic mastocytosis, a rare mast cell disorder with systemic involvement. The code is used when the disease is causing organ damage or significant clinical deterioration.
Clinical signs
Clinical features vary; refer to documentation. Common findings may include mast cell-related symptoms plus evidence of organ dysfunction, such as cytopenias, liver or spleen enlargement, or bone involvement.
When to use this code
Use C96.21 when the provider documents aggressive systemic mastocytosis, especially when there is clear evidence of mast cell infiltration affecting organs. Code it from the confirmed diagnosis, not from suspicion alone. Check documentation for the specific organ findings supporting aggressiveness.
Do not use for
Do not use this code for cutaneous mastocytosis, indolent systemic mastocytosis, or mast cell activation symptoms without confirmed aggressive disease. If the record is unclear, check documentation.
Coding tip
Assign C96.21 only when the chart supports systemic mastocytosis with aggressive features, and code any documented organ complications separately when appropriate.