ICD-10 Code D89.41 – Monoclonal mast cell activation syndrome (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code D89.41 – Monoclonal mast cell activation syndrome
What it is
Monoclonal mast cell activation syndrome is a mast cell disorder in which a clonal mast cell population causes recurrent mediator-related symptoms. Use this code when documentation identifies this specific diagnosis.
Clinical signs
Clinical features vary; refer to documentation. Commonly reported findings include recurrent episodes of flushing, itching, hives, abdominal symptoms, or anaphylaxis-like reactions, with evidence of mast cell activation and clonality.
When to use this code
Use D89.41 when the provider documents monoclonal mast cell activation syndrome by name. It may also apply when the record clearly describes a clonal mast cell disorder with recurrent mediator symptoms consistent with this diagnosis. If the chart only says “mast cell disorder,” check documentation.
Do not use for
Do not use this code for unspecified mast cell activation syndrome, systemic mastocytosis, or other allergic reactions without confirmation of a monoclonal process. Check documentation.
Coding tip
Assign D89.41 only when the record supports both mast cell activation and a monoclonal/clonal finding.