Monoclonal mast cell activation syndrome

This code represents a rare, systemic mast cell activation disorder characterized by the presence of a clonal population of mast cells that exhibit aberrant expression of CD25, but without meeting the diagnostic criteria for systemic mastocytosis. Patients experience recurrent episodes of mast cell mediator release, leading to symptoms such as flushing, urticaria, gastrointestinal issues, and anaphylaxis.

Use this code for patients diagnosed with monoclonal mast cell activation syndrome (MMAS) based on bone marrow biopsy showing clonal mast cells (typically identified by CD25 expression) and evidence of mast cell mediator release symptoms, but without the major or multiple minor criteria for systemic mastocytosis. Documentation should clearly state the diagnosis of MMAS and supporting pathological findings.

• Distinguish carefully from systemic mastocytosis (D89.83) and idiopathic mast cell activation syndrome (D89.42).
• Ensure documentation specifies "monoclonal" to differentiate from other forms of mast cell activation syndrome.
• Code any associated symptoms or complications separately, such as anaphylaxis (T78.0-).