ICD-10 Code D89.42 – Idiopathic Mast Cell Activation Syndrome (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code D89.42 – Idiopathic mast cell activation syndrome
What it is
D89.42 identifies idiopathic mast cell activation syndrome, a condition where mast cells release mediators without a known trigger or another defined cause. Use it when documentation specifically states the idiopathic form.
Clinical signs
Clinical features vary; refer to documentation. Commonly reported findings include recurrent episodes involving flushing, itching, hives, abdominal symptoms, or lightheadedness, but the diagnosis should be documented by the clinician.
When to use this code
Use D89.42 when the record clearly names idiopathic mast cell activation syndrome or MCAS with an idiopathic cause. It is appropriate for confirmed encounters, problem lists, and specialist assessments when the condition is being evaluated or managed.
Do not assign it for vague allergic symptoms, suspected mast cell disease, or mastocytosis unless the provider documents this diagnosis. Check documentation if the cause is secondary, hereditary, or otherwise specified.
Do not use for
Do not use this code for allergic reactions, anaphylaxis, or mastocytosis unless idiopathic MCAS is specifically documented. Avoid it when the chart only says “mast cell disorder” or “possible MCAS.”
Coding tip
Code only from the provider’s stated diagnosis and avoid inferring idiopathic MCAS from symptoms alone.