ICD-10 Code D89.40 – Mast cell activation, unspecified (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code D89.40 – Mast cell activation, unspecified
What it is
D89.40 identifies mast cell activation when the record does not specify a more exact type or cause. Use it for documented mast cell activation disorder or syndrome without further detail.
Clinical signs
Clinical features vary; refer to documentation. Commonly recorded findings may include episodic flushing, itching, hives, abdominal symptoms, or other mediator-related complaints linked to mast cell activation.
When to use this code
Use this code when the clinician documents mast cell activation and does not identify a specific subtype, trigger, or underlying condition. It is appropriate when the chart supports the diagnosis but lacks further detail. Check documentation if the provider names a more specific mast cell disorder.
Do not use for
Do not use this code for a confirmed, more specific mast cell disorder when documentation supports a different code. Avoid it when mast cell activation is only suspected or not clearly stated.
Coding tip
Code D89.40 only when the provider explicitly documents mast cell activation and the record does not support a more specific diagnosis.