ICD-10-CM 2027
ICD-10 code for mast cell activation
From the official ICD-10-CM alphabetic index entry “Activation › mast cell (disorder) (syndrome)”. Page updated September 29, 2026.
About coding mast cell activation
The ICD-10-CM code for mast cell activation is D89.40 (Mast cell activation, unspecified). The official index lists 4 more specific codes, so check the documentation for details such as type, cause, site or severity before settling on D89.40.
Within D89.4, choose D89.40 (Mast cell activation, unspecified) only when documentation doesn't support a more specific option: monoclonal mast cell activation syndrome (D89.41), idiopathic mast cell activation syndrome (D89.42), secondary mast cell activation (D89.43), hereditary alpha tryptasemia (D89.44) and other mast cell activation disorder (D89.49).
It is not a CC or MCC, so as a secondary diagnosis it doesn't change the MS-DRG severity level.
In MS-DRG v44.0, it is part of the grouping logic for DRG 814 (Reticuloendothelial and Immunity Disorders with MCC, relative weight 2.0729), DRG 815 (Reticuloendothelial and Immunity Disorders with CC, relative weight 1.0444) and DRG 816 (Reticuloendothelial and Immunity Disorders without CC/MCC, relative weight 0.7381), in MDC 16 (Diseases and Disorders of the Blood and Blood Forming Organs and Immunological Disorders), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.
More specific mast cell activation codes (official index)
Common questions about mast cell activation ICD-10 codes
What is the ICD-10 code for mast cell activation?
- D89.40 — Mast cell activation, unspecified. More specific codes apply when the documentation supports them: D89.42, D89.41, D89.43, D89.49.
What is the ICD-10 code for idiopathic mast cell activation?
- D89.42 — Idiopathic mast cell activation syndrome.
What is the ICD-10 code for monoclonal mast cell activation?
- D89.41 — Monoclonal mast cell activation syndrome.
What is the ICD-10 code for secondary mast cell activation?
- D89.43 — Secondary mast cell activation.
Is D89.40 billable?
- Yes. D89.40 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).
Can D89.40 be used as a principal diagnosis?
- Yes. The Medicare Code Editor doesn't restrict D89.40 as a principal diagnosis.
Is D89.40 a CC or MCC?
- No. D89.40 is neither a CC nor an MCC under MS-DRG v44.0.