C96.21 (Aggressive systemic mastocytosis) compared with D89.40 (Mast cell activation, unspecified), from the official CMS tabular data. Do not report these codes together: the official tabular list marks them Excludes1.
Not coded here — these two are mutually exclusive.
“aggressive systemic mastocytosis (C96.21)”
An Excludes1 note means the two conditions cannot occur together, so the pair should not be reported for the same encounter. The sole exception is when the conditions are documented as unrelated to each other.
Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a
No — do not report together. D89.40 carries an Excludes1 note covering C96.21: “aggressive systemic mastocytosis (C96.21)”
Sole exception: when the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).
| C96.21Aggressive systemic mastocytosis | D89.40Mast cell activation, unspecified | |
|---|---|---|
| Billing status | Billable | Billable |
| Classification | C00-D49 — Neoplasms (C00-D49) | D50-D89 — Diseases of the Blood and Blood-forming Organs and Certain Disorders Involving the Immune Mechanism (D50-D89) |
| Definition | Aggressive systemic mastocytosis is a billable ICD-10-CM diagnosis code (C96.21). | Mast cell activation, unspecified is a billable ICD-10-CM diagnosis code (D89.40). |
| Includes | — | defects in the complement system immunodeficiency disorders, except human immunodeficiency virus [HIV] disease sarcoidosis Mast cell activation disorder, unspecified |
Derived from the official CMS ICD-10-CM tabular data (FY2026). Not billing advice; verify sequencing rules for the encounter. All data sources