D89.40 (Mast cell activation, unspecified) compared with M35.9 (Systemic involvement of connective tissue, 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.
“autoimmune disease (systemic) NOS (M35.9)”
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 M35.9: “autoimmune disease (systemic) NOS (M35.9)”
Sole exception: when the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).
| D89.40Mast cell activation, unspecified | M35.9Systemic involvement of connective tissue, unspecified | |
|---|---|---|
| Billing status | Billable | Billable |
| Classification | D50-D89 — Diseases of the Blood and Blood-forming Organs and Certain Disorders Involving the Immune Mechanism (D50-D89) | M00-M99 — Diseases of the Musculoskeletal System and Connective Tissue (M00-M99) |
| Definition | Mast cell activation, unspecified is a billable ICD-10-CM diagnosis code (D89.40). | Systemic involvement of connective tissue, unspecified is a billable ICD-10-CM diagnosis code (M35.9). |
| Includes | defects in the complement system immunodeficiency disorders, except human immunodeficiency virus [HIV] disease sarcoidosis Mast cell activation disorder, unspecified | autoimmune disease NOS collagen (vascular) disease NOS systemic autoimmune disease systemic collagen (vascular) disease |
Derived from the official CMS ICD-10-CM tabular data (FY2026). Not billing advice; verify sequencing rules for the encounter. All data sources