D89.40
Mast cell activation, unspecified
D89.40 (also written D8940) is a billable ICD-10-CM diagnosis code for mast cell activation, unspecified. It is part of the FY2027 code set, effective October 1, 2026. It is one of the 24 codes in category D89 (Other disorders involving the immune mechanism, not elsewhere classified). The Alphabetic Index lists it under 1 entry. Its coding notes include 15 Excludes1 entries and 11 Excludes2 entries.
Excludes1
Not coded here. The listed conditions generally should not be reported together.
- aggressive systemic mastocytosis (C96.21)
- congenital cutaneous mastocytosis (Q82.2)
- (non-congenital) cutaneous mastocytosis (D47.01)
- (indolent) systemic mastocytosis (D47.02)
- malignant mast cell neoplasm (C96.2-)
- malignant mastocytoma (C96.29)
- mast cell leukemia (C94.3-)
- mast cell sarcoma (C96.22)
- mastocytoma NOS (D47.09)
- other mast cell neoplasms of uncertain behavior (D47.09)
- systemic mastocytosis associated with a clonal hematologic non-mast cell lineage disease (SM-AHNMD) (D47.02)
Excludes2
Not included here. Both codes may be reported if the patient has both conditions.
- transplant failure and rejection (T86.-)
Also from chapter/block level (10 notes)
- human immunodeficiency virus [HIV] disease (B20)
- autoimmune disease (systemic) NOS (M35.9)
- certain conditions originating in the perinatal period (P00-P96)
- complications of pregnancy, childbirth and the puerperium (O00-O9A)
- congenital malformations, deformations and chromosomal abnormalities (Q00-Q99)
- endocrine, nutritional and metabolic diseases (E00-E88)
- human immunodeficiency virus [HIV] disease (B20)
- injury, poisoning and certain other consequences of external causes (S00-T88)
- neoplasms (C00-D49)
- symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)
Care settings
How CMS programs treat this code. These are CMS claim edits and list entries, not coverage or eligibility rules.
Home Health Primary
Accepted as the primary (principal) diagnosis on a home health claim. The Home Health Grouper assigns it to the MMTA - Infectious Disease, Neoplasms, and Blood-Forming Diseases clinical group.
Source: CMS Home Health Grouper Software v07.2.26, effective October 1, 2026Includes
Further defines or gives examples of the content of the category.
Also from chapter/block level (3 notes)
- defects in the complement system
- immunodeficiency disorders, except human immunodeficiency virus [HIV] disease
- sarcoidosis
Inclusion terms
Conditions classified to this code.
- Mast cell activation disorder, unspecified
- Mast cell activation syndrome, NOS
Alphabetic Index terms
Terms the official Alphabetic Index lists for this code. 1 entry