D89.40 – Mast cell activation, unspecified

ICD-10-CM 2027 diagnosis code · Certain disorders involving the immune mechanism

Billable code

ICD-10-CM code D89.40

Code: D89.40 (claims format: D8940)
Description: Mast cell activation, unspecified
Billable: Yes – valid for HIPAA-covered transactions
Valid for: Dates of service October 1, 2026 – September 30, 2027 (FY2027)
Chapter: 3. Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism (D50-D89)
Block: D80-D89 Certain disorders involving the immune mechanism
Parent codes: D89 › D89.4

Notes for D89.40

Applicable to:

Mast cell activation disorder, unspecified
Mast cell activation syndrome, NOS

Notes inherited from parent codes

Instructional notes at a category or block level apply to every code below it.

D89 – Other disorders involving the immune mechanism, not elsewhere classified

Excludes1 (not coded here):

hyperglobulinemia NOS (R77.1)
monoclonal gammopathy (of undetermined significance) (D47.2)

Excludes2 (not included here):

transplant failure and rejection (T86.-)

D89.4 – Mast cell activation syndrome and related disorders

Excludes1 (not coded here):

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)

Block D80-D89

Includes:

defects in the complement system
immunodeficiency disorders, except human immunodeficiency virus [HIV] disease
sarcoidosis

Excludes1 (not coded here):

autoimmune disease (systemic) NOS (M35.9)
functional disorders of polymorphonuclear neutrophils (D71-)

Excludes2 (not included here):

human immunodeficiency virus [HIV] disease (B20)

Index terms for D89.40

Entries in the ICD-10-CM alphabetic index that lead to this code:

Activation, mast cell

Related codes in D89.4

D89.41 – Monoclonal mast cell activation syndromeBillable
D89.42 – Idiopathic mast cell activation syndromeBillable
D89.43 – Secondary mast cell activationBillable
D89.44 – Hereditary alpha tryptasemiaBillable
D89.49 – Other mast cell activation disorderBillable

Frequently asked questions

What is ICD-10 code D89.40?

D89.40 is the ICD-10-CM code for mast cell activation, unspecified, in the block D80-D89 (Certain disorders involving the immune mechanism).

Is D89.40 a billable code?

Yes. D89.40 is a billable/specific code that can be used to indicate a diagnosis for reimbursement purposes.

What category does D89.40 belong to?

It belongs to category D89 – Other disorders involving the immune mechanism, not elsewhere classified.

Source: CDC/NCHS ICD-10-CM FY2027 code files, tabular list and index. Reference only – code assignment must follow the official ICD-10-CM guidelines and documentation in the medical record.