2026 ICD-10-CM Diagnosis Code D89.40Mast cell activation, unspecified
ICD-10-CM Codes›D50–D89›D80-D89›D89
- Billable — Valid for Submission
- Chronic Condition
D89.40 is a billable ICD-10-CM diagnosis code for mast cell activation, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 814 through 816. Coders also document this condition as anaphylaxis due to mast cell disorder. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Immunity disorders.
Code Identity
Code Classification
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Anaphylaxis due to mast cell disorder
- Mast cell activation syndrome
Tabular List NotesGuidance
Coding notes and annotation back-references applicable to this code.
Inclusion Terms
- Mast cell activation disorder, unspecified
- Mast cell activation syndrome, NOS
These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of "other specified" codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
- mast cell (disorder) (syndrome) - D89.40
External Cause of Injuries IndexGuidance
References for this code in the External Cause of Injuries Index.
- Activation
- mast cell (disorder) (syndrome)
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Mast Cell Activation Syndrome
a clinically and genetically heterogeneous group of mast cell disorders in which there is aberrant release of mast cell mediators with little to no accompanying proliferation of mast cells.Mast Cells
granulated cells that are found in almost all tissues, most abundantly in the skin and the gastrointestinal tract. like the basophils, mast cells contain large amounts of histamine and heparin. unlike basophils, mast cells normally remain in the tissues and do not circulate in the blood. mast cells, derived from the bone marrow stem cells, are regulated by the stem cell factor.
Convert D89.40 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code History & ChangesHistory
Replacement D89.40 replaces the following previously assigned code(s):
- D89.89 - Oth disrd involving the immune mechanism, NEC
Questions About D89.40Overview
Is D89.40 (Mast cell activation syndrome and related disorders) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report mast cell activation, unspecified on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does D89.40 group to?
When mast cell activation, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 814, 815, 816, with relative weights from 0.6320 to 2.1267 depending on complications. Higher weights mean higher Medicare reimbursement.
What is the ICD-9 equivalent of D89.40?
Under the General Equivalence Mappings, mast cell activation, unspecified converts to ICD-9-CM 279.8 (immune mechanism dis NEC). The mapping is approximate, so confirm the match fits the documentation.
Footnotes
[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:
- The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
- The condition places limitations on self-care, independent living, and social interactions.
