D89.40 Mast cell activation, unspecified

The ICD-10-CM code for Mast cell activation, unspecified is D89.40 (FY2026). It is a billable, claim-ready diagnosis code.

Classification

Chapter

3: Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism (D50-D89)

Section

D80-D89: Certain disorders involving the immune mechanism (D80-D89)

Category D89

29 codes (24 billable)

FY2026 Status

Stable since FY2024

Also Known As

ICD-10-CM Alphabetic Index entries that lead to D89.40:

• Activation › mast cell (disorder) (syndrome)

Inclusion Terms

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

U.S. Hospital Utilization

• An estimated 4,775 U.S. inpatient stays in 2023 included D89.40 among the documented diagnoses.
• 210 stays listed it as the principal diagnosis.

Source: National Inpatient Sample (NIS), Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality, 2016–2023. National survey-weighted estimates.

Official Coding Guidelines

Chapter 3 TOC entry

3. Chapter 3: Disease of the blood and blood-forming organs and certain disorders involving the immune mechanism (D50-D89) ............................................................................................. 39

Reserved for future guideline expansion

Reserved for future guideline expansion ...................................................................................... 39

Source: CMS — ICD-10-CM Official Guidelines for Coding and Reporting, FY2026

References

• CMS — ICD-10-CM Official Code Set and Guidelines, FY2026
• CDC/NCHS — ICD-10-CM Files and Browser Tool
• AHRQ — HCUP National Inpatient Sample, Frequencies by Diagnosis Code (2016–2023)