ICD-10-CM 2027
ICD-10 code for reaction hypersensitive
From the official ICD-10-CM alphabetic index entry “Hypersensitive, hypersensitiveness, hypersensitivity › reaction”. Page updated September 29, 2026.
About coding reaction hypersensitive
The ICD-10-CM code for reaction hypersensitive is T78.40 (Allergy, unspecified). The official index lists 1 more specific code, so check the documentation for details such as type, cause, site or severity before settling on T78.40.
Within T78.4, choose T78.40 (Allergy, unspecified) only when documentation doesn't support a more specific option: Arthus phenomenon (T78.41) and other allergy (T78.49).
Codes in this range require a 7th character: A for initial encounter, D for subsequent encounter and S for sequela. Use placeholder "X" characters to fill any empty positions before the 7th character.
It has been part of ICD-10-CM since the code set took effect on October 1, 2015 and hasn't changed since.
More specific reaction hypersensitive codes (official index)
- upper respiratory tract NEC J39.3 — Upper respiratory tract hypersensitivity reaction, site unspecified
Common questions about reaction hypersensitive ICD-10 codes
What is the ICD-10 code for reaction hypersensitive?
- T78.40 — Allergy, unspecified (a header code; report a more specific subcode). More specific codes apply when the documentation supports them: J39.3.
What is the ICD-10 code for upper respiratory tract nec reaction hypersensitive?
- J39.3 — Upper respiratory tract hypersensitivity reaction, site unspecified.
Is T78.40 billable?
- No. T78.40 is a header code. Report one of its more specific subcodes instead.