T78.40 – Allergy, unspecified

ICD-10-CM 2027 diagnosis code · Other and unspecified effects of external causes

Non-billable header

ICD-10-CM code T78.40

Code: T78.40 (claims format: T7840)
Description: Allergy, unspecified
Billable: No – use a more specific code below
Valid for: Dates of service October 1, 2026 – September 30, 2027 (FY2027)
Chapter: 19. Injury, poisoning and certain other consequences of external causes (S00-T88)
Block: T66-T78 Other and unspecified effects of external causes
Parent codes: T78 › T78.4

Billable codes under T78.40

T78.40 is a header code and cannot be used on claims. Choose the most specific code:

T78.40XA – Allergy, unspecified, initial encounterBillable
T78.40XD – Allergy, unspecified, subsequent encounterBillable
T78.40XS – Allergy, unspecified, sequelaBillable

7th character values

Codes in this category need a 7th character to describe the encounter or episode of care (use placeholder X for empty positions).

Character	Meaning
`A`	initial encounter
`D`	subsequent encounter
`S`	sequela

Notes for T78.40

Applicable to:

Allergic reaction NOS
Hypersensitivity NOS

Notes inherited from parent codes

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

T78 – Adverse effects, not elsewhere classified

Excludes2 (not included here):

complications of surgical and medical care NEC (T80-T88)

7th character note:

The appropriate 7th character is to be added to each code from category T78

T78.4 – Other and unspecified allergy

Excludes1 (not coded here):

specified types of allergic reaction such as:
allergic diarrhea (K52.29)
allergic gastroenteritis and colitis (K52.29)
dermatitis (L23-L25, L27.-)
food protein-induced enterocolitis syndrome (K52.21)
food protein-induced enteropathy (K52.22)
hay fever (J30.1)

Block T66-T78

Code also:

, if applicable, place of occurrence, such as:
highway rest stop (Y92.523)
school (private) (public) (state) as the place of occurrence of the external cause (Y92.21-)
service areas as the place of occurrence of the external cause (Y92.52-)
single-family non-institutional (private) house as the place of occurrence of the external cause (Y92.01-)
transport vehicle as the place of occurrence of the external cause (Y92.81-)

Index terms for T78.40

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

Related codes in T78.4

T78.41 – Arthus phenomenonHeader
T78.49 – Other allergyHeader

Frequently asked questions

What is ICD-10 code T78.40?

T78.40 is the ICD-10-CM code for allergy, unspecified, in the block T66-T78 (Other and unspecified effects of external causes).

Is T78.40 a billable code?

No. T78.40 is a header code with more specific child codes; report one of the billable codes listed above.

What category does T78.40 belong to?

It belongs to category T78 – Adverse effects, 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.