Coding and coverage · ICD-10-CM
T78.40XA Allergy, unspecified, initial encounter
7th character A: initial encounter
Instructional notes
Chapter 19
Excludes2Not included here: the patient may have both; report both when both are present.
Use additional codeAdd a second code for the full picture.
- code to identify any retained foreign body, if applicable (Z18.-)
Notes
- Use secondary code(s) from Chapter 20, External causes of morbidity, to indicate cause of injury. Codes within the T section that include the external cause do not require an additional external cause code
- The chapter uses the S-section for coding different types of injuries related to single body regions and the T-section to cover injuries to unspecified body regions as well as poisoning and certain other consequences of external causes.
T78
Excludes2Not included here: the patient may have both; report both when both are present.
7th charactersThe appropriate 7th character is to be added to each code from category T78
- A initial encounter
- D subsequent encounter
- S sequela
T78.4
Excludes1Not coded here: never report with this code, except when the two conditions are unrelated.
T78.40
Inclusion terms
- Allergic reaction NOS
- Hypersensitivity NOS
Chapter 19: Injury, poisoning and certain other consequences of external causes · T07-T88 Injury, poisoning and certain other consequences of external causes (T07-T88)
Source: ICD-10-CM FY 2027 code descriptions and tabular list (CMS and CDC National Center for Health Statistics), effective 2026-10-01 through 2027-09-30 · CMS ICD-10 files. HCC mapping: CMS-HCC V28, payment year 2027. As of 2026-11-04.
Coverage and payment depend on the claim details; confirm with the Medicare Administrative Contractor.