T78.40XA — Allergy, unspecified, initial encounter
Is T78.40XA billable?
Yes, but T78.40XA is an unspecified code. T78.40XA is a valid, billable ICD-10-CM code for encounters from October 1, 2026 through September 30, 2027. However, T78.40XA is an unspecified or catch-all code. Payers apply additional scrutiny to unspecified codes and many deny them when the medical record supports a more specific option, so check the documentation before selecting T78.40XA.
T78.40XA at a glance
| Code | T78.40XA |
|---|---|
| Description | Allergy, unspecified, initial encounter |
| Billable | Yes |
| Code set | ICD-10-CM FY2027 |
| Valid for encounters | October 1, 2026 – September 30, 2027 |
| 7th character "A" | initial encounter |
What does the 7th character "A" mean in T78.40XA?
In T78.40XA, the 7th character A means initial encounter. The 7th character is required for T78.40XA to be a valid, complete code.
Same condition, different episode of care — switch the 7th character:
Which MS-DRGs does T78.40XA group to?
T78.40XA sits in MDC 21 and helps define the logic of 2 MS-DRGs (version 44, FY2027). Which one a stay actually groups to also depends on procedures and secondary diagnoses.
- 915Allergic Reactions with MCCmedical
- 916Allergic Reactions without MCCmedical
How is T78.40XA listed in the Alphabetic Index?
The Alphabetic Index lists these routes under T78.40, the nearest indexed parent of T78.40XA. Use them to find the family, then select the specific code.
- Allergy, allergic (reaction) (to)
- Allergy, allergic › drug, medicament & biological (reaction) (to)
- Hypersensitive, hypersensitiveness, hypersensitivity › reaction
Inclusion terms
Alternative wording in documentation that is classified to T78.40XA.
- Allergic reaction NOS
- Hypersensitivity NOS
Excludes1 — never code together — inherited from T78.4
The conditions below can never be reported together with T78.40XA on the same claim. An Excludes1 note means the two conditions cannot occur in the same patient, so reporting both is a coding error.
Use additional code — inherited from Chapter 19
Report an additional code alongside T78.40XA to fully describe the condition. T78.40XA is sequenced first.
- code to identify any retained foreign body, if applicable (Z18.-)
Excludes2 — not included here — inherited from Chapter 19
The conditions below are not part of T78.40XA, but a patient may have both at the same time. When documentation supports it, T78.40XA and the excluded code may both be reported.
Excludes2 — not included here — inherited from T78
The conditions below are not part of T78.40XA, but a patient may have both at the same time. When documentation supports it, T78.40XA and the excluded code may both be reported.
- complications of surgical and medical care NEC (T80-T88)
Code also — inherited from T66-T78
Two codes may be needed to describe the condition fully. Whether T78.40XA is sequenced first depends on the reason for the encounter.
- , 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-)
How long has T78.40XA existed?
T78.40XA has been in ICD-10-CM since at least FY2016, the earliest fiscal year on record here.
Derived from the official order files for FY2016–FY2026. FY2017 and FY2020 publish no order file, so those years are not covered.