2027 ICD-10-CM Diagnosis Code T78.40XAAllergy, unspecified, initial encounter
T78.40XA is a billable ICD-10-CM diagnosis code for allergy, unspecified, initial encounter. The 7th character A marks it as an initial encounter code, used while the patient is receiving active treatment. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 915 through 916. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Allergic reactions.
Code Identity
Code Classification
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Acute allergic reaction
- Adverse cutaneous reaction to diagnostic procedure
- Adverse reaction caused by detergent
- Adverse reaction to bone resorption inhibitor
- Adverse reaction to oil
- Allergic bronchitis
- Allergic condition
- Allergic cough
- Allergic disorder
- Allergic disorder - symptom change
- Allergic disorder monitoring status
- Allergic disorder of digestive system
- Allergic disorder of respiratory system
- Allergic disorder of respiratory tract
- Allergic disorder treatment started
- Allergic disposition
- Allergic encephalitis
- Allergic facies
- Allergic gingival disease
- Allergic otitis externa
- Allergic reaction
- Allergic reaction after allergen immunotherapy
- Allergic reaction caused by analgesic
- Allergic reaction caused by anesthetic
- Allergic reaction caused by antiinfective agent
- Allergic reaction caused by Hevea brasiliensis latex protein
- Allergic reaction caused by hymenoptera venom
- Allergic reaction caused by insect bite and/or insect sting
- Allergic reaction due to allergy skin test
- Allergic reaction to adhesive
- Allergic reaction to bee sting
- Allergic reaction to colophony
- Allergic reaction to detergent
- Allergic reaction to drug
- Allergic reaction to hair dye
- Allergic reaction to nickel
- Allergic reaction to oil
- Allergic reaction to platinum
- Allergic reaction to venom
- Allergic reaction to wasp sting
- Allergic sensitization
- Allergic vaginitis
- Allergy skin test positive
- Allergy test positive
- Allergy treatment changed
- Allopurinol adverse reaction
- Allopurinol hypersensitivity syndrome
- Angry back syndrome
- Atopic reaction
- Cell-mediated immune reaction
- Chronic allergic otitis externa
- Chronic non-infective otitis externa
- Complication of diagnostic procedure
- Complication of patch testing
- Contact hypersensitivity reaction
- Cutaneous hypersensitivity
- Cutaneous hypersensitivity reaction caused by Hymenoptera venom
- Cutis laxa following hypersensitivity reaction
- Cutis laxa, acquired type
- Drug reaction with eosinophilia and systemic symptoms
- Drug reaction with eosinophilia and systemic symptoms caused by strontium ranelate
- Emollient adverse reaction
- Hypersensitivity condition
- Hypersensitivity disposition
- Hypersensitivity finding
- Hypersensitivity gingivitis
- Hypersensitivity reaction
- Hypersensitivity reaction mediated by antibody
- Hypersensitivity reaction mediated by immune complex
- Hypersensitivity reaction with blocking antibody
- Hypersensitivity reaction without blocking antibody
- Immune hypersensitivity reaction by mechanism
- Injection site hypersensitivity
- Laryngotracheobronchitis
- Late effect of accidental injury
- Late effect of superficial injury
- Myocarditis due to hypersensitivity state
- Non-allergic hypersensitivity condition
- Pain provoked by allergen
- Pruritic dermatitis
- Recurrent allergic croup
- Recurrent bronchitis
- Recurrent croup
- Skeeter syndrome
- Skin: type 3 delayed reaction
- Type 1 hypersensitivity response
- Type 1 skin hypersensitivity reaction
- Type 4 hypersensitivity response
- Ulcer of esophagus due to allergic disorder
- Xanthine oxidase inhibitor adverse reaction
Coding GuidelinesGuidance
The appropriate 7th character is to be added to each code from block Adverse effects, not elsewhere classified (T78). Use the following options for the applicable episode of care:
- A - initial encounter
- D - subsequent encounter
- S - sequela
Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027, published by CMS and the National Center for Health Statistics.
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to T78.40XA: its own notes plus those printed at T78.4, T78, block T66-T78, and Chapter 19. A note printed at a category, block or chapter applies to every code under it.
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
Use Additional Code
Add a second code, after this one, to fully describe the condition.
- code to identify any retained foreign body, if applicable (Z18.-)
Code Also
A second code may be needed to fully describe the condition; the note gives no sequencing direction.
- 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-)
Notes
General instructions on how these codes are used.
- 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.
7th Character
Every code in the category takes a 7th character from the listed values, with the placeholder X filling any empty positions before it.
- The appropriate 7th character is to be added to each code from category T78
- Ainitial encounter this code
- Dsubsequent encounter
- Ssequela
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name T78.40XA, its category, or a range that includes it.
Excludes1 5
These codes carry an Excludes1 note naming T78.40XA: they are not reported together with it, unless the two conditions are unrelated.
- L23 Allergic contact dermatitisallergy NOS (T78.40) names T78.40, which includes this code
- L24 Irritant contact dermatitisallergy NOS (T78.40) names T78.40, which includes this code
- L25 Unspecified contact dermatitisallergy NOS (T78.40) names T78.40, which includes this code
- L27 Dermatitis due to substances taken internallyallergy NOS (T78.40) names T78.40, which includes this code
- T75 Other and unspecified effects of other external causesadverse effects NEC (T78.-) names T78.-, which includes this code
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Hypersensitivity
altered reactivity to an antigen, which can result in pathologic reactions upon subsequent exposure to that particular antigen.
Patient EducationClinical
Allergy
An allergy is a reaction by your immune system to something that does not bother most other people. People who have allergies often are sensitive to more than one thing. Substances that often cause reactions are:
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T78.40XA to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T78.40XAOverview
What is the ICD-10 code for allergy, unspecified, initial encounter?
The ICD-10-CM code for allergy, unspecified, initial encounter is T78.40XA (sometimes written as T7840XA). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is T78.40XA (Allergy, unspecified) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report allergy, unspecified, initial encounter on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What does the 7th character A in T78.40XA mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for allergy, unspecified, such as an emergency visit or first evaluation.
What MS-DRG does T78.40XA group to?
When allergy, unspecified, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 915, 916, with relative weights from 0.6926 to 1.6559 depending on complications. Higher weights mean higher Medicare reimbursement.
Can T78.40XA be reported with K52.29?
Not as a rule. The Excludes1 note at T78.4, which applies to T78.40XA, lists "allergic diarrhea (K52.29)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
