2026 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 2026 (October 1, 2025 through September 30, 2026) 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 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 2026, published by CMS and the National Center for Health Statistics.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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
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, 2025 through September 30, 2026.
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 0.6663 to 1.6816 depending on complications. Higher weights mean higher Medicare reimbursement.
What is the ICD-9 equivalent of T78.40XA?
Under the General Equivalence Mappings, allergy, unspecified, initial encounter converts to ICD-9-CM 995.3 (allergy, unspecified). The mapping is approximate, so confirm the match fits the documentation.
Footnotes
[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.
