T78.40XA — Allergy, unspecified, initial encounter

T78.40XA

ICD-10 Chapter 19: Injury & Poisoning — illustrative image — Chapter 19: Injury, poisoning and certain other consequences of external causes (S00–T88) — illustrative image

What this code means

An allergy recorded without the substance or reaction type being specified, at the first encounter.

Also called: allergy, allergic reaction.

When this code is used

Used for allergy, unspecified, initial encounter.

Where it sits in ICD-10-CM

Code T78.40XA belongs to Chapter 19: Injury, poisoning and certain other consequences of external causes, which covers codes S00–T88 in the FY2027 ICD-10-CM tabular list. The official description above is copied verbatim from the CDC/NCHS FY2027 code file.

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Frequently asked questions

What is ICD-10-CM code T78.40XA?

T78.40XA is the ICD-10-CM diagnosis code for “Allergy, unspecified, initial encounter”. An allergy recorded without the substance or reaction type being specified, at the first encounter.

When is code T78.40XA used?

Used for allergy, unspecified, initial encounter. It belongs to Chapter 19 of ICD-10-CM (Injury, poisoning and certain other consequences of external causes).

This guide explains ICD-10-CM codes in plain language for patients, students and other readers. It is educational information only — not coding, billing or medical advice. The code recorded for any real encounter is assigned from the full clinical documentation by the treating team and qualified coders, and only they can decide which code fits a specific case.