In simple words: This code is used when someone has an allergic reaction, but the doctor doesn't know what caused it yet.It's used the first time the person sees the doctor for this reaction.

This code classifies an unspecified allergy during the initial encounter with the patient.It is used when a patient presents with an allergic reaction, but the specific allergen is unknown or cannot be determined at that time.Additional codes may be necessary to specify the type of allergic reaction and any associated complications.

Example 1: A patient presents to the emergency room with symptoms of anaphylaxis (hives, swelling, difficulty breathing) after eating an unknown food.The attending physician documents the anaphylactic reaction using T78.40XA, pending identification of the allergen., A patient visits their primary care physician with a rash and itching that started after exposure to a new plant at work. The physician, unsure of the exact cause, codes the encounter with T78.40XA for the unspecified allergy while further investigation is conducted., A patient experiences a sudden onset of respiratory distress with symptoms suggesting an allergic reaction. Initial assessment reveals unknown cause, resulting in use of T78.40XA.

Detailed documentation of the allergic reaction including symptoms, onset, and any known exposures is necessary for accurate coding.Any treatments administered, such as epinephrine or antihistamines, should be recorded.If the allergen is identified later, this should be documented as well.

** This code is for initial encounters only.Subsequent encounters where the allergen is known or further specified should be coded accordingly using more specific ICD-10-CM codes.