How can I differentiate between a true drug allergy and an adverse drug reaction in my patients?

To differentiate between a true drug allergy and an adverse drug reaction (ADR), take a detailed history and perform a clinical examination, focusing on the timing, nature, and pattern of the reaction relative to drug exposure.

True drug allergy typically presents with characteristic signs such as anaphylaxis (erythema, urticaria, angioedema, hypotension, bronchospasm) occurring usually within 1 hour of drug exposure, or non-immediate reactions like widespread rash, fever, and systemic involvement appearing days to weeks after exposure.

In contrast, adverse drug reactions may lack these immunological features and can include non-allergic side effects such as gastrointestinal symptoms alone, which are less likely to be allergic in origin.

Consider a drug allergy more likely if the reaction occurred during or after drug use, the drug is known to cause that reaction, or the patient has had a similar reaction previously to the same drug or drug class.

Conversely, suspect a non-allergic ADR if there is a plausible non-drug cause for symptoms or if symptoms are limited to gastrointestinal effects.

For suspected anaphylaxis, measuring serum tryptase can help confirm an allergic mechanism.

Referral to a specialist drug allergy service is recommended for severe or uncertain cases to confirm diagnosis through specialist investigations.