How should I approach the diagnosis of a suspected hypersensitivity reaction in a patient with a recent history of medication use?

To approach the diagnosis of a suspected hypersensitivity reaction in a patient with recent medication use, start with a detailed clinical history and examination, focusing on the timing of symptom onset relative to drug exposure and the nature of the reaction (immediate or non-immediate) to guide suspicion of drug allergy.

Use clinical features and timing to classify the reaction: immediate reactions (e.g., anaphylaxis, urticaria, angioedema) typically occur within 1 hour of drug exposure, while non-immediate reactions (e.g., widespread maculopapular rash, DRESS, Stevens-Johnson syndrome) usually develop days to weeks after first or subsequent exposures.

Consider whether the drug is known to cause the observed reaction and if the patient has had similar reactions previously to the same or related drugs, as this increases the likelihood of drug allergy.

Exclude non-drug causes, especially if symptoms are gastrointestinal only or if similar symptoms have occurred without drug exposure.

In cases of suspected anaphylaxis, measure serum mast cell tryptase according to NICE anaphylaxis guidelines.

If drug allergy is suspected, stop the suspected drug and manage acute symptoms. Document the suspected allergy in the medical record and provide the patient with information about the reaction and drugs to avoid.

Refer patients with severe or anaphylactic reactions to specialist drug allergy services for further investigation and confirmation.