Drug Provocation Test

A drug provocation test gives a suspected drug in graded doses under observation to confirm or exclude hypersensitivity. It is most often used to remove unnecessary drug allergy labels, particularly penicillin, and for NSAIDs and sulphonamide antibiotics.

Two phenotypes are assessed. Immediate reactions are IgE-mediated and usually appear within an hour. Delayed reactions are T-cell-mediated and typically appear as a rash over several days.

It is performed when the history is low risk and the decision to remove a label would change treatment. NSAID hypersensitivity can be selective, with some patients reacting to one class of NSAID while tolerating another, so testing is directed to the agent of interest and to the class that matters clinically.

Indications. A low risk history of penicillin reaction, NSAID tolerance assessment, sulphonamide antibiotic tolerance, and label removal in children.

Preparation and technique. Penicillin testing may begin with skin tests when the risk is higher. Direct oral challenge is used for low risk histories. A small initial fraction of the therapeutic dose is given, followed by the full dose, with observation between steps. Antihistamines and beta blockers are stopped before the test where appropriate. Doses are given following the drug's product label, adjusted for weight and renal function.

Contraindications and precautions. Absolute contraindications include previous severe cutaneous adverse reactions to the same drug, including Stevens-Johnson syndrome, toxic epidermal necrolysis and drug reaction with eosinophilia and systemic symptoms. Previous anaphylaxis to the same drug requires specialist assessment. Uncontrolled asthma, organ involvement in a previous reaction, and recent systemic illness are further cautions.

Interpretation.

• A negative challenge at full dose confirms tolerance, and the label can be removed.
• A positive challenge is characterised as urticaria, angioedema, bronchospasm or rash, and the drug is recorded as a confirmed allergy.
• Immediate reactions are observed for at least one hour after the final dose.
• Delayed reactions are followed over several days, with review for rash, fever, mucosal involvement or organ involvement.

Causes of a false positive reaction. Viral exanthems coinciding with the challenge, anxiety driven subjective symptoms, and vasovagal symptoms.

Critical values and findings needing immediate action.

• Anaphylaxis, bronchospasm and hypotension require immediate treatment with adrenaline and supportive care.
• Any sign of a severe cutaneous reaction stops the challenge and triggers specialist review.

Limitations and artefacts. Delayed rashes may take days to evolve, so the follow up period matters. A reaction to an excipient can mimic a drug reaction. Confounding symptoms from other medicines are common in older adults.

Follow up tests. Specific allergy testing for related drugs, and further challenge where cross-reactivity is uncertain, depending on the history.

Special groups. Children generally tolerate challenges well but require weight based dosing. Pregnancy requires case-by-case decisions. Older adults on multiple medicines may have confounding symptoms.