In a drug challenge test a patient is administered a drug to watch if it causes a reaction. Allergy to drugs is studied with skin or blood tests, but the results of these are not always conclusive. Also, skin and blood tests are not available for all drugs. Thus, sometimes the doctor will suggest performing a drug challenge test.
This may be done to ascertain that the patient is allergic and must avoid the drug, or to asses if the drug is tolerated and the patient can receive it. Sometimes it is used to assess tolerance to an alternative drug (for example, if a patient is allergic to an antibiotic, another antibiotic of the same or different family is tested).
The patient is administered small doses of the drug, which are gradually increased until a normal dose is given. The drug is administered orally, inhaled or injected. The patient is watched for several hours. Sometimes the patient is instructed to take the drug for several days. The observation period may last up to two weeks, as some drugs cause delayed reactions.
The risk of the test is the appearance of symptoms, which might be severe. Therefore it must be performed in a medical setting ready to recognise and treat reactions. When very severe or life-threatening reactions are anticipated, the challenge test will not be performed.
Sometimes the patients complain of subjective symptoms that may be due to fear of reactions. In a “blind” test the patient is administered, in occasions apart, either the drug or a placebo (a substance with no effect but looking just like the drug) to check whether reactions appear. In a “double blind” test, neither the patient nor the doctor knows if the patient is receiving the drug or the placebo (this is known by a third person who does not evaluate the reactions).
Some medication, especially antihistamines, may change the result of the test, and must be withdrawn 4-7 days before. An informed consent with details of benefits and risk of the test is needed to perform the procedure.