Challenge testing is the gold standard for diagnosing allergy. Challenge tests commonly performed by allergists include food challenge, drug challenge, inhalation challenge and physical challenge (heat, cold, exercise).
Skin prick tests and specific IgE blood tests are often used to aid in the diagnosis of food allergies. However, having positive skin tests or specific IgE to foods does not necessarily mean that the patient is allergic to those foods. The reason is because the body can develop an immune response called oral tolerance that suppresses the food allergy while the skin or blood test remains positive. Therefore, young children with food allergies often develop tolerance when they grow older while their test results remain positive for a few more years. On the other hand, negative skin or blood tests can reliably exclude immediate-type food allergy.
Allergists therefore use food challenge to determine whether a patient is allergic to a particular food when there is suspicion that skin or blood test results are incorrect, or when the patient might have outgrown his/her food allergy. An open challenge is performed by simply giving the patient increasing amounts of the food under close medical supervision. However, a significant amount of psychological overlay is sometimes present, which will bias the result of the challenge. Therefore the food is often hidden in a vehicle such as apple sauce and given to the patient in a single-blinded (subject is blinded to the test condition) or double-blinded (both observer and subject blinded) manner, alternating with plain vehicle.
Skin testing can only be used to diagnose allergy to a few drugs such as penicillin and insulin. Drug challenge, where increasing amounts of a drug is given orally, subcutaneously or intravenously, is sometimes used to confirm or exclude drug allergy.
Allergy to food additives, which are chemicals and therefore behave more like drug allergy, also requires oral challenge for diagnosis.
The defining characteristic of asthma is reversible lower airway obstruction. As asthma symptoms are often intermittent unless triggered by external factors, diagnosis can be difficult. Drugs such as methacholine or histamine can trigger airway narrowing in asthmatics, and are used in inhalation challenge to diagnose asthma. Other specific asthma triggers such as aspirin and allergen can also be used.
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