The diagnosis of an allergy begins with careful questioning to compile what doctors call a history. Do symptoms appear when the patient is at a particular location, or at a given time of day, day of week, or season of the year? What kind of work does the patient do? Are there pets in the home? What are the patient’s hobbies?
Standard skin tests are the most reliable method of testing for allergies. These tests produce immediate results and are simple and inexpensive. The prick method is the most common type of skin test. A drop of standardized allergen extract is placed on the skin – usually the back – and a needle is passed through the extract to make a tiny puncture in the skin. If the patient has IgE antibodies to the allergen, a hive will appear at the site within about 15 minutes. This method is safe and causes very little discomfort.
The intradermal test is another type of skin test. A syringe is used to inject allergen extract into the top layer of skin, which raises a small bubble on the surface. The result is similar to the prick test. A hive will appear in about 15 minutes if IgE antibodies are present. Intradermal tests are usually placed on the upper arm, so fewer allergens can be tested simultaneously. The intradermal method is many times more sensitive in identifying allergies than the prick test, but this can be a disadvantage. The test may show positive for allergens that don’t cause any significant problems for the patient under real-life circumstances.
A third kind of skin test, called a patch test, can be used to look for substances that might be causing contact dermatitis. A suspected allergen, such as a cosmetic, is placed on the skin and covered with a bandage for 48 hours. If the patient is allergic, the skin will become red and peel.
Antihistamines and a few other types of medicines can interfere with prick and intradermal skin tests, so before being skin tested, the patient should stop taking these drugs for a period of time specified by the physician.
The radioallergosorbent (RAST) test, a blood test done in a laboratory, can be used instead of the skin tests to detect allergies, but this test is less sensitive than skin tests, and much more expensive. Still, the test is valuable in a few situations. Some skin conditions make skin testing impossible, or the physician might suspect that a patient is so sensitive to an allergen that a skin test could be dangerous.
Positive skin or RAST tests will help narrow the list of suspected allergens, but the patient’s history is vital in determining which of the positive results are relevant. A challenge test may also be needed. If the patient’s allergic reactions haven’t been severe, the physician might deliberately expose the patient to one or more substances that produced a positive skin or RAST test. This is particularly true with food allergies.
The double-blind food challenge has become the standard method for diagnosing food allergies. In double-blind testing, the patient swallows capsules at regular intervals. Neither the patient nor the physician knows what they contain. A third party keeps records of the capsules’ contents. Sometimes the capsules contain a food the patient is known to tolerate, sometimes they contain a food suspected of being an allergen, and sometimes they contain an inert substance (placebo) incapable of causing an allergic reaction. Such a test enables the patient and the physician to judge reactions objectively.
A final word about diagnosis: People who suspect they suffer from allergy should make sure that this important phase of their care is handled only by qualified medical personnel, using scientifically valid tests. Some testing techniques are highly questionable: these include cytotoxic testing, in which reactions of white blood cells to suspected allergens are examined under the microscope, and monitoring of symptoms after various substances are placed under the tongue (sub-lingual testing) or injected under the skin (provocation and neutralization testing). These and other unproven methods can be expensive, uncomfortable, or even dangerous in some cases. They do not provide reliable diagnostic information and often lead to inappropriate treatment.