The diagnosis of type I hypersensitivities starts with a careful review of the person's symptoms, family history, and personal history, including: the age of onset and seasonal symptoms as well as those that appear after exposure to animals, hay, or dust or that develop in specific environments, such as home and work. Other environmental and lifestyle factors such as exposure to pollutants, smoking, alcohol, drugs, exercise, and stress may worsen symptoms and should be taken into consideration. Once the list of possible allergens has been narrowed, specific testing can be done.
Laboratory testing may include:
- Allergen-specific IgE blood testing: this is testing that is used to help diagnose allergies. The test measures the amount of allergen-specific IgE antibodies in the blood in order to detect an allergy to a particular substance. It involves taking a blood sample and checking for each allergen suspected. Allergens may be selected one at a time or by choosing panels such as food panels, which contain the most common adult or child food allergens, and regional weed and grass panels, which contain the most common airborne allergens in the location where the person lives. Individual selections are very specific; for example: bumble bee versus honeybee, egg white versus egg yolk, common ragweed versus western ragweed.
The health practitioner will select the most appropriate allergens. Usually someone will only be truly allergic to a few substances (4 or less). If a specific IgE test is negative, then chances are that the person tested is not allergic to that substance, but a positive test must be evaluated in the context of the person's clinical history. Someone can have a low level of IgE and still have a severe reaction to actual exposure to the allergen or an elevated level and never experience a reaction. Children who outgrow a food allergy may continue to have positive IgE test results for many years.
Note: The traditional method for blood testing was the RAST (radioallergosorbent test), but it has been largely replaced with newer IgE-specific immunoassay methods. Some health practitioners, however, still refer to all IgE allergy blood tests as RAST even though it is not the methodology that the laboratory uses.
- Total IgE testing is sometimes done to look for an ongoing allergic process. It is a blood test that detects the total amount of IgE protein (including allergy antibodies) but does not identify specific allergens. Conditions besides allergies can also cause the IgE level to rise.
- Complete blood count (CBC) and WBC differential—these tests include the measurement of eosinophils, a type of white blood cell. The level of eosinophils may be increased in a person with allergies.
- Histamine and/or tryptase blood tests may be used to help diagnose anaphylaxis or mast cell activation.
Other types of allergy tests:
- Skin prick or scratch tests are done in an allergist's or dermatologist's office and must be done by a trained professional. They are often used to detect airborne allergies such as pollens, dust, and molds. Because of the potential for a severe reaction, skin prick tests are not usually used for food allergies. The person being tested must not have significant eczema or be taking antihistamines or certain antidepressants for several days before the skin prick test. False positives can arise with even a non-allergic person if the dosage of the allergen is high enough.
- Intradermal allergy skin tests, using injections that form a bubble under the skin, may be done but they are not widely accepted because of a high false-positive rate.
- Patch testing. Delayed hypersensitivity skin and patch tests are the easiest methods of testing for type IV delayed hypersensitivity. A concentration of the suspected allergen is applied to the skin under a nonabsorbent adhesive patch and left for 48 hours. If burning or itching develops more rapidly, the patch is removed. A positive test consists of redness with some hardening and swelling of the skin and sometimes vesicle (blister-like) formation. Some reactions will not appear until after the patches are removed, so the test sites are also checked at 72 and 96 hours.
- Oral food challenges are considered the "gold standard" for diagnosing food allergies. They are labor-intensive and require close medical supervision because reactions can be severe, including life-threatening anaphylaxis. Food challenges involve giving a person small amounts of unmarked potential food allergens in capsule or intravenous form and watching for allergic reactions. Negatives are confirmed with larger meal-sized portions of food.
- Food elimination is another way to test for food allergies: eliminating all suspected foods from the diet, then reintroducing them one at a time to find out which one(s) are causing the problem.
Some other tests that are not standardized and not recommended or widely accepted as useful include:
- Allergen-specific immunoglobulin G/G4 (IgG/G4) antibody
- Cytotoxic testing
- Applied kinesiology
- Basophil histamine release/activation
- Hair analysis
- Electrodermal test (Vega)