The ELISA is a useful and powerful method for estimating ng=mL to pg=mL ordered materials in the solution, such as serum, urine, and culture supernatants (i.e., any cell-derived material that is used in the test, depending on the ELISA that is being run). This is the most reliable test for detecting antibodies and is commonly used in diagnostic testing for allergies. Antibodies to various antigens are detected easily by this test. Extracted and purified antigens are fixed to a surface to which the patient's serum is added. After washing and centrifugation, adherent immunoglobulin is then detected when a second antibody couple to an enzyme is added to the original surface. The last stage of the test involves adding the enzyme's substrate, causing a color reaction that is then measured by a spectrophotometer. This test is also performed by placing the antibody on the plate surface. ELISA assays are very sensitive and can measure IgA, IgE, IgG, and IgG4 antibodies, yielding disclosure of both immediate and long-term hypersensitivity reactions.
Allergy Detection Tests (continued)
The radioallergosorbent test (RAST)
For the RAST, possible allergens are affixed to a plate that is then saturated with a sample of serum from the patient. If the patient's serum contains an antibody that is specific to an affixed antigen, it will link to the antigen. Next, a small amount of radioactive, polyclonal antireagenic antibody is then added to the plate. Following a reaction and washing period, residual radiation is measured to determine what percentage of the radioactive antibody is bound to the linked antigen-antibody complex. Higher amounts of radioactive bonding equate to a greater amount of reactive antibody that is specific to tested allergens in the patient's serum. The RAST primarily measures IgE-mediated allergies, that is, immediate hypersensitivity. Being that a growing body of evidence cites IgG in food allergies, the RAST may not detect all food allergen reactions accurately.
The radioallergosorbent procedure (RASP)
The RASP is similar in nature to RAST, but follows a different protocol. The RASP is a mild variant of the RAST that identifies IgG results in a meaningful and reproducible way. The RASP also has greater sensitivity and specificity for detecting food allergens than the RAST, which may be the result of ability of this test to detect IgG complexes in addition to IgE.
Epicutaneous or cutaneous allergen testing produces a localized pruritic wheal and erythema that is maximal at 15-20 minutes post-introduction. It is used most commonly for diagnosing of allergic respiratory diseases in patients with symptoms of pruritis, congestion, sneezing, and chronic coughs with wheezing. Skin testing for allergy is appropriate only if a patient has symptoms that are consistent with IgE-mediated allergy within two hours of eating a suspected food.
Oral provocation with suspected allergenic foods administered in a double-blinded, placebo-controlled test is widely considered to be the definitive test for food allergy. This type of testing is not performed in patients with suspected food-induced anaphylaxis. Testing via this method will reveal immediate hypersensitivity to foods that cause symptomology. However, the relationship between certain foods ingested and resulting symptoms is not entirely clear-cut. In such cases, patients are placed on limited diets that involve the removal of commonly allergenic foods (e.g., foods with corn, wheat, soy, and dairy products) and the addition of hypoallergenic foods or foods that are rarely consumed by patients.
Patients are maintained on the diet for several weeks, on the theory that allergy symptoms that have been caused by previously removed allergenic foods will be reduced or removed and that reintroduction of such foods at a later date will cause significant clinical symptoms that are reportable by patients. This type of testing for food allergies is advisable because singular tests are unable to identify specific triggers for patients who experience a wide array of symptoms (such as nausea, abdominal pain, vomiting, cramping, and diarrhea), which may be the result of various mediators of allergy, such as mast cells, eosinphils, IgE, and IL-4. To achieve specificity, however, more testing may be required.
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