There are different tests to determine if a symptom(s) is related to ingestion or inhalation of a food or an airborne. The appropriate test will be chosen based on the type of symptoms, severity of reaction and time between the exposure and symptoms manifestation.
Blood test, skin prick test, patch test, food challenge test and provocation test are the only valid tests for identifying allergic or non-allergic reactions.
Blood test is used for identifying, confirming or excluding the cause of immediate allergy, also known as IgE-mediated, type I allergy or True allergy.
Blood test is used for diagnosing the cause of allergy symptoms.
A small sample of blood is drawn (from fingertip or vein depending on the number of allergen tested) and sent to a specialist laboratory for analysis. The test measures the amount of specific immunoglobulin E antibodies (IgE) to inhalants and food in the blood. Results are graded from grade 0 (negative) and grade 1 (weak positive) to grade 6 (strong positive), depending on the level of the allergen's specific IgE antibody in the blood. The higher the grade, the more likely of allergy to that allergen.
The RAST or ImmunoCAP-RAST is the most common blood test used for allergy testing. Specific IgE RAST can detect allergy to food, inhalants and other allergens. The result of RAST test is an indicator of sensitization, not clinical allergy. Therefore, the result must be interpreted in conjunction with clinical history.
A more recent and advanced method of blood testing called Component-Resolved Diagnostic (ImmunoCAP-ISAC) is more accurate in distinguishing between sensitization and clinical allergy, however the cost of ImmunoCAP-ISAC is significantly higher than a RAST test and interpretation of the result could be very complex even for an experienced clinician.
Skin Prick Test
Skin prick test (SPT) is used for identifying, confirming or excluding the cause of immediate allergy, also known as IgE-mediated, type I allergy or true allergy.
Skin prick test is used for diagnosing the cause of allergy symptoms.
Skin prick test is the oldest and the common test used for allergy diagnosis. The test is quite safe and straightforward, and the results are immediately available (within 15-20 minutes).
A skin prick test involves tiny puncturing or scratching the upper layer of the skin ( forearm or back) , to introduce a very small amount of a suspected allergen to the body. In case of positive allergy, a reaction similar to a mosquito bite may appear, usually within 15-20 minutes on the site of the prick. Skin prick test is used for food and inhalant allergy, though it has a better accuracy rate in diagnosis of the cause of asthma, hay fever and rhinitis.
Skin prick test can be used from infancy (4-6 months of age) to old age, though occasionally some small children might find it irritating. The test is not suitable for people with active eczema at the site of testing and for those with antihistamine intake in the past 72 hours. Although skin prick test is a very safe test, it might not be suitable for individuals with severe allergic reaction, anaphylaxis and those with uncontrolled asthma. Using fresh food, especially fresh fruit and vegetables, for skin prick testing often produces more accurate result. Commercial inhalant allergen extracts are usually used for the skin prick testing as they can't be prepared fresh.
Intradermal test is usually used for diagnosing drug/medicine allergy.
Intradermal allergy testing is another method of skin testing to help determine whether an individual is allergic to a specific allergen. The test involves injection of a small amount of the suspected allergen under the surface of the skin. After about 15-20 minutes the area is examined for a reaction at the site. A typical reaction looks like a mosquito bite with swelling and redness. The intradermal test is more sensitive than the skin prick test and usually used for drug allergy testing.
Patch testing is usually used for identifying reaction to a substance that comes in contact with the skin (soaps, detergents, preservatives, metals, perfumes, cosmetics and so on) . It is also occasionally used for conditions such as atopic dermatitis (eczema) and Eosinophilic esophagitis.
Patch test is used in patients with dermatitis to identify allergy to cosmetics, metals, perfumes and latex.
Patch testing is usually carried out by a dermatologist on the upper back to find out whether the skin condition is caused or aggravated by an allergy to substances which have come into contact with the skin. This is called contact allergy. There are approximately 40 substances which are most frequently in contact with the skin such as natural rubber latex, preservatives, metals, perfumes, cosmetics, leather chemicals, lanolin and plants among others.
Patch testing requires 3 visits to the hospital or dermatologist clinic. On day one of testing, tiny amounts of up to 25 or more substances are applied as small patches to the upper back of skin. After two days and visiting the dermatologist the patches are removed. The skin is examined to see if there is a reaction to any of the tested substances. After a further two days the skin is examined again in case there is a delayed reaction to any substance.
An elimination diet is a short-term avoidance dietary plan to determine food sensitivities, food intolerances and delayed food allergies. This method, combined or in absence of conclusive blood test or skin prick test can be helpful in diagnosing immediate or IgE-mediated food allergies. The elimination diet is also used to assess the relation between consumption of certain food and presence and severity of certain symptoms. The diet generally lasts 2-4 weeks. During this period suspected foods will be eliminated from the diet or a specific dietary plan will be recommend to follow. If the excluded food is causing the reaction, the symptoms often disappear by the end of this period. The type of the food that should be eliminated or the special diet to be followed will be decided by the clinician. The elimination diet will be determined according to the symptoms, clinical history and other factors, and it may involve eliminating one or several common food allergens, avoiding certain suspected food culprits, following a special dietary plan or a combination of all of them. Following the elimination period, the excluded food items will be reintroduced to the diet individually and according to the specific priority list. If the symptoms return by reintroducing a specific food item, then that food will be considered as the contributor to the reaction/symptom and should be avoided for certain period of the time.
Provocation or Challenge Test
Provocation or Challenge test is typically administered orally, nasally or via lung to identify immediate allergy to food, drug, inhalants or stimuli.
Provocation or Challenge test is used to identify allergy to food, drug or inhalants.
In some instances performing blood test or skin prick test for identifying reaction to food, inhalants or stimuli is either inadequate or the result is inconclusive. Under such circumstances, and to confirm, exclude or identify the cause reaction a direct oral, nasal or lung provocation test is performed using suspected food, inhalant allergen extracts or various stimuli. Similarly, if the result of blood test and skin prick test for food allergy is inconclusive, or if there is no suitable test for a specific substance (such as preservatives, colourings and drugs) an oral challenge test may need to be performed. Sometimes the provocation or challenge test is merely performed to confirm the result of blood test or skin prick test. Oral food challenge or provocation test for severe and life threatening reactions is only performed in hospitals or clinics where emergency medical staff are handy and there is instant access to full resuscitation equipments.