By Dr Ananya Mandal, MD
Allergies come in variety for types and forms and diagnosis is often important in order to avoid the possible allergy causing substances that trigger allergic reactions that may be life threatening.
Diagnosis is primarily based on history and certain laboratory tests. (1-4)
History of Allergy Symptoms
Diagnosis begins with a detailed history of the symptoms of allergies, possible triggers as felt by the patient and duration and severity of the symptoms.
Symptoms of allergies after intake of certain foods, inhalation of dust or mites or animal hair and in certain seasons when there is profusion of pollen in the environment are significant.
History of contact with latex, nickel plating etc. in patients with contact dermatitis or skin allergies is significant.
Diagnosis of Food Allergies
For detection of food allergies questions are asked, these include:
onset time from intake of the suspected food and reaction
persistent association with a particular food
amount of this potentially allergenic food
anyone else who took the food getting ill
if any benefits were derived from allergy medicines
Patient is asked to maintain a diet diary and regular elimination of suspected foods is done to detect the suspected allergen in food.
History of Allergies in Family
The physician enquires about history of allergies in any of the family members. First degree relatives like parents, siblings who suffer from allergies is a significant finding.
This type of propensity for allergies is termed atopy and often runs in the family. It is determined both by genes as well as environment.
Other history like growing up in a home with smokers, male children, obese and overweight children, children from small families and those on frequent antibiotics are also relevant in diagnosis of allergies.
Skin Prick Test
This is the first test to be done to look for a potential allergen leading to the allergic reaction.
In this test the skin usually on the inner side of the forearm or over the back is cleaned. The skin is pricked with a tiny amount of the suspected allergen from a battery of allergens available in the laboratory. Each of the different allergen sites are marked with codes.
In case a person is allergic to a certain allergen, there is a skin reaction around the site of injection. The skin around the prick quickly turns itchy, red and swollen.
Since the amount of the allergen is very tiny there is a very rare risk of a more severe allergic reaction. It is safe in all age groups including babies.
Skin prick tests are not performed in patients with eczema and those who are on medications like antihistaminics which may mitigate the allergic reaction to the tested allergens.
Blood tests (radio-allergosorbent tests – RAST) are performed to measure the amount of antibodies like Immunoglobulin E or IgE. These are antibodies that are produced in response to a suspected allergen.
The results of the IgE are usually provided as a number between 0 and 6 with 0 indicating no sensitivity or propensity to allergies and 6 indicating highest sensitivity.
These tests are used when a rare allergen is suspected that may lead to future life threatening attacks of allergic reactions or anaphylactic reactions.
This is a test that is used to find the allergen that is causing skin allergies or contact dermatitis or eczema. In this a small amount of the suspected allergen is put on metal discs. These discs are taped to the skin and left for 48 hours.
After removal the skin reaction is noted. Patch test is usually conducted by a dermatologist or a skin disease specialist.
Oral Food Challenge
This is performed to detect a specific food allergy. This is a very dangerous test and needs to be performed under supervision.
An oral food challenge is the final method healthcare professionals use to diagnose food allergy.
Mast Cell Tryptase Levels Assessment
The serum level of β-tryptase can be used to diagnose anaphylaxis and mast cell activation. Tryptase levels peak at 45 - 60 minutes and may remain and may remain elevated for up to 24 hours.
CAST testing (cellular antigen stimulation test)
This is done to test for allergies to food additives and colors. These are not mediated by IgE.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)