Seeing your child have an allergic reaction is a difficult thing to deal with, especially when you don’t exactly know what’s causing it. Fortunately, there are some medical tests and methods available to accurately identify those pesky allergens. Dr Chinedu Nwokoro gives us a rundown of the tests available and what they can tell us.
1. Allergy-focused history
A doctor will normally start by asking questions about the timing, circumstances and the specific features of any reactions and of response to treatment. They will also need to know about your child’s diet, medications and about other medical conditions such as asthma, hayfever and eczema in your child and in other family members. Asking these questions helps your doctor assess the likelihood, the type and the severity of allergy, and thus guides the choice, interpretation, and safety of the available diagnostic tests.
2. Allergy skin prick test
During allergy skin tests the skin is exposed to the suspected allergen (a substance that causes allergy) and is then observed for signs of an allergic response caused by histamine release in the skin.
The skin prick test will identify allergic sensitisation and indicates a risk of Type 1 (immediate-type) allergy. Children should be well with no fever, viral illness or breathing difficulty at the time of testing. They should also have been off all antihistamines for 72-96 hours before testing and the skin should be clear of all moisturisers and steroid creams.
You can expect to have the results on the same day. A positive test on its own is not sufficient to diagnose allergy. Some children will exhibit dermatographia (a condition where the skin releases histamine in response to minor physical contact) and will need different testing.
How is the test performed?
Before the test, medication and equipment to quickly treat anaphylaxis or minor reactions must be at hand if needed. During the test, you can expect the following to happen:
- A labelled grid is drawn on the arm and a small drop of an allergen extract is applied in each square about 2cm apart, alongside a drop of positive (histamine solution) and negative (salt solution) control.
- The liquid drops are then pricked very lightly and after patted dry.
- After 15 minutes your doctor or nurse will measure the diameter of the wheal (the raised area of skin caused by histamine release) associated with each allergen or control solution and record it on a proforma.
3. Specific IgE blood testing
Immunoglobulin E (IgE) is an antibody (a protein molecule produced by the immune system) in response to an allergen. Different types of IgE react to different allergens, these are referred to as allergen specific IgE. People with immediate type allergy have high levels of allergen specific IgE in their blood. A needle is inserted into a vein (usually the hand or arm) and a small amount of blood is extracted. The sample is sent to the laboratory where the concentration of the different types of allergen specific IgE is measured.
- High levels of specific IgE indicate allergic sensitisation and possible allergy.
- High background levels of total IgE can lead to falsely high specific IgE in the absence of allergy
- The ISAC test uses microchip technology to test for 112 different allergen-specific IgE antibodies on one blood sample. It is more costly than a simple specific IgE and results can take several weeks to come back.
- Neither specific IgE blood testing nor skin prick testing will detect non-IgE mediated (delayed type) food allergies which cause gastrointestinal symptoms such as bloating, vomiting and diarrhoea).
4. Food elimination diet
The delayed-type allergy can be identified by excluding suspected allergens from the child’s diet for six weeks while monitoring symptom levels. Non-IgE-mediated allergy is likely if symptoms resolve and then reappear on reintroduction of the suspected allergen. A dietician can support this process. There are no laboratory tests for delayed-type food allergy.
5. Oral food allergen challenge
If you and your doctor believe that your child has outgrown their food allergy, then the oral food allergy challenge may be considered. This is a process by which a food allergen is eaten slowly and in gradually increasing amounts under medical supervision, with close observation for signs of allergic reaction. This is the gold standard for diagnosis of immediate type food allergy and should only occur under specialist medical supervision.
How are allergies treated?
Prevention - There is a lot of reliable evidence to suggest that early exposure to potential food allergens can protect your child against the development of an allergy in the future. Therefore, we recommend early exposure to a wide range of food in your child’s early life, including in the womb and via breast milk.
Avoidance - There is no evidence to suggest that avoiding allergenic foods in pregnancy while breastfeeding or in early infancy will benefit your child. If your child has already established an allergy then you should, of course, make sure they avoid these triggers, ensuring to take care and maintain a balanced diet with the assistance of specialist paediatric allergist where necessary.
Removal of aeroallergens (allergens carried in the air – like dust, pollen, animal dander) can improve other allergy-related conditions such as asthma. Animal dander can linger for several months after removal, and house dust mite allergen avoidance has not been shown to improve asthma. Regular antihistamine can be helpful in these settings.
Treatment of acute symptoms
Mild symptoms can be treated with oral antihistamines such as cetirizine or chlorpheniramine as and when needed.
Children who are at a high risk of suffering anaphylaxis should be provided with an adrenalin auto-injector ‘pen’ to be used at school and at home if needed. An anaphylaxis care plan (and training where needed) should also be given to their school so they treat severe reactions in a timely fashion.
Treatment of long-term complications
Long term complications, such as allergy-induced asthma, should be controlled both with medication provided by your doctor and by managing aeroallergens. Uncontrolled asthma dramatically increases the risk of anaphylaxis in response to allergen exposure.
Immunotherapy involves the reduction of allergic sensitisation by stepwise exposure to gradually increasing amounts of allergen, and can be given by injection or by mouth.
- Injection immunotherapy is available for hay fever and certain bee and wasp venoms. It is delivered in specific allergy clinics and has variable results.
- Oral immunotherapy is available for milk and peanut in a small number of highly specialised sites. It is unclear whether the induced immunological tolerance of these allergens will continue if patients stop taking the immunotherapy.
If you suspect your child may have developed an allergy and would like an allergy test, visit Dr Chinedu Nwokoro’s profile and book an appointment to see him at his clinic. Alternatively, he offers an e-Consultation service, so you can talk to him about any doubts you have from your own home.