Friday Dr Q&A with Dr Liew – Allergy Tests

This was an original 4 week Friday Q&A, combined into a single more informative post.

Children’s Allergy Basics

Thank you Dr Liew for taking time to help answer these questions, we’ll start with the basic information on allergy.

What is an allergy? For children with eczema, should parents send their children to allergy tests or should only those with eczema of a certain severity do so?

Dr Liew: An allergy is simply an abnormal immune reaction to a common protein. Symptoms are varied depending on trigger and organ involvement. The most common reaction is on the skin, resulting in itchy rashes like urticaria (hives) or eczema flares.

Atopic eczema starts essentially as a skin barrier defect, with resultant dryness, itch and allergen sensitisation later. It is not a pure allergic disease. Skin tests for eczema patients are generally not necessary, as the most common allergen is house dust mites. Food triggers are more commonly in young infants with significant eczema despite good skin therapy.

Allergy vs Intolerance

MarcieMom: Allergy and intolerance are often mixed up; can you explain the difference between the two, specifically:

How a parent can correctly identify if the child is allergic or intolerant and what follow-up action they should take in each case?

Dr Liew: Allergy and intolerance result in adverse reactions, but the key difference is that the former involves the immune system, whilst the latter do not. If the immune system is involved, there is a potential for severe allergic reaction called anaphylaxis with continued exposure due to immune memory. There is no risk of anaphylaxis in intolerance. Eg. Cow’s milk allergy can result in hives, vomiting and wheezing; in contrast, cow’s milk intolerance presents with diarrhea in lactose deficient individuals.

Skin Prick Test for Kids

Common questions on allergy tests: There are a few allergy tests available – skin prick test, blood IgE test and patch test.

Can you explain a little more about these tests, how it is done on a child and how can a parent decide which to bring their child for?

Dr Liew: The type of allergy test recommended is based on the allergy symptom of the patient. As eczema is a mixed IgE/non-IgE driven disease process, tests for IgE like SPT and blood tests may be useful if positive, as targeted elimination may be attempted. If the tests are negative, empiric elimination of 2-3 weeks duration may still be considered if there appears to be a consistent food trigger.

Prolonged food avoidance however is not necessary and may result in malnutrition.

A skin prick test (SPT) is probably commonest and simplest allergy test performed in outpatient clinics. Essentially it is a scratch test with either a needle or plastic device, thus introducing a tiny amount of allergen protein under the skin surface. The test is completed within 15 minutes and measurement taken. A positive reaction appears as a small hive-like wheal and surrounding redness. It is good for identification of IgE mediated allergic reactions, and also for trending allergy sensitisation over time. Antihistamines would need to be ceased for 5-7 days, and there must be an area of clear skin (preferably the back in young children) for the test to be done. This is my preferred allergy test as there is minimal pain (especially with plastic devices), and results immediate. SPT also correlate better with allergic reactions if properly performed, as it takes into account the blocking antibody responses when the body is “outgrowing” an allergy.

Blood IgE Test

Allergen specific IgE test can also be easily performed for IgE mediated reactions. This is sometimes preferable if the patient has severe eczema and unable to tolerate cessation of antihistamines, or has minimal normal skin to perform the SPT. Blood tests are generally more expensive, as each allergen protein tested cost around $30. Results are usually known within a week.

Patch Testing

Patch tests are designed to test for delayed allergic reactions, rather than acute IgE reactions as the above 2 tests. Patch tests involved placing small area of allergen protein coated on wells or filter paper, and leaving them as a skin patch for 48 to 72 hours. A reading is then performed to look for delayed hypersensitivity reactions and the skin reaction graded. As there are several confounding factors for a successful test, patch tests are currently not recommended for clinical use but confined to research studies.

What to do after Your Child is Tested Positive for Allergens?

Given that allergy tests are not 100% accurate, parents may start to rely on an elimination diet strategy. 

Can you explain how much you would rely on each allergy test and whether they serve a different purpose in your diagnosis?

Dr Liew: Standard allergy tests are accurate, but have their limitations. Unvalidated tests for food allergy include blood IgG testing, intradermal skin testing, applied kinesiology, electrodermal testing, hair mineral testing, and iridology, and should not be performed. Eczema is a chronic medical condition with no curative treatment currently, and some patient would undertake extreme measures to look for the “elusive trigger”.

I would remind your readers that eczema is a skin disease, and not an allergic disease. Removing triggers can reduce the eczema, but will not cure it.

Food Elimination for Kids

Empiric food elimination may be considered for 2-3 weeks if a consistent food trigger is suspected. An objective assessment should be made if food elimination has resulted in any change. Food triggered eczema would improve significantly with elimination. If there is no improvement, as in the majority of cases, the food should be introduced and assessment made if there is a change. I would caution regarding multiple food elimination as I have seen really malnourished and stressed out patient and families.

Young Children’s Developing Allergy Profile

MarcieMom: I understand that it’s good to send the child for a re-test, to check if he or she has outgrown any allergy or developed new ones. 

What’s the reason for the change in the allergy profile of the child? Also, how often do you recommend a re-test and would your recommendation differ for a child who has different type of allergies and/or differ for a child who has different level of severity in eczema?

Dr Liew: Retesting is sometimes required in food allergies, but generally not necessary for eczema. The allergic profile of an individual changes according to his/her immunity and exposure to environmental proteins. We often see food allergens being “outgrown”, but a gain of house dust mite sensitisation with time. Retesting is usually considered if there is a new allergic disease eg. Allergic rhinitis, rather than based on fluctuation in eczema severity.

Common Pediatric Allergens

Based on your experience and knowledge of research studies, what are the common allergens in children? 

Dr Liew: The most common allergens depend on the allergic conditions. For eczema, older patients tend to be sensitised to house dust mites, but not food allergens. Some young infants with eczema has food sensitisation and trigger. The most common food allergens in eczema children include egg, cow’s milk, wheat, peanuts and soy. Some forms of food allergy eg, allergic enteropathy presents with isolated intestinal symptoms without skin involvement.

If a parent suspect a food is causing an allergy, should the parent insist on it being an item to be included in allergy test? 

As a pediatrician, would you test the child on what the parent suspect?

Dr Liew: A detailed history would be obtained prior to allergy testing. If there is a suspected food trigger, I would usually test it. I would usually encourage targeted testing, whereby only the relevant food items are tested. For eg, should the child be tolerating cow’s milk formula for the past 3 years, and eczema commenced after 3 years old, cow’s milk protein is unlikely to be a food trigger, and should not be tested.

Gluten and Eczema

MarcieMom: Gluten has been gaining more awareness and with many food packaging labeling gluten-free, some parents may think it’s gluten that’s causing the eczema. 

Again, how can a parent identify if it’s gluten (is it even a common trigger?) that’s triggering the eczema?

How can this be differentiated from celiac disease?

Dr Liew: There should be a close temporal relationship between food ingestion and eczema flare. If there is inconsistent relationship, it is unlikely a food allergy. Celiac disease is an immune-mediated reaction to glutens, resulting in malabsorption in the gut, and resultant bloatedness, diarrhea and weight loss.