Friday Dr Q&A with Dr Liew – Allergy Tests

Dr Liew Woei Kang

Marcie, who inspired MarcieMom to start this blog, doesn’t have any allergy and thus, this blog has been focused on eczema. Recognizing that there are many parents whose child also have allergy, MarcieMom invites Dr Liew Woei Kang, Paediatrician with special interest in Allergy, Immunology & Rheumatology to share more about managing allergy for eczema children.

More about Dr Liew: Dr. Liew practices at the SBCC Baby & Child Clinic and is also a visiting consultant to KK Hospital. He was also awarded several research grants from the National Medical Research Council, Singhealth Foundation and KKH Research centre to pursue clinical research in paediatric anaphylaxis, drug allergy, primary immunodeficiencies and Kawasaki disease. He is also the President of Singapore’s Asthma & Allergy Association which is currently administering the very first eczema fund (initiated by MarcieMom’s donation) for low income patients in Singapore.

MarcieMom: Thank you Dr Liew for joining me again for another Friday Dr Q&A. I’ve written about the inaccuracies of skin prick test, though I’d still recommend parents to get their child tested in order to avoid being paranoid of everything in this world that may be an allergen.

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 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.

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 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.

MarcieMom: Thank you Dr Liew, next week we’ll learn more on allergy test and elimination diet.

Friday Dr Q&A with Prof Hugo – Skin Prick Test

Prof Hugo Van Bever

Prof. Hugo Van Bever is the Head of National University Hospital’s Pediatric Allergy, Immunology & Rheumatology Department. He is also an active member of the board APAPARI (Asian Pacific Association of Paediatric Allergy, Respirology and Immunology) and has published more than 250 papers in national and international journals. His main research interest areas are paediatric allergy and paediatric respiratory infections.

Marcie Mom: Morning Prof Hugo, today’s questions is on skin prick test, something I always recommend parents to do (in this post).

In a skin prick test, typically a few common food allergens such as egg and milk will be tested instead of all possible food that the child may eat. Why is testing a few of these food allergens sufficient to diagnose if there’s an underlying food allergy?

How often should a child get retested for food allergy? And what type of patients needs to be retested regularly?

Anti-histamines should be avoided before a skin prick test so as to avoid incorrect results (due to no reaction seen when there should be one). What else should a parent take note before bringing the child for allergy test?

Prof Hugo: Because only a few foods are involved in eczema, mainly in young children. These are: cow’s milk, egg, wheat and soy. Other foods are very seldom involved in eczema. In older children food allergy is usually not involved in eczema. Older children have mainly an allergy to house dust mites.

If the child improves there is no need to repeat skin prick test.

Skin prick test should be performed only when the child is in good health (no ongoing infection, no fever) and did not take antihistamines for at least 3 days. Antihistamines may suppress the skin prick test’s results, leading to false negative results. Skin prick test also needs an area of normal skin. In cases of severe eczema, skin prick test’s results are difficult to interpret. A blood test might be an alternative.

Marcie Mom: Thanks, for adults who can take a patch test, can refer to this post.

Questions I asked at webinar hosted by DrSearsLean

Dr Sears L.E.A.N. webinar on The Pros and Cons of Soy (image from DrSearsLean.com)

It’s another first for me, participating in a webinar, this time hosted by DrSearsLean. The webinar was conducted by Dominique Hodgin, MA, NE, the Executive Director of Education at DrSearsLean and Nutritionist. The presentation covered the definition of soy as well as the positive and negative effects of soy. I’ll leave to DrSearsLean to update you on the presentation which will be made available on his website; for me, I managed to get my questions related to eczema children answered and presenting to you as fast as I can!

Marcie Mom:   For infants who are diagnosed with eczema, should they switch to cow’s milk alternative like soy or goat or partially hydrolysed milk?

Dominique: A lot of the soy-based infant formula are isolated soy proteins which can potentially create some problems associated with soy isolates. If non-soy formula can be consumed by the infant, it’ll be better to consume a non-soy one. Dr Sears recommended in his book that even if the infant has to use a soy formula, it is good to try to reintroduce lactose at a later age.

Marcie Mom: Can a skin prick test accurately detect a soy allegy? How young can an infant take a skin prick test?

Dominique: Yes, a skin prick test can detect a soy allergy. Infant, as young as 6 month old can take a skin prick test (younger than that, may not be accurate).  Of course, to be sure, you can check for symptons of allergy whether present/absent after eliminating soy from the diet.

Marcie Mom: For moms with cow’s milk allergy, would drinking soy during pregnancy instead of cow’s milk, reduce chance of baby developing soy allergy? (actually, I typed in the incorrect question, wanted to ask if it’d reduce chance of baby developing milk allergy)

Dominique: Not necessary. Milk allergy is a broad term, as there are various components that one may be allergic to, say whey. In any case, no definite on moms’ diet on baby’s development of allergy/not. Do also note that one does not only consume soy through drinking soy, as soy is present in many packaged food. Key is not to overconsume soy.

Is your Eczema Child (Wrongly) Avoiding a Tested-Positive Food in SPT?

Skin Prick Test (SPT)

If your child has eczema and has been avoiding a food that he/she has been tested positive in the Skin Prick Test (SPT), it may be time to review if there is indeed a true food allergy. Research indicates, as I understand from various online articles, that the skin prick test is a test of sensitization, and not clinical relevance. Below are some points on the accuracy of skin prick test:

1.      Skin Prick Test is more accurate for non-food allergen, such as dust mite and pollen, than for food allergen.

2.      Skin Prick Test is More Accurate in predicting when there is NO food allergy and Less Accurate in predicting a Positive food allergy. The implication of this is that children who have been avoiding food (for example more than 50% of those tested positive for milk and eggs, and more than 70% of those tested positive for peanuts) actually didn’t have an allergic reaction to these foods. (There is only a 0.5-10% chance that a child who is allergic to a food will test negative for it.)

3.      Follow-up tests are required for skin prick test for children at different age, sometimes about 1-2 years later. The results can be different as children may ‘outgrow’ some allergy or develop new ones? (hopefully not…)

4.      An oral food challenge is the most accurate (less than 5% incorrect).

5.      For children with eczema, there is an 80% possibility that food that the child is avoiding turn out not to be one he/she is allergic to. Refer to this article for detailed research.

Despite above, Skin Prick Test still remains the most recommended first test for allergy, as it is by comparison simpler, lower cost, faster results, more tolerated (children won’t feel much pain, you can read this post on my baby Marcie’s experience) and more accurate than blood test. If anyone has an experience to share on skin prick test for your child, do comment below this post.

Taking the fear out of the Skin Prick Test

Skin Prick Test (image from eatstudy.co.uk)

Sending a baby or a young child to a skin prick test may be fearful for you or your child.

My baby girl, Marcie, had her skin prick test about 7 months old when I gave up trying to identify what triggered her eczema flares (some days I thought it’s the stage 1 food she was having and some nights I drove myself paranoid researching all sorts of allergies). Prior to the skin prick test taken at Singapore’s National University Hospital, I had some fears of the skin prick test myself; I imagined that Marcie’s skin will be pricked all over with many holes to test for all possible food she will eat in her lifetime and I also imagined the needle to be like a syringe that will inject the possible allergens into her skin.

A Quick Prick that tells a lot

It turned out that the skin prick test is about 20 minutes, starting with the medical staff cleaning Marcie’s inner forearm and applying the possible allergens (egg, dust mite, cat’s hair, dog’s hair, milk, mold) onto her arm. While one staff proceeded to prick Marcie’s skin with a needle to allow the substance to enter her skin, another staff was playing with Marcie. Marcie did not cry or even whine but I think that’s because her usual itch is far more ‘painful’ than that little prick. We had to wait 20 minutes or so before the medical staff examined the size of the wheal, i.e. reddish swelling to determine if Marcie is allergic to any of the substance.

What the Skin Prick Test Results say

All the results tested negative and as mentioned, this takes a lot of stress out – no worries over food allergy (apparently, testing for milk and egg is sufficient for food allergy; no need to prick 100 holes to test for all possible food to be consumed in a lifetime).

Don’t try pricking at home

It was a blessing that Marcie did not show any allergic reaction. However, if there is a reaction to the allergen, there will be swelling that is bigger than the dummy test and also itching. The test is not to be done at home without medical staff in case of severe reactions to the allergen. The skin area to be tested must also be free from rashes, otherwise, the reactions will not be visible. Plus, consumption of anti-histamines should be avoided otherwise the skin may not show a reaction (when there should be one).

I strongly encourage parents to bring their child for the skin prick test, otherwise, you may drive yourself paranoid guessing what causes the eczema..

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