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.

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