Diagnostic Tests and Procedures
Diagnostic Tests and Procedures
Skin Prick Tests

Skin Prick Testing (SPT) is the most commonly performed allergy test. It is used to test for Immunoglobulin E (IgE) type of allergy to environmental allergens such as house dust mites, pollen, animal dander and also food allergens such as shellfish, cow’s milk, soy, wheat, peanuts, treenuts, eggs and fish.

How it is performed

Standardised extracts of the many various allergens are available in our hospital. One droplet of each extract is placed on the inner aspect of the forearm about 3cm apart and a specially modified lancet is used to prick the surface of the skin through the droplet. Care is taken not to draw any blood. A swelling of at least 3mm in diameter is a positive reaction. This test can be performed within the same clinic visit and the result can be read at around 20 minutes after the test is performed.

Intradermal Skin Tests

An intradermal skin test may be done when a substance does not cause a reaction in the skin prick test but is still suspected to be an allergen for that person. During this test, a small amount of the allergen solution is injected into the skin under its top-most layer. The intradermal test is more sensitive than the skin prick test but can be slightly more uncomfortable. A positive result is a swelling of more than 3mm in diameter.

Specific Serum Immunoglobulin E (IgE)

This is a blood test that measures the level of immunoglobulin E (IgE), which is the antibody that is produced by the immune system and is raised in an allergic person. Specific IgE levels measure the level of antibodies present against a specific allergen in the allergic person’s blood. This test is more sensitive than the skin tests but require blood to be drawn and results may only be available after a few days to a week.

Component Resolved Testing

Skin Prick Tests may occasionally show a positive reaction in persons who are actually sensitized to another cross-reactive allergen which shares the same chemical structure as the original allergen being tested. Thus the person may not truly be allergic to the original allergen.

Component Resolved Testing involves a blood test which measures IgE antibody levels to specific proteins in allergens that are highly associated with true allergy. Thus it helps to distinguish between true allergy and cross-reactivity to another substance that shares similar chemical properties.

The tests available in our institution include omega-5-gliadin (wheat) and Ara-H-2 (peanut)

Atopy Patch Tests

Patients who have eczema (atopic dermatitis) or contact dermatitis may have atopy patch tests done. These tests are used to determine if a specific substance causes allergic inflammation of the skin. Tiny amounts of the suspected chemicals will be applied to the upper part of the back and covered with adhesive tape. This should not be disturbed for about 48 hours, thus showering is not recommended. Your test results will be read by your allergy doctor 48 hours later. Development of redness, blisters or a bumpy rash over an area covered by a suspected allergen is a positive reaction.

Oral food challenge / drug challenge

The best way to diagnose whether a food or drug allergy is truly present or absent is a food/drug challenge. If required, your doctor will arrange a separate appointment for your child to return to our Day therapy centre where he/she will be closely supervised while consuming the suspected food or drug. Only minute amounts of the food will be administered initially while your child is closely observed for any signs of allergy. If he/she tolerates the minute amount without any reaction, the amount will be gradually increased in a series of steps. If your child develops any symptoms of allergy at any point during the challenge, it will be stopped immediately and your child will be treated for the allergic symptoms appropriately.

The total duration of a challenge could last as long as 2-3 hours including the periods required for observation.

Lung Function Test

This is a test which measures lung volumes as well as airflow obstruction. It requires blowing through a tube equipped with a sensor with good effort. Results are used to support the diagnosis of asthma, assess severity as well as response to treatment on follow up. This test is usually performed in children above the age of 6 who are able to follow instructions well.

Desensitization

Desensitization is a method to help patients with an allergy to tolerate exposure to a specific allergen. Desensitization to a certain medication involves giving the medication in slowly increasing amounts, starting with small doses, and ending with the full dose required by your child. The medication is given in the same format (oral or intravenous) but starts more slowly and the process takes much longer. Not all reactions to medications are able to be desensitized. These will have to be evaluated by your doctor. The whole process may take several hours. Your child will also be required to be observed for a period of time after the procedure. This may take 1-2 hours. In some cases, your doctor may recommend admission for overnight observation for the safety of your child.

Immunotherapy (Sublingual and subcutaneous)

Immunotherapy is a treatment which aims to reduce or completely remove your child’s allergic symptoms and the need for longterm medications. This is performed when your child has exhausted all conventional medications but still suffers from significant morbidity from his allergic condition. In this method, a weak concentrations of the allergen extract is given as drops under the tongue or as injections under your child’s skin. Increasing concentrations are gradually given to reach the ‘full dose’. The total duration of treatment is usually 3-5 years. However, improvements in symptoms are usually seen in the first six months. Currently we conduct immunotherapy for allergic rhinitis and house dust mite allergies and insect venom. Research studies are also underway for immunotherapy for food allergens such as peanut.

Links

List of antihistamines:

  • Cetirizine/ levocetirizine (Zyrtec)
  • Chlorpheniramine (Piriton)
  • Hydroxyzine
  • Loratadine (Clarityn)
  • Desloratadine (Clarinex)
  • Fexofenadine (Telfast)

 

 
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