Diagnosis of Allergy
 Diagnosis of Allergy |  Treatment for Allergy |  Joint Collaborative Severe Asthma Service |  Pre-vaccination Screening Service |
- Skin Prick Test
- Intradermal Skin Test
- Blood Tests
Allergy Tests
- Spirometry
- Reversibility Test
- Peak Flow
Lung Function Tests
- Drug Challenge
- Food Challenge
- Mannitol Challenge
Challenge Tests
One-stop Investigation with State-of-the-Art Facilities
HKSH Lee Tak Hong Allergy Centre offers a comprehensive range of assessment tools to investigate allergic conditions. Our specially trained team, including Specialists in Allergy and Immunology, and Registered Dietitians and nurses with dedicated training in allergy management, conducts all necessary tests and interprets the results for our patients. Thorough investigations of any possible allergic cause(s) of a patient’s symptoms from a detailed medical history, specific investigations including food diaries, blood tests, skin tests, lung function tests, elimination diets, and food challenge tests. Tests are conducted as appropriate to either confirm the causes of allergy, or to ascertain that the problem is not allergy at all.
Skin Prick Test
A skin prick test (SPT) is performed by placing a drop of an allergen solution on the skin and then gently pricking the skin with a sterile lancet to introduce the allergen into the top layer of the skin. A similar procedure is done with salt water as a negative control.
Compared to the salt water control if the skin reacts to the allergen with an itchy raised surface area (wheal) and a surrounding redness (flare) after 15 minutes, it suggests that the person is allergic to that substance.
It is good practice to also perform a test with the chemical histamine as a positive control in the same session; everyone reacts to this chemical and a positive result is indicative that the person has not been taking antihistamines recently. Antihistamines if taken will prevent any allergic reaction from occurring and give rise to a false negative result in the test.
Intradermal Skin Test
Intradermal skin tests are for allergy analysis, using used in the diagnosis of drug allergies. A small amount of allergen is injected into the skin with a syringe and needle in the intradermal skin test. This form of testing is more sensitive than SPT but can produce more frequent adverse reactions.
Blood Test
RAST (radioallergosorbent test) is a blood test used to detect specific IgE antibodies to suspected or known allergens.
(1) Lung Function Tests
Spirometry is the most commonly used lung function test and it will show up any obstruction to airflow, such as in asthma. Occasionally more detailed lung function tests are required.
(2) Reversibility Test is done in some cases where asthma may be suspected and to differentiate asthma from other conditions. For this test, the patient will be asked to perform spirometry before and after being given a drug (bronchodilator) to open up the airways. As asthma has a reversible element to the airways obstruction, significant improvement in airflow after using a bronchodilator drug suggests a diagnosis of asthma.
(3) Peak Flow: A peak flow meter is a small device that measures the fastest rate of air that you can blow out of the lungs. Like spirometry, it can detect airways narrowing. It is more convenient than spirometry and is commonly used to help diagnose asthma. Many people with asthma will use a peak flow meter to monitor their asthma status.
Challenge Tests
(1) Drug Challenge: The primary aim is to exclude or confirm allergy to a drug. Drug challenge involves introducing gradually increasing amounts of suspected drug by oral, inhaled or other routes. It carries a risk of severe allergic reactions, therefore must be carried out in an appropriate setting by competent staff and only if clinically indicated.
(2) Food Challenge: A food challenge test is performed when history, skin prick test and blood test show ambiguous results or if intolerance rather than allergy is suspected. It may also be performed to see whether a patient has outgrown a food allergy. A food challenge involves eating incremental amounts of the suspected food allergen, often under supervision to see whether there is any reaction.
(3) Mannitol Challenge: To assess the lung’s irritability, a mannitol test may be performed as hyperirritability of the airways in response to environmental changes and irritations is a characteristic of asthma. The test is also useful in documenting improvements in asthma after treatment. The test is conducted by inhaling increasing doses of powdered mannitol under supervision and measuring spirometry or peak flow after each dose to assess response.
Allergy Kitchen for Food Challenge
Oral food challenges are the gold-standard for diagnosis of food allergy and intolerances. A food challenge involves eating incremental amounts of the suspected food, often under supervision to see whether there is any reaction, thereby pinpointing the culprit.
The dedicated Allergy Kitchen for investigation of food allergy is rarely available in other HKSH Lee Tak Hong Allergy Centre around the world. Foods are prepared separately without seasonings and in different ways for food challenge tests. This allows pinpointing of the exact allergen and/or preparation method which gives rise to the allergic reaction. The dedicated kitchen is crucial to the accuracy of food challenge tests by preventing cross-contamination between different food substances and cooking utensils.
Food challenges or skin prick tests (SPT) are performed in a specially designed negative pressure room to ensure that no allergen enters the general clinic area from the challenge room to affect other patients.