Allergy skin prick test
HOW DOES AN ALLERGIC REACTION OCCUR?
Allergic people have been “sensitised” to an allergen (e.g. pollen) when it enters the person’s body… say through the nose. The pollen causes the body to make allergic antibodies, called IgE. The IgE antibodies then go around the blood and attach itself to cells called mast cells. This person is now “sensitised” and much more likely to have an allergy attack. The second time the pollen enters the person’s body it connects with the IgE antibodies attached to the surface of the allergy cells. This makes the allergy cell burst and releases its chemicals into the blood, causing symptoms and leading to “inflammation”: swelling of the parts of the body involved. (See what is an allergy)
What is a skin prick test?
Skin prick tests (SPT) are a way of checking which substances may cause an allergic reaction. SPTs use your own body to show us whether it has IgE antibodies to the allergens in the skin prick drops.
SPTs can be used to help with the diagnosis of environmental allergens in the air (that cause hay fever or asthma symptoms) and for food allergies. Drug prick testing is done in specialised units. Insect allergies cannot be diagnosed using SPT
Are there any alternatives to a skin prick test?
Yes. The amount of allergic response in your body against allergens can also be measured with a blood test. However, blood test results take approximately 1-2 weeks to return and are more costly. SPT results are seen 15 minutes after the test is performed.
Are allergy tests foolproof?
These tests show the presence of the IgE antibodies. If these tests are negative, an immediate type allergy is almost always ruled out.
But, a positive skin prick test DOES NOT always mean that the person is allergic, it means the person has the potential to be allergic. Because the tests pick up the specific allergy antibody, they will pick up the presence of antibodies even when the person is only sensitised to an allergen, even when they are not actually allergic to it.
The larger the size of the skin prick test , the more likely it is that the person is truly allergic to that allergen rather than just being sensitised.
For environmental allergens an SPT of 3mm or more is usually considered a positive test.
For food allergy a positive test varies for different foods and in people of different ages, but usually it is much larger than for environmental allergens. In some cases the test cannot give the final answer and other tests may need to be done.
Allergy tests are only a guide as to whether the person has the potential to be allergic but they cannot predict whether the reaction will be mild or severe.
Before you come for a skin test
Antihistamines should be stopped 3-5 days before the allergy skin test is done as they interfere with the accuracy of the test.
If you/your child suffers an allergic reaction prior to the appointment you should still treat them using antihistamines if needed, and the test can be delayed.
SPT should not be performed if you/your child is wheezy or very ill. Please let the nurse or doctor know if the reliever inhaler has been used on the day of the appointment.
SPT for environmental allergens is done with specially-made allergen solutions. They are also available for some, but not all foods. Ask the doctor whether you will need to bring any foods with you on the day of the skin test.
Testing
The SPT is performed by placing a drop of the allergen on the skin (usually the forearm or upper back). A lancet is then used to prick the top layers of skin through the drop so that the allergen is introduced under the skin surface. The drop is then wiped away. This process is repeated for each allergen.
AFTER THE TEST The results of the SPT may be negative, slightly raised or clearly positive. The result must always be discussed with the doctor or specialist nurse for advice as to what the test means and what the implications are. IS THE TEST RISKY? Typical reactions are the wheal and flare at the site of the test that was described earlier. Reactions from the allergens after the test are rare. The itch and swelling normally goes away within an hour. Very occasionally patients will feel dizzy or light-headed and need to lie down. Severe allergic reactions from allergy testing are extremely rare. If you/your child has any generalized reactions after the test such as hives (urticaria, bommels), wheeze or a generalized itchy rash please contact your GP or emergency department straight away. www.allergyfoundation.co.za PAGE 3 | ALLERGEN SKIN TESTING ALLERGEN SKIN TESTING After 15 minutes we look at the skin for signs of a reaction. A negative reaction is when the skin remains normal. If we see a white wheal (also sometimes called a hive; a bump similar to that seen in an insect bite) surrounded by redness (also known as ‘flare’) we measure it to see how large the reaction is. This wheal will disappear within 30-60 minutes.
To make sure the test is working properly we always do a test to histamine in one of the drops. Remember histamine is the chemical released from the allergy cells and that causes symptoms. So everyone should react to the histamine test … this is normal. Skin tests may be slightly uncomfortable, but are usually well tolerated, even by small children and infants. Skin prick tests may cause very mild discomfort when the skin is pricked. If the test is positive the wheal may feel itchy for up to 20 minutes. Once the test has been read, a cold compress can be applied or some antihistamine given.
After the test
The results of the SPT may be negative, slightly raised or clearly positive. The result must always be discussed with the doctor or specialist nurse for advice as to what the test means and what the implications are.
IS THE TEST RISKY?
Typical reactions are the wheal and flare at the site of the test that was described earlier. Reactions from the allergens after the test are rare. The itch and swelling normally goes away within an hour. Very occasionally patients will feel dizzy or light-headed and need to lie down. Severe allergic reactions from allergy testing are extremely rare. If you/your child has any generalized reactions after the test such as hives (urticaria, bommels), wheeze or a generalized itchy rash please contact your GP or emergency department straight away.