Skin Prick Testing
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Skin Prick Testing
As part of your child’s appointment the doctor, nurse or dietitian may request a skin prick test (SPT). This leaflet explains what this test involves and what will happen when you and your child come to our clinic. If you have any questions, please speak to a member of staff caring for your child.
Why does my child need a SPT?
This test will help us to identify which allergen (the substance that causes an allergic reaction) your child may be allergic to. It can also help us monitor the progress of your child’s allergies.
An allergy is when the immune system responds to an allergen, such as food or pollen, as if it is a harmful substance. This can cause the following symptoms:
• A runny nose
• Itchy eyes
• Hives (rash)
• Difficulty in breathing
Skin prick testing (SPT) is an accurate way to test individual substances to see if they cause an allergic reaction. It can test for food and aeroallergen (causing hay fever-type symptoms) allergies. However, skin prick testing cannot diagnose drug or venom allergies.
What Substances can be Tested?
We can perform skin prick tests for a range of foods, such as:
• A range of nuts
• Fish and Shrimp
If you think your child may have had an allergic reaction to any other food, you can bring a small piece of it with you to the clinic. The food should not be mixed with any other products, for example sauces. It should also be individually packaged. For example, bring a whole piece of fruit rather than a pre-prepared fruit salad. If the food is usually eaten cooked, such as chicken, please bring both raw and cooked samples.
If you cannot bring the food you want tested with you, we can perform a blood test to see if your child is allergic or not (see ‘Are There any Alternatives to a SPT?’ below).
We can also test for a wide range of aeroallergens, for example grass and tree pollen, house dust mites, and animals.
Are There any Alternatives to a SPT?
Yes. A blood test can be used to measure your child’s allergic response to a specific allergen. However, skin prick testing is quicker than a blood test. Results are seen in less than 15 minutes, but blood test results take about four weeks.
How can I Prepare my Child for the Test?
If your child is taking antihistamines such as chlorphenamine (Piriton®), cetirizine (Zirtek®) or loratadine (Clarityn®), they need to stop taking them before their appointment. This is because
antihistamines may interfere with the test. Please follow the instructions below:
• If your child is taking short-acting antihistamines (usually taken two-three times a day), such as chlorphenamine (Piriton®), they should stop taking them two days before the appointment.
• If your child is taking long-acting antihistamines (usually taken daily), such as cetirizine (Zirtek®), or loratadine (Clarityn®), they should stop taking them five days before the appointment.
If your child has an allergic reaction before their appointment, treat them as you usually would. Use antihistamines if you need to.
Please contact Paediatric Day Care Unit via 996373 if you:
• Are unsure when to stop your child’s antihistamines
• Are unsure whether your child is taking an antihistamine
• Feel that stopping your child’s antihistamine would not be appropriate.
• Have needed to give antihistamines for an allergic reaction in the days before your child’s appointment.
A SPT should not be performed if your child if wheezy. If your child has a blue (reliever) inhaler and needs it on the day of their appointment, you should still attend the appointment, but let a nurse or doctor know that the child has been wheezy.
What Happens Before the Test?
Before any allergy testing, the doctor or nurse will ask about your child’s medical history. You will need to give details about any allergic reactions or symptoms your child has been experiencing.
You may also be asked questions about your child’s:
• Current or previous illnesses
• Emotional and social conditions
• Schooling, entertainment
• Eating habits
Consent – Asking for your Consent
We want to involve you in decisions about your child’s care and treatment. We will ask you if you are happy for us to perform the test. This means that you agree for your child to have the treatment and you understand what it involves. If you would like more information about our consent process, please speak to a member of staff caring for your child.
What Happens During the Test?
A nurse will place a drop of each allergen on your child’s skin (usually their forearm or upper back). The skin under the drop of allergen is then pricked with a lancet (a type of needle) that scratches the top layer of the skin. By doing this, the allergen gets under the surface of the skin. The nurse will then wipe away the drop. If we perform a test with the food you have brought with you, we will prick a piece of the food with a lancet and then immediately prick your child’s skin with it. This method is called ‘prick to prick testing’.
We can test for several allergens at your appointment. The nurse will repeat the same process for each allergen. We will make a different mark on your child’s skin with a washable pen for each test. This means we can identify the different allergen tests. After 15 minutes, the nurse will check your child’s skin for a reaction.
There are two types of reaction:
• A positive reaction – the skin under the drop of allergen becomes red and itchy. A white, raised swelling called a weal is surrounded by a red area. This takes about 15 to 20 minutes to reach a maximum size and then fades after a few hours. This means that your child is likely to be allergic to the substance in the solution.
• A negative reaction – the skin under the drop of allergen remains normal. This means that your child is not likely to be allergic to the substance in the solution.
Does the Skin Prick Testing Hurt?
Skin tests may be slightly uncomfortable when the skin is pricked, but they should not hurt. If the test is positive, the weal may feel itchy. This will usually start to settle after 20 minutes – most children and infants cope with it well.
If your child is upset, a cold compress/ice pack can be used on the area to help relieve the itching once we have the results of the test. Alternatively, we can give your child some antihistamine medicine.
What Happens After the Test?
If the SPT is performed in allergy clinic the results of the SPT must always be considered with your child’s specific history of reaction or tolerance in mind. Because of this, the consultant, specialist nurse or dietician who runs the clinic will discuss the test results and what they mean with you. Please feel free to ask any questions you have at this time. If the SPT performed on Paediatric Day care (PDCU), the results will be sent to the requesting clinician for interpretation and advice on future management. The parents must be informed that although we have the results immediately, we will not be offering any advice on this on PDCU.
We will also send a letter to your GP about your child’s test results.
What Are the Risks?
Typical reactions to having the test include:
• A red and itchy skin at the site of the test.
• A weal can develop at the site of the test.
Reactions from the allergens after your child has left the clinic are rare. In general, any symptoms of an allergy are noticed soon after the skin test. Local itching and swelling normally settle down within a few hours. If your child becomes upset or uncomfortable once you have left the clinic, you can use a cold compress/ice pack to help relieve symptoms on the affected area.
Alternatively, you can give your child some antihistamine medicine. Very occasionally, children feel dizzy or light-headed and need to lie down. Again, we will be on hand and able to treat this if it occurs. Severe allergic reactions from allergy testing are extremely rare. In the unlikely event that your child feels unwell after skin prick testing, or if they show signs of an allergic reaction, we will keep them in the department for observation.
If your child becomes unwell after you have left the clinic, please contact your GP or go to your local Emergency Department (A&E) immediately.
Information has been provided by Guy’s and St Thomas’s NHS Foundation Trust Evelina London.