Skin Prick Testing
Skin prick testing or SPT demonstrates an allergic response to a specific allergen. In conjunction with an allergy focused history, SPT can help to confirm the presence of an allergy to either a food or inhaled substance (allergen).
SPT is the most common allergy test performed in an allergy clinic by specially trained staff. SPT is a simple, safe and quick test, providing results within 15-20 minutes. This will enable you to receive a diagnosis and management plan at your appointment.
The skin prick test introduces a tiny amount of allergen into the skin, eliciting a small, localised allergic response, in the form of a wheal (bump) and flare (redness) at the site of testing. These tests can be carried out on all age groups, including babies.
- SPT is usually carried out on the inner forearm, but in some circumstances may be carried out on another part of the body, such as back or thigh. For example, there is a larger area on the back or thigh to perform testing on a baby, similarly, for those with troublesome eczema the test can be performed on any clear patch of skin
- The test allergens are selected following a discussion with your clinician and based on your history
- As few as 3 or 4 or up to about 25 allergens can be tested
- The skin is coded with a marker pen to identify the allergens to be tested
- A drop of the allergen (extract) solution is placed on the skin
- The skin is then pricked through the drop using the tip of a lancet – this can feel a little sharp but should not be painful and should not bleed.
The patient needs to avoid taking anti-histamines and certain other medications before the test. Long acting antihistamines (those that do not cause drowsiness) should be stopped for five days, short acting antihistamines can be stopped 48 hours beforehand. Many cough mixtures contain an antihistamine, therefore please tell the healthcare professional who is performing the test any medication you have taken. The results will not be reliable if anti-histamines are still in the body when the test is performed as antihistamines can prevent the body from reacting to the SPT. If the patient has not stopped taking anti-histamines before their appointment they may not be able to have the skin prick test.
Two control samples are included to make sure that the test has worked; one of the controls will cause a reaction in all people, and the other should not cause a reaction in anyone. This helps the nurses and doctors ensure the test has been conducted properly.
The skin may become itchy within a few minutes and become red and swollen with a “wheal” in the centre (very much like the reaction to a nettle sting). The wheal has a raised edge which slowly expands to reach its maximum size in about 15 minutes, clearing for most people within an hour. Depending upon what you have discussed with your doctor, this may be a ‘positive’ response and your doctor will discuss the results and what they mean for you.
No reaction to the SPT (a negative response) may indicate that the patient is not sensitive to that allergen. Negative reactions may occur for other reasons, for example; if the patient is taking anti-histamines or medications that block the effect of histamine. Which is why it is important the results are interpreted by a healthcare professional experienced in allergy. In some cases, the doctor may request a blood test to help clarify the results.
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