Skin Prick Test (SPT)
Skin Prick Test (SPT) is a method of testing a person to determine whether or not they have an (IgE mediated) allergic response to a specific allergen. It is a simple and safe test; gives an immediate result (15 to 20 minute). This test helps an allergist/ doctor to reach a diagnosis in subjects with rhino-conjunctivitis, asthma, urticaria, anaphylaxis, atopic eczema,food and drug allergies. thereofre, an allergist can provide the patient with an appropriate treatment plan. However, for the diagnosis of an allergic disease, in addition to the result of SPT, an allergy focus clinical history is compulsory.
SPT should be performed by trained practitioners/individual who also trained in resuscitation techniques. This test can be carried out to anybody including babies with some exceptions.
A tiny drop of allergen is placed usually onto volar/inner aspect of the forearm at least 1.5 cm apart (ALK), followed by a gentle pin/lancet prick through the drop. A new lancet with a point length of 1mm can be used to prick through the drop at 90 degrees to the skin, which can be felt but not painful or bleed. In addition to forearm, other parts of the body can be used such as back or outer side of thigh.
The prick allows the antigen into the epidermis and non- vascular superficial dermis, where they can meet the IgE primed mast cells if an individual is allergic to this substance. As a result of the antigen and IgE interaction, must cells release histamine and other mediators leading to a visible ‘wheal and flare’ at the site of testing within 15 minutes and it is usually itchy. In most of the people, the wheal disappear within an hour. The discomfort of SPT is least and the risk of systemic reactions is minimal.
Before the test is performed it is important to carry out physical examination and take the medical history of the patients as some medicines may reduce or prevent skin reactivity to the SPT/allergens. Therefore, the patient must be stopped such medicines prior to the test.
Before the appointment the patients must stop/withheld long acting antihistamines for 5 days and short acting antihistamines for 2 days. Some antidepressants such as, doxepin, tricyclics and tetracyclic etc have antihistamine activities may need to be stopped 1 to 2 weeks prior to the test. Many cough mixtures may contain antihistamines therefore, it is important to get the medical history carefully. Long term topical corticosteroids may reduce the skin reactivity, H2 receptor antagonist may also affect the test and some study suggest to stop for 24 hour.If the patient for any reason could not stop such medication before the test they may not be able to have the SPT. There is no restrictions for topical moisturisers, however they may cause extracts to run or disperse which creates a practical difficulty.
Someone may think how many allergens could be tested on an individual at the same time? Doctor can make a decision following a clinical history for the numbers of allergens which can be applied in single test. It could be as few as 3 or up to about 25. It is important to note that individuals may have a positive SPT without having allergy symptoms.
SPT can be done for:
1. Food allergy tests – including milk, wheat, nut, gluten, and egg.
2. Pet allergy tests – including cat, horse, and dog dander
3. Asthma and Hay fever tests – including tree pollen,grass pollen and house dust mites etc,
4. Skin allergy tests – for dermatitis and eczema.
Most commonly used allergens extracts in SPT are from inhalant, insect venoms, certain food, and drugs.
In addition to the allergens extract, two controls samples are used to make sure that the test has worked. Positive control (histamine) will cause reaction in all people and the negative control (saline) should not cause a reaction in anyone.
Positive response: A wheal/ bump appear within 15 minutes of the test which may be itchy and red. This indicates the patient is sensitive or allergic to that allergen. Positive test result is considered while the mean wheal diameter is ≥ 3 mm at the prick test, and ≥ 5 mm at the intradermal test. A positive allergy test indicates sensitisation to an allergen but does not necessarily predict that a clinically relevant allergic reaction will occur. That is why general allergy testing in otherwise healthy individuals is discouraged. If a skin test is positive, one will have to distinguish reactions, which are clinically relevant, from those, which are not. History and/or challenge tests help to clarify the relevance of a sensitization. Usually, a clinically irrelevant sensitization does not lead to practical consequences.
Negative response: there is no reaction to the test may indicate that the patient is not sensitive to that allergen. If a patient use antihistamines/ other medications that block the effect of histamine prior to the test that may give a negative result. Skin test results may be negative although patients are allergic. In other words, while the result of SPT do not correlate with the history, in that case doctor may request a blood test to help clarify the results. Skin prick testing in children treated with topical glucocorticoids or tacrolimus is associated with a risk of false-negative test results.
Anaphylactic reactions at skin testing are very rare. If an individual is known to have severe allergic reaction to anything or has suffered with severe asthma, pregnancy and using beta blocker then the test should be performed in a specialised centre where complete resuscitation technique with specialist advice facilities available.