Written by Rebecca Woolley, BSc Hons, reviewed by Sophie Mullany, BSc
In the event of an allergic reaction, a range of tests are available to determine which substance is the primary cause. Various combinations of skin, blood or challenge tests may be used, depending on symptoms and the patient’s medical history.
Skin tests are often the first choice due to their convenience and speed. They are used to screen potential allergens (allergy-causing substances) by applying them to the skin and observing whether they trigger a reaction.
Skin Prick Test
A skin prick test is normally used to detect allergies that cause an immediate reaction on contact with the skin, such as hay fever or food allergies. Drops of a solution containing an allergen (such as animal hair or pollen) are applied to different marked areas of the skin. The skin is then pricked at each site to allow the allergen to enter the skin. For food allergies, the lancet is dipped in the food before pricking the skin.
Doctor doing allergy tests. Image Credit: Monika Wisniewska / Shutterstock
An allergic reaction is indicated by skin reddening or a small bump (wheal) at the marked site.
An intradermal test is similar to the skin prick test, except that the allergen solution is injected directly into the skin. The primary advantage of an intradermal injection is that it can detect weaker allergic reactions.
The disadvantages, however, are the unpleasantness of the injection and the stronger allergic reactions that it can trigger. Therefore, intradermal tests are usually only performed when the response to a skin prick test is insufficient.
Skin Scratch/Scrape Tests
A skin scratch test is generally used if the skin prick test is inconclusive. A small section of skin is removed so that the allergen can be rubbed over the exposed flesh. Substances are able to penetrate deeper layers of tissue than with the skin prick test and, therefore, trigger stronger reactions. The skin scrape test is similar but removes only the top layer of skin.
These tests are less reliable than the skin prick test as they cannot control the amount of allergen applied and are more likely to trigger non-allergic skin irritation.
To reduce the risk of strong reactions, the substance can be rubbed on without breaking the skin. Reactions take longer to detect than the skin prick test, evidenced as red, bumpy skin after about 20 minutes if allergy occurs.
The patch test is used to screen substances if allergic symptoms are expected to be observed between one and three days after exposure to the allergen.
Patches containing different allergens are applied to a person’s back and left in place for 48 hours. The patches are then removed, and the skin examined for signs of a hypersensitive reaction. A second assessment is performed after 96 hours, and sometimes additionally after 7 to 10 days in some people.
Triggers are often single substances, found in a variety of materials, such as medications (e.g. lidocaine), cosmetics (preservatives or fragrances), jewelry (nickel or cobalt) or gloves (latex). If a contact allergy occurs, the skin swells, turns red, itches and small blisters may form.
Skin Allergy Patch Test on Back of Patient Showing Redness and Swelling. Image Credit: Andy Lidstone / Shutterstock
Advantages of skin tests:
- Fast and cheap
- Sensitive at detecting low-level allergies
Disadvantages of skin tests:
- Some medications affect how skin reacts to allergens and need to be discontinued prior to testing, e.g. antibiotics (amoxicillin or penicillin), or painkillers (aspirin or ibuprofen)
- Interpretation of skin reactions requires a good deal of experience and training, therefore the skill of the tester can affect the results
- Mild discomfort from the reactions generated, e.g. itch
Blood tests may be conducted in addition to, or instead of, skin tests. A skin prick test may not suitable when:
- There is high risk of triggering a known severe allergy
- The patient is taking a medicine that can interfere with skin testing
- The patient has signs of a severe skin condition, such as eczema or psoriasis
A blood test may also be used to confirm results if skin tests are not clear. There are a variety of blood tests available, including: enzyme-linked immunosorbent assay (ELISA), in-vitro basophil histamine release assay and radioallergosorbent test.
Blood is drawn from a vein in the arm to provide a blood sample. An allergen is added to the sample and the number of immunoglobulin E antibodies produced in response to the allergen are measured. Although allergies are likely to increase the number of circulating IgE antibodies, this is only an indication.
High numbers of IgE antibodies can also result from parasite infections and smoking, therefore it is difficult to form definitive conclusions based on blood tests alone. In addition, the number of antibodies cannot be correlated with symptom severity.
Advantages of blood tests:
- Requires only one, simple needle prick
- Medication does not affect results
Disadvantages of blood tests:
- Longer time taken to obtain results
- Less sensitive and more expensive
Challenge tests are used to identify a trigger when a person’s allergy does not produce a strong skin reaction. The body is exposed to the allergen in its natural form, via a typical route, and then monitored for signs of a reaction.
For example, to determine the cause of hay fever, a specific type of pollen can be applied to the nasal mucosa with the patient subsequently monitored for sneezing, a stuffy nose and watery eyes. Food allergies can be tested by feeding the patient the suspected trigger. Allergens can also be used to challenge the lungs or eyes in a similar way.
Patients should not have any allergic symptoms before a challenge test and medical supervision is necessary throughout as they can stimulate serious allergic reactions.
Steps to Identifying an Allergy Trigger
In general, to identify an allergy trigger, a skin test is the first step. If this is inconclusive, too risky, or the allergen cannot be made into an appropriate solution, a blood test is performed.
A challenge test is the third option if the former two are not possible, or results are inadequate. Whichever test is used, the results must be interpreted along with the patient’s medical history in order to provide a formal diagnosis of the allergy.