Allergy Skin Testing is the most commonly used and easiest method of identifying patients who suffer from allergies. Furthermore, it is a method by which a specific allergen can be determined. When properly performed, skin testing is considered to be the most convenient and least expensive test for detecting allergic reactions. Since the early 1900s, skin testing has been a common practice for establishing a diagnosis of allergy by reexposure of the individual to a specific allergen. Skin testing provides useful confirmatory evidence when a diagnosis of allergy is suspected on clinical grounds. The simplicity, rapidity, low costs, sensitivity, and specificity explain the crucial position skin testing has in allergy testing.
In an allergic patient, an immediate wheal (swelling) and flare (redness) reaction follows injection of the specific allergen (that substance to which the person is allergic). This reaction is initiated by immunoglobulin E (IgE) antibodies and is mediated primarily by histamine secreted from mast cells. This usually occurs in about 5 minutes and peaks at 30 minutes. In some patients a “late-phase reaction” occurs; this is highlighted by antibody and cellular infiltration into the area that usually occurs within 1 to 2 hours.
There are two commonly accepted methods of injecting the allergen into the skin. The first method is called the prick-puncture test. In this method, the allergen is injected into the epidermis. Life-threatening anaphylaxis reactions have not been reported with this method. The second method is called the intradermal test. Here the allergen is injected into the dermis (creating a skin wheal). Large local reactions and anaphylaxis have been reported with this latter method.
Patients with dermographism (nonallergic response of redness and swelling of the skin at the site of any stimulation) develop a skin wheal with any skin irritation, even if nonallergic. In these patients, a false-positive reaction can occur with skin testing. To eliminate these sort of false positives, a “negative control” substance consisting of just the diluent without an allergen is injected at the same time as the other skin tests are performed. Patients who are immunosuppressed because of concurrent disease or medicines may have a blunted skin reaction even in the face of allergy. This would cause false-negative results. To avoid false negatives, a “positive control” substance consisting of a histamine analogue is also injected into the forearm at the time of skin testing. This will cause a wheal and flare response even in the nonallergic patient, unless the patient is immunosuppressed.
For inhalant allergens, skin tests are extremely accurate. However, they are less reliable for food allergies, latex allergies, drug sensitivity, and occupational allergies. Although there is considerable variability in accuracy of skin testing because of poor injection techniques, when performed correctly, skin testing represents one of the major tools in the diagnosis of allergy.
Allergy Skin Testing can be Life Threatening
Allergy Skin Testing may lead to Anaphylaxis, medications and equipments required to handle anaphylaxis must be available and ready to use in case such a life threatening reaction occurs as a result of performing the test. The test must be performed with caution for patient who already show symptoms of Allergy.
Allergy Skin Testing must not be used if the patient has a history of Anaphylaxis. Instead, Allergy Blood Testing would be the option to use.
Allergy Skin Testing Normal Findings
Less than 3 mm Wheal Diameter and less than 10 mm Flare Diameter.
Causes of Allergy Skin Testing False Results
- False-positive results may occur in patients with Dermographism.
- False-positive results may occur if the patient has a reaction to the diluent used to preserve the extract.
- False-negative results may be caused by poor-quality allergen extracts, diseases that attenuate the immune response, or improper technique.
- Infants and the elderly may have decreased skin reactivity.
- Drugs that may decrease the immune response of skin testing include Angiotensin-converting Enzyme (ACE) Inhibitors, Beta Blockers, Corticosteroids, Nifedipine, and Theophylline.
Performing Allergy Skin Testing
- Applying allergens on the patient to lead to serious allergic reactions that have to be treated immediately. The availability of medical doctor and the ability of instant medical treatment must be assured before starting the test.
- A syringe that contain 0.05 mL of 1:1000 Aqueous Epinephrine must be ready before testing as a precaution for the event of an exaggerated allergic reaction. In the event of a systemic reaction, a tourniquet should be placed above the testing site and epinephrine should be administered subcutaneously.
- A drop of the allergen solution is placed onto the volar surface of the forearm or back.
- A 25-gauge needle is passed through the droplet and inserted into the epidermal space at an angle with the bevel facing up.
- The skin is lifted up and the fluid is allowed to seep in. Excess fluid is wiped off after about a minute.
- A negative prick-puncture test should be performed before an intradermal test.
- With a 25-gauge needle, the allergen solution is injected into the dermis by creating a skin wheal. In this method, the bevel of the needle faces downward. A volume of between 0.01 and 0.05 mL is injected.
- In general, the allergen solution is diluted 100- to 1000-fold before injection.
Indication of Seeing Weals and Flares after performing Allergy Skin Testing
All of the following diseases are immunoreactive (allergic) in their pathophysiology. Specific allergens, when injected or applied to the skin, will cause an allergic reaction of wheal and flare depending on the condition:
- Food Allergy.
- Drug Allergy.
- Occupational Allergy.
- Allergic Rhinitis.