Type of test Skin
Normal findings
< 3 mm wheal diameter
< 10 mm flare diameter
Test explanation and related physiology
When properly performed, skin testing is the most convenient and least expensive test for detecting allergic reactions. 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, immediate wheal (swelling) and flare (redness) reactions follow injection of the specific allergen (that substance to which the person is allergic). This reaction is initiated by IgE 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, which is highlighted by antibody and cellular infiltration into the area. This usually occurs within 1 to 2 hours.
There are three commonly accepted methods of injecting the allergen into the skin. The first method is called the prick- puncture test or scratch 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 der mis (creating a skin wheal). Large local reactions and anaphylaxis have been reported with this latter method. For these two tests, the allergen placement part of the test takes about 5 to 10 minutes. The third method is called the patch test. This takes much longer because the patient must wear the patch for 48 hours to see if there is a delayed allergic reaction. With this method, needles are not used. Instead, an allergen is applied to a patch that is placed on the skin. It is usually done to detect whether a particular substance (e.g., latex, medications, fragrances, preservatives, hair dyes, metals, resins) is causing an allergic skin irritation, such as contact dermatitis.
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 sorts 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 dis ease 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 analog 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, for food allergies, latex allergies, drug sensitivity, and occupational allergies, skin tests are less reliable.
Contraindications
• Patients with a history of prior anaphylaxis Potential complications
• Anaphylaxis Interfering factors
• False-positive results may occur 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 ACE inhibitors, beta-blockers, corticosteroids, nifedipine, and theophylline.
Procedure and patient care
Before
* Explain the procedure to the patient.
• Observe skin testing precautions:
1. Be sure that a physician is immediately available.
2. Evaluate the patient for dermographism.
3. Have medications and equipment available to handle anaphylaxis.
4. Proceed with caution in patients with current allergic symptoms.
5. Render great detail to the injection technique.
6. Avoid bleeding due to injection.
7. Avoid spreading of allergen solutions during the test.
8. Record the skin reaction at the proper time.
• Obtain a history to evaluate the risk of anaphylaxis.
• Identify any immunosuppressive medications the patient may be taking.
• Evaluate the patient for dermographism by rubbing the skin with a pencil eraser and looking for a wheal at the site of irritation.
• Draw up 0.05 mL of 1:1000 aqueous epinephrine into a syringe before testing in the event of an exaggerated allergic reaction.
• A negative prick-puncture test should be performed before an intradermal test. During Prick-puncture method
• 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 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.
Intradermal method
• 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.
Patch method
• Clean the skin area (usually the back or arm).
• Apply the patches to the skin (as many as 20-30 can be applied).
• Instruct the patient to wear the patches for 48 hours. Tell the patient to avoid bathing or activities that cause heavy sweating.
• Tell the patient the patches will be removed at the doctor’s office. Irritated skin at a patch site may indicate an allergy.
After
• Evaluate the patient for an exaggerated allergic response.
• In the event of a systemic reaction, a tourniquet should be placed above the testing site, and epinephrine should be administered subcutaneously.
• With a pen, circle the area of testing and mark the allergen used.
• Read the skin test at the appropriate time.
• Skin tests are read when the reaction is mature, after about 15 to 20 minutes. Both the largest and smallest diameters of the wheal are determined. The measurements are averaged.
• The flare is measured in the same manner.
• Observe the patient for 20 to 30 minutes before discharge.
Abnormal findings
- Allergy-related diseases
- Asthma
- Dermatitis
- Food allergy
- Drug allergy
- Occupational allergy
- Allergic rhinitis
- Angioedema
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