overview of skin...
OVERVIEW OF SKIN TESTING FOR ALLERGIC DISEASE
© H. Nolte, K. Kowal, L. DuBuske, 2007
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The incidence of atopic disease has increased 10-fold in developed countries in the last two decades. Almost 30 percent of Americans have symptoms of upper respiratory allergy, including up to 40 percent of children and 10 to 30 percent of adults. Inflammatory processes involving IgE (IgE-mediated allergies) account for the majority of clinically significant environmental, food, and medication allergies.
There are three components to the diagnosis of an IgE-mediated allergic disorder. These are:
Thus, allergy testing is an important element in the evaluation of allergic disease; however, by itself, it is not sufficient to make a diagnosis. A positive test (skin or in vitro) must be supported by an appropriate clinical history of reactivity and, in some cases, allergen challenge to confirm that the suspected allergen causes symptoms.
This topic review will discuss general principles of skin testing, including indications, contraindications, factors influencing results, techniques, and accuracy. The two major methods of skin testing currently used, the prick/puncture technique and the intradermal technique, are described. This review includes the types of skin testing used in the diagnosis of IgE-mediated allergy. Other forms of skin testing, such as patch testing for contact dermatitis or atopy patch testing for eosinophilic gastrointestinal disorders, are reviewed separately.
OVERVIEW OF SKIN TESTING
Skin testing is the most rapid, sensitive, and cost effective testing modality for the detection of IgE-mediated disease. Results are obtained in less than an hour with minimal patient discomfort. However, there are some situations in which in vitro testing is preferable. (See "Contraindications" below).
Skin testing and other diagnostic procedures in the field of allergy (eg, challenges) should be performed by specialists trained in the methods and interpretation of the test results. Any procedure that involves the deliberate exposure of a patient to a substance that may cause significant harm should be undertaken by someone with expertise in the emergency treatment of allergic reactions and anaphylaxis. In addition, because each type of allergy has unique characteristics and implications for the patient's health, an allergy expert can be an invaluable source of information for both the patient and the primary provider.
General approach to testing
Testing may be very specific or more general, based upon the clinical setting:
A person with demonstrable IgE to a specific allergen is said to be "sensitized" to that allergen. Both skin testing and in vitro testing are used to demonstrate sensitization. However, a sensitized person is considered "allergic" to an allergen only if they react with representative symptoms when exposed to it.
This distinction is made because not all sensitized patients will develop actual symptoms on exposure. With some food skin tests, for example, the percentage of people who react to a food for which they have skin tested positive can be as low as 50 percent. Thus, food challenge procedures are often necessary to clarify a patient's status. By comparison, the proportion of individuals with positive pollen skin tests who truly react is much higher; therefore, a positive skin test and a history of symptoms during the appropriate season is sufficient to make the diagnosis. As a result, the confirmation of reactivity is crucial to making an accurate diagnosis and each type of allergy must be approached individually.
Skin testing is a bioassay that detects the presence of allergen-specific IgE on a patient's mast cells. When allergen is introduced into the skin of a patient during skin testing, it comes into contact with cutaneous mast cells. Binding of the allergen occurs if the patient's mast cells are coated with IgE recognizing that specific allergen. If both IgE and allergen are present in sufficient quantities, then adjacent IgE molecules directed against the allergen may be cross-linked on the cell surface and initiate intracellular signaling.
These events lead to mast cell activation, release of the contents of intracellular granules (degranulation), and the de novo generation of inflammatory mediators. Degranulation releases pre-formed vasoactive mediators and enzymes, such as histamine, tryptase, chymase, and carboxypeptidase. Histamine is the major mediator of the wheal and flare response, but other mediators (eg, prostaglandin D2) are also involved, as the size of the wheal does not correlate directly with the concentrations of histamine released.
The clinical result of these cellular events is a positive skin test, or a transient "wheal-and-flare" reaction. This reaction consists of a central area of superficial skin edema (wheal) surrounded by erythema (flare). This pruritic reaction represents the immediate phase of the allergic reaction.
Late phase reactions (LPRs) may develop at skin test sites in some individuals. These consist of deep tissue swelling, warmth, pruritis, and erythema beginning one to two hours after testing and resolving in 24 to 48 hours. LPRs are mast cell-mediated and IgE-dependent, although they do not predict symptoms on exposure and are not used in allergy diagnosis.
Skin testing is useful in the diagnosis of a variety of IgE-mediated allergic disorders, including:
Skin testing should not routinely be performed in patients who are at high risk for an anaphylactic reaction to testing, have experienced a recent anaphylactic event, are taking medications that may interfere with the treatment of anaphylaxis, or have certain skin conditions.
High risk patients
Patients who are at higher risk for an anaphylactic reaction in response to skin testing include:
Patients with significant cardiovascular disease, including coronary artery disease and cardiac arrhythmias, and elderly patients may also present an enhanced risk potential for skin testing related to the adverse effects inherent in treating anaphylaxis in such patients.
In vitro allergy testing should be the initial diagnostic modality in such patients, or in the case of uncontrolled asthma, skin testing can be performed after symptoms have been stabilized.
An episode of anaphylaxis within the previous month is a contraindication to skin testing because it may yield falsely negative results. Anaphylaxis can render the skin temporarily non-reactive. Full restoration of reactivity can take two to four weeks. This is especially important in assessing patients who have had anaphylaxis from insect stings. In vitro allergen testing may be more reliable during this refractory period if immediate diagnosis is necessary, because free allergen-specific IgE in the serum is less affected by anaphylaxis.
Presence of interfering medications
Beta 2-antagonists and angiotensin converting enzyme antagonists may inhibit the management of anaphylaxis. Thus, in vitro assessment of serum antigen-specific IgE may be the safest method of testing for individuals who cannot discontinue these medications.
Patients with certain skin conditions, including dermatographism, urticaria, and cutaneous mastocytosis, cannot be skin tested because false positive results are common. Skin test results are also difficult to interpret in patients with atopic dermatitis affectin the areas where testing is performed, both because of changes in skin cellularity and the application of topical medication. However, patients with atopic dermatitis in other areas can be skin tested normally.
FACTORS AFFECTING RESULTS
The results of skin testing are influenced by various factors, including medications and physiologic characteristics of the patient, as well as the reagents and devices used.
The patient's full medication list should be reviewed in advance of testing. Several types of medications can interfere with skin testing, although only some are routinely discontinued. Non-interfering allergy medications that can be continued are also discussed below.
Several physiologic characteristics of the patient should be considered, although the effects of these factors are usually minor.
The results of skin testing can be influenced by the potency and quality of allergenic extracts, testing devices, and techniques.
Conversely, false positive results can arise from naturally occuring histamine and other vasoactive amines in some allergen extracts (ie, insect venoms, molds, foods) as extracts produced in the United States are not dialyzed to remove histamine, which may occur as a natural contaminant, or from cross-reactivity with clinically irrelevant proteins. An example of the latter phenomenon can occur in foods containing the profilin protein Bet v 2, which can cause positive skin tests to foods in birch allergic patients without symptoms of food allergy. In the testing of non-standardized drug preparations, false positives can occur from irritating properties of the drug in question. If these issues are suspected, repeating the test in one or more volunteers to assure that the test is negative in others is sometimes required.
The allergens relevant to human allergic disease are either glycoproteins and lipoproteins from other living organisms, or in the case of some drug and occupational allergens, a combination of small chemical moieties conjugated (or "haptenated") to serum proteins. Allergens have in common the ability to trigger the formation of IgE in genetically susceptible individuals. Hundreds of clinically significant allergens have been identified from tree, grass, and weed pollens, molds, dust mites, foods, parasites, animal danders, insect venoms, drugs, occupational chemicals, and other biologic materials.
At this time, the allergens used in clinical practice are obtained almost exclusively from natural sources. In some cases, the more important allergenic proteins from a given substance have been identified, cloned, sequenced, and produced through recombinant technology. However, recombinant allergens do not necessarily reproduce the clinical response observed to natural allergens. Allergens from natural sources are heterogeneous. Individual allergic patients can be sensitive to different allergens from the same source, and can even respond differently to isoallergens that are identical except for minor differences in primary amino acid composition or substituted side chains. Thus, although recombinant allergens offer the advantages of being readily available and uniform in composition, their use is currently limited to research settings.
The reliance on allergens derived from natural sources introduces immense variability into the commercial preparation of these products. Allergen content in plant, animal, and insect materials can vary because of the season, growth conditions, genetic differences between geographic regions, or other variables. For this reason, commercial allergen extracts are not uniform among manufacturers or between lots. Each preparation will detect a slightly different population of IgE antibodies.
Efforts have been made to "standardize" the commonly-used extracts. In standardized extracts, the important allergenic proteins have been identified and the manufacturer has verified their content to be within a specified range. Based upon clinical testing, the standardized extracts are labeled in biological potency units, termed allergy units (AU), biological allergy units (BAU), or in micrograms of major allergen content (for venoms). Currently, standardized versions of some pollen, dust mite, animal dander, and venom extracts are available, and their use is preferred over non-standardized extracts. Unstandardized products are labeled in protein nitrogen units (PNU) or weight of protein per volume.
The manufacturer and concentration of the skin test reagents used in testing must always be recorded in the patient record. Withouth this information, precise interpretation of the results by other providers is not possible.
CHOICE OF SKIN TEST METHOD
The two major methods of skin testing currently in use are the prick/puncture technique are the intradermal technique. In most cases, prick/puncture method is the appropriate initial procedure.
The prick/puncture of epicutaneous method of skin testing involves application of droplets of 1:10 or 1:20 weight/volume allergen extract solutions on the volar surface of the forearm or upper back, after the skin has been cleaned with a 70 percent alcohol solution. Each droplet contains a single allergen extract, although for screening purposes, mixes of closely related allergens (eg, four tree pollens) are sometimes used. One of a number of commercially available test devices are then used to prick through the droplets of allergen. Test should be placed at least two centimeters apart to avoid overlapping reactions.
Prick by prick testing
Prick-by-prick testing is a variation of epicutaneous skin testing in which fresh food is used as a source of the allergen, instead of a commercially-prepared extract. This is often done to evaluate allergies to fruits, many of which occur only in the fruit is eaten in the raw form. To perform this, the food and the patient's skin are both cleaned, and the test device is used to prick first the food and then the patient's skin. If a positive test is obtained using this method, the test should be repeated on a non-allergic individual, to assure that the result is not a false positive irritant response.
Both a positive control of histamine dichloride (10 mg/mL for epicutaneous use) and a negative control of diluent identical to that of the allergen extracts (usually glycerinated saline) should always be applied in order to verify that the patient's skin is normally responsive.
Definition of a positive test
A positive reaction appears as a raised wheal with surrounding erythema. A positive reaction is defined in one of two ways:
Positive results are recorded by measuring the greatest diameter of both the wheal and the erythema (separate measurements for each) in millimeters, at 10 minutes for the histamine control, and at 15 to 20 minutes for the allergen extracts. For research purposes, a more precise measurement is usually used, in which the longest wheal diameter (D) and the diameter perpendicular to D (d) are measured. The result is then expressed as a mean (D + d) / 2.
Skin test reactions usually begin to resolve within 30 minutes. A permanent visual record of the results can be made by tracing the reactions with a fine-tipped marker and transferring them to clear tape, which is then placed on paper.
Although allergy skin testing is considered a safe procedure, it is not without risk. It is recommended that full emergency equipment and medications, (including epinephrine) be present for treatment of the rare, yet potentially life-threatening anaphylactic event.
Generalized reactions can occur with prick/puncture skin testing. The systemic reaction rate for prick/puncture tests has been estimated at 33 reactions per 100,000 tests. One study examined systemic reactions to prick testing in infants and found a rate of 0,5 percent, all of which occured when fresh food was used.
A survey of fatal reactions related to skin testing and immunotherapy performed during 1990 to 2001 was returned by 25 percent (646) of members of the American Academy of Allergy, Asthma, and Immunology. There was one fatality due to skin prick testing, which was noted in a woman with allergic rhinitis, food allergy, and moderate persistent asthma that was not optimally controlled. This is the first reported fatality due to prick/puncture testing.
The exact sensitivity and specificity of skin prick testing are dependent on the allergens used as well as the different variables inherent in this bioassay. (See "Factors affecting results" above). For skin testing in general, the overall sensitivity is low, and often estimated at approximately 50 percent. Thus, a positive skin test only suggests the possibility of allergy. However, the negative predictive value of skin prick testing is very high and skin testing can exclude allergy with relative certainty. These characteristics are explained in more detail below.
A positive skin test to a particular allergen, taken alone, only indicates the presence of IgE specific to that allergen. It still must be demonstrated that the patient develops symptoms upon exposure to the allergen.
When standardized inhalant extracts with high potency are used, prick/puncture tests generally have high sensitivity and specificity (>85 percent). The diagnostic efficacy is lower for molds and certain foods (See "Allergen Preparation" below for an explanation of standardization of extracts).
A positive skin test in the absence of a relevant clinical history may be indicative of sub-clinical sensitization (ie, may be a true positive) that may eventually become clinically apparent, or it may be a false positive reaction.
Compared with other allergens, the performance of inhalant extracts in the diagnosis of allergic rhinitis is superior. A large body of literature has examined this issue. The following are a few representative examples:
Skin testing for foods is much less reliable, and positive reactions that do not correlate to clinical reactivity are more common. This is discussed in detail separately.
The negative predictive accuracy of prick/puncture skin testing is high. A negative skin test confirms the absence of an IgE-mediated reaction with greater than 95 percent accuracy. As previously described, the negative predictive accuracy of food testing using commercial extracts is lower.
Intradermal skin testing is performed by injection of 0,02 to 0,05 mL of a 1:500 to 1:1000 weight/volume allergen extract into the skin. A 26 of 27 gauge needle, positioned at a 45 degree angle, is used to make a two to three mm "bleb" of extract intradermally. The technique is identical to that used to place the intradermal PPD test for tuberculosis.
An intradermal negative control is included, in order to control for reactions in response to the injection method. A positive histamine control is not needed if reactivity to histamine has already been demonstrated by prick method; however, if required, histamine can be injected intradermally at a dilute concentration of 0,001 mg/mL.
Intradermal tests are usually performed following negative prick/puncture tests. Because they are approximately 100- to 1,000-fold more sensitive, the starting test dose of intradermal extract solutions in patients with a preceding negative prick/puncture test should range between 100 and 1,000-fold more dilute than the prick/puncture test solution.
This technique is commonly used among otolaryngology practitioners in the diagnosis of allergy, however, use among allergy specialists is rare, having been largely replaced by prick/puncture followed by standard intradermal testing. The technique may be useful for evaulation of patients in whom nasal or bronchial challenges must be performed, as it can be used to assess the initial concentration for challenge. It is also used for research purposes, especially in studies of immunotherapy efficacy, because it provides a more precise measure of a patient's sensitivity. It is used in commercial laboratories for standardizing the strength of extracts.
Intradermal testing is commonly performed with environmental allergens and drugs. It is not generally done with food allergens or latex, due to an unacceptably high rate of false positive results and systemic reactions with these allergens.
Systematic reactions can occur at a low rate (less than 0,5 percent of patients). Fatalities have been reported that were primarily caused by allergens no longer in common use. In addition, in one survey, five of six fatalities associated with skin testing were observed in patients who underwent intradermal testing without prior prick/puncture testing.
Intradermal tests are more reproducible than prick/puncture skin tests and are more sensitive. However, false-positive reactions are more common, and for some allergens, have been demonstrated to add little to diagnostic accuracy.
False positives may be related to naturally-occurring histamine, endotoxins, and other skin irritants in some mold, venom, and inhalant extracts. Intracutaneous bleeding can be falsely interpreted as a positive reaction.