INTRODUCTION — Immunoglobulin E (IgE)-mediated allergies account for the majority of clinically significant environmental, food, and medication allergies. Skin testing is an important element in the diagnosis of IgE-mediated allergy.
This topic review will discuss general principles of skin testing used in the diagnosis of IgE-mediated allergy, including indications, contraindications, factors influencing results, techniques, and accuracy. The two major methods of skin testing used, the prick/puncture technique and the intradermal technique, are described. The use of skin testing to diagnose specific disorders is presented in the appropriate topic reviews.
Other forms of skin testing, such as patch testing for contact dermatitis or atopy patch testing for eosinophilic gastrointestinal disorders, are reviewed separately.
ROLE IN DIAGNOSIS — There are three components to the diagnosis of an immunoglobulin E (IgE)-mediated allergic disorder. These are:
- Identification of the possible culprit allergen(s), usually through a careful clinical history.
- Demonstration of IgE specific to the allergen(s) by skin testing or in vitro testing.
- Determination that exposure to the allergen(s) results in symptoms, either by history or with a challenge procedure.
Allergy testing to demonstrate allergen-specific IgE fulfills the second component and is an important element in the evaluation of allergic disease. However, a positive skin test, by itself, is not sufficient to make a diagnosis. Positive skin tests must be supported by an appropriate clinical history of reactivity and, in some cases, allergen challenge, to confirm that the suspected allergen causes symptoms.
REFERRAL — 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.
OVERVIEW OF SKIN TESTING — Skin testing is the most rapid, sensitive, and cost-effective testing modality for the detection of immunoglobulin E (IgE)-mediated disease. The procedure lasts less than an hour with minimal patient discomfort. There are several published practice parameters for allergen skin testing. The material in this topic is consistent with, although not identical to, these publications.
Advantages over in vitro tests — The primary advantages of skin testing over in vitro testing are:
- The results are available within 15 to 20 minutes (compared with several days for in vitro tests).
- In most settings, skin testing is less costly than in vitro testing.
- For an individual patient, the relative size of positive reactions gives some indication of the patient’s sensitivity to specific allergens, although the sizes of positive reactions vary significantly among individuals.
- Patients can see the reactions and this often helps them understand that they are/are not allergic to a given substance. In addition, watching the swelling and erythema develop in the skin helps patients understand what happens elsewhere in the body in response to allergen exposure.
- Solutions for skin testing can be made from a wide variety of allergenic inhalants, foods, and medications, whereas commercial in vitro tests are available for a more limited number of allergens. Thus, skin testing is often useful for evaluating possible allergies to unusual allergens.
Biologic mechanism — Skin testing is a bioassay that detects the presence of allergen-specific IgE on a patient’s mast cells. A positive reaction implies that mast cells within other target organs (ie, eyes, nose, lungs, and gastrointestinal tract) would also react upon exposure to that allergen.
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 crosslinked 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 preformed 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 localized 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, pruritus, 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 the diagnosis of IgE-mediated allergy.
Sensitization versus clinical allergy — 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 IgE 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, only about 50 percent of positive tests are true positives. Thus, food challenge procedures are often necessary to clarify a patient’s allergic status.
- By comparison, the proportion of individuals with positive pollen skin tests who react to inhalation of that pollen is much higher. Therefore, a positive skin test and a history of symptoms during the appropriate season are sufficient to make the diagnosis.
Thus, the confirmation of reactivity is crucial to making an accurate diagnosis and this is more important for certain types of allergy (eg, food) than for others. The relationship between sensitization and clinical allergy is reviewed in more detail separately.
INDICATIONS — Skin testing is useful in the diagnosis of a variety of allergic disorders that involve immunoglobulin E (IgE)-mediated type I immediate reactions, including:
- Allergic asthma, rhinitis, and conjunctivitis – Skin testing using panels of indoor and outdoor allergens is the primary diagnostic modality for these conditions and is well validated.
- Food allergy – Skin testing is considered the primary diagnostic technique for a variety of food allergies, although in vitro testing is also well validated for several childhood food allergies. In a small number of clinical settings, the results of skin testing may be accurate enough to circumvent the need for food challenges to confirm the diagnosis, although in many cases, challenge procedures are still needed if a definite diagnosis is required.
- Some medication allergies – Skin testing is used in the diagnosis of IgE-mediated drug allergies. However, it has been validated and standardized only for the evaluation of penicillin allergy.
For allergies to other medications, including other antibiotics, biologicals, chemotherapy agents, insulin, and drugs used in anesthesia, skin testing is often used as part of the evaluation, but the significance of a positive or negative result is not as well defined.
- Venom allergies – For allergy to the venoms of Hymenoptera species (wasps, honey bees, yellow jackets, hornet, imported fire ants, and others), skin testing is the diagnostic test of choice and is well validated.
- Latex allergy – Skin testing can be used to evaluate for latex allergy and biologically-standardized extracts of latex allergen for skin prick testing are commercially available in the European Union and Canada, although not in the United States. Some allergists produce their own extracts for testing, although the concentration of allergens in these preparations can vary dramatically and the protein content should be measured and standardized to avoid systemic reactions. The diagnosis of latex allergy is reviewed in detail separately. (See “Latex allergy: Epidemiology, clinical manifestations, and diagnosis”.)
General versus focused testing — Testing may be very focused or more general, based upon the clinical setting:
- A focused approach is most appropriate for patients whose history and/or physical examination strongly indicates the identity of the specific offending allergen(s). As an example, a patient who developed urticaria and wheezing while taking penicillin would be evaluated with tests for IgE specific to penicillin and its immediate derivatives.
- A broader approach is used for patients with perennial rhinitis and asthma symptoms. In this setting, it would be appropriate to test with a selected panel of outdoor and indoor allergens pertinent to the geographical region:
- Tree, grass, and weed pollens, with choices reflecting regional flora.
- Molds, including Alternaria alternata, Penicillium notatum, Aspergillus fumigatus, and Cladosporium.
- Dust mites, including Dermatophagoides pteronyssinus and Dermatophagoides farinae, and cockroach.
- Animal danders, including cat pelt and dog epithelium.
CONTRAINDICATIONS — 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. Rates of systemic reactions to skin testing are discussed below. (See ‘Safety’ below.)
High risk for anaphylaxis — Patients who are at higher risk for an anaphylactic reaction in response to skin testing include:
- People with poorly-controlled asthma and reduced lung function.
- People with clinical histories of severe reactions to minute amounts of allergen.
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.
Relative contraindications to skin testing include significant cardiovascular disease (including active angina and cardiac arrhythmias), frail health in older adults, and pregnancy, because patients with these conditions (or in the case of pregnancy, the fetus) are at greater risk for adverse effects from anaphylaxis and its treatment, were it to occur. However, in such patients, there may be situations in which the benefits of the information gained from skin testing outweigh the risks.
Recent anaphylaxis — 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 nonreactive. Full restoration of reactivity can take two to four weeks. However, if skin testing needs to be performed earlier than one month, positive results are still useful.
In situations in which rapid diagnosis is necessary, in vitro allergen testing, if available for the allergen in question, can be performed instead. Free allergen-specific IgE in the serum is less affected by recent anaphylaxis.
Skin conditions — Patients with certain skin conditions cannot be skin tested because false-positive results are common. These conditions include:
- Acute or chronic urticaria
- Cutaneous mastocytosis
Skin test results are also difficult to interpret in patients with atopic dermatitis affecting the areas where testing is performed, both because of changes in skin cellularity and the application of topical medications. However, patients with atopic dermatitis can be skin tested normally if the tests are applied to unaffected areas.
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 source of allergen and skin test device used.
Medications that should be discontinued — 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.
- H1 antihistamines can suppress skin reactivity for one to seven days, depending upon the specific drug. First-generation nonselective antihistamines (eg, diphenhydramine, clemastine) generally suppress skin reactivity for one to three days, whereas second-generation antihistamines (eg, cetirizine, loratadine) may blunt skin tests for up to seven days. A general approach is to discontinue all antihistamines for one week prior to testing. It is important to recognize that if a patient has not withheld antihistamines for the full recommended time, that interference can still be present even though the histamine reaction is positive. In some cases, residual antihistamines will reduce the histamine control response, but completely negate the response to allergens. Thus, if a weak positive response to the histamine control or responses to allergens are unexpectedly negative, a longer medication-free period and repeat testing are suggested.
- Antihistamine nasal sprays, which can become absorbed systemically, should be withheld for three days.
- One study of antihistamine eye drops found no effect on skin testing. However, because other ophthalmologic medications can be systemically absorbed, we suggest caution and ask our patients to withhold these agents for three days.
- Antihistamines used for nonallergic diseases, such as phenothiazine antiemetics (eg, promethazine, prochlorperazine) or agents given for vertigo/motion sickness or insomnia (eg, meclizine, doxylamine) may block skin reactivity for up to two weeks.
- H2 receptor antagonists (eg, ranitidine, cimetidine) are generally discontinued at least 48 hours before testing, as these medications may reduce the skin reactivity.
- The use of topical glucocorticoids for longer than one week can partially suppress responses to both allergens and histamine in treated areas of skin by locally reducing the number of skin mast cells. In this setting, skin testing should be performed on noninflamed skin that has not been treated with topical agents.
- Treatment with omalizumab, an anti-IgE antibody used for asthma treatment, may depress skin reactivity for up to six months, although skin reactivity in some individuals may return more quickly (eg, in four to eight weeks after treatment has been discontinued).
- Tricyclic antidepressants may block skin reactivity for up to two weeks. Because antidepressants are difficult for patients to discontinue and/or have long half-lives, testing is often done despite them, especially if the dose is relatively low. However, interference must be considered if a weak or negative skin response is obtained. In such patients, assessment of allergen-specific IgE in serum may be preferable. In contrast, selective serotonin reuptake inhibitors (SSRIs), such as citalopram, dapoxetine, escitalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline, do not affect skin testing.
- Limited data concerning topical calcineurin inhibitors are conflicting. No effect on skin testing was reported with pimecrolimus. However, tacrolimus (0.1 percent applied twice daily for two weeks) was found to significantly reduce allergen skin prick test results in a small group of children, without affecting responses to histamine . These agents should be held for one week before skin testing.
- Higher doses of methotrexate may inhibit basophil-histamine release . In vitro allergen-specific IgE testing would be preferable in patients requiring these medications.
Medications used in therapy of allergic diseases that do not impair skin reactivity include:
- Leukotriene receptor antagonists.
- Decongestants, short- and long-acting inhaled beta agonists, theophylline, and cromolyn preparations (oral, nasal, or inhaled). Cromolyn ointment (used in the treatment of cutaneous mastocytosis) would impair skin testing, but this disorder is one of the contraindications to skin testing.
- Inhaled or intranasal glucocorticoids.
- One study of 33 patients showed that oral glucocorticoids (10 to 60 mg daily) administered for two years had minimal effect on the results of skin prick testing. Similar results were found in a second study. Therefore, skin testing may be performed in patients taking systemic glucocorticoids, although a negative or small response to the positive histamine control should raise the possibility of interference.
- Oral theophylline.
Beta blockers and ACE inhibitors — Beta blockers and angiotensin-converting enzyme (ACE) inhibitors do not interfere with the performance of skin testing, but could potentially make it more difficult to treat anaphylaxis if it were to occur. Thus, in vitro assessment of serum antigen-specific IgE may be the safest method of testing for individuals who are thought to be at an increased risk for a systemic reaction as a result of testing and cannot discontinue these medications.
In patients who are not at an increased risk for a systemic reaction, the decision to discontinue these medications prior to skin testing depends upon the patient’s dose and the condition for which they are receiving the beta blocker or ACE inhibitor. Such decisions should be made on an individual patient basis. The impact of beta blockers and ACE inhibitors on the treatment of anaphylaxis is discussed elsewhere.
Physiologic variables — Several physiologic characteristics of the patient should be considered, although the effects of these factors are usually minor.
- The age of the patient can affect the results of skin testing. Skin testing can be performed in people of any age, although there are differences in responsiveness with age. Infants have smaller positive reactions to histamine and allergens, although clear positives can be obtained in very young children. Skin reactivity increases gradually throughout childhood and plateaus in the mid-adolescent years. A gradual decline in reactivity is often seen after the age of 50 to 60 years, although skin testing is still accurate. Older adults may also respond less vigorously to histamine controls. In younger children, a significant discordance between allergen-specific skin prick test and serum allergen-specific IgE has been reported.
- The skin of the forearm generally produces smaller wheals than the skin of the back. This effect is more pronounced with allergens than with histamine, although it is rarely of significant clinical consequence. Performing testing on the arms has the advantage of allowing for the placement of a tourniquet proximal to the testing site (to slow absorption of allergen into the central vasculature) in rare cases of anaphylaxis in response to testing. Moreover, it allows patients to watch the response to individual allergens.
- The size of skin test reactions to some pollen antigens in sensitive patients may increase during the pollen season, although positive results are present throughout the year.
Technical variables — The results of skin testing can be influenced by the potency and quality of allergenic extracts, testing devices, and techniques.
Quality of the extract — The potency and quality of the allergen extract used for testing can influence the results of skin testing. In most cases, standardized extracts are preferred when available. The major inhalant allergens (eg, grasses, dust mites, ragweed) are well characterized and some are standardized in potency units (eg, grasses, dust mites, ragweed).
In contrast, most commercially-available food extracts are not standardized. However, extracts of milk, eggs, peanuts, soy, fish, shellfish, and tree nuts are reliable. In contrast, commercial extracts of fruits and vegetables are sometimes inadequate because the responsible allergen may be labile and altered during processing. Thus, in the event of a negative skin test for these foods, or when there is suspicion that the allergenicity of the food depends on its preparation (raw versus cooked), the fresh food should be used for skin testing using the “prick-by-prick” method.
Conversely, false-positive results can arise from naturally-occurring histamine and other vasoactive amines in some allergen extracts (i.e., 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 nonstandardized 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.
Skin test device — The testing device used can influence the results and the practitioner should consider the strengths and weaknesses of different devices when deciding which method to use. A review of five testing devices widely used in Europe concluded that metal lancets and intravenous needles were more sensitive than the plastic lancet tested. Careful consideration of the device used is of particular importance in research settings. As an example, multiheaded devices (those that allow for application of several antigens simultaneously) produce more false negatives than single-headed devices, and the inner heads may produce weaker responses than the heads on the corners of the devices. However, multiheaded devices can be very useful when testing children by allowing rapid application of multiple antigens. All staff who perform skin testing should be assessed for reproducibility of their skin test results using a given device and histamine-positive and diluent-negative controls.
ALLERGEN PREPARATION — Allergens are characterized by the ability to trigger the formation of immunoglobulin E (IgE) in genetically-susceptible individuals. The allergens relevant to human-allergic disease are usually 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. 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 commercially-prepared 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, they may not be appropriate in all settings. Recombinant allergens are used for both testing and treatment in Europe, although they are still limited to research applications in the United States.
SKIN TESTING METHODS — The two major methods of allergen skin testing in use are the prick/puncture technique and the intradermal technique. In most cases, the prick/puncture method is the appropriate initial procedure.
Prick/puncture method — The prick/puncture method is the primary diagnostic method and is the most appropriate initial test, in the absence of contraindications.
Technique — The prick/puncture or 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. Standardized allergenic extracts labeled by potency units, such as bioequivalent allergy units (BAU) should be used at the recommended strength per package insert. 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. Tests should be placed at least 2 centimeters apart to avoid overlapping reactions.
The earlier “scratch” method of skin testing, in which allergen was placed on the skin and then a lancet or needle was used to make a linear excoriation in the skin, is rarely used, as the technique is more traumatic to the skin, variable in performance, and carries a higher risk of systemic reaction.
Controls — 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:
- It is defined most commonly as a wheal that is equal or larger in size to that associated with the histamine control, with the histamine control normally producing a wheal of at least three millimeters in diameter.
- Alternatively, a wheal diameter larger than 3 millimeters may be considered positive.
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, with a result of 3 mm or greater being considered positive.
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.
Prick-by-prick testing with fresh food — 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 if 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 nonallergic individual, to assure that the result is not a false-positive irritant response.
Accuracy and interpretation — Prick/puncture testing is sensitive although not very specific. Overall positive predictive accuracy is less than 50 percent if all types of allergens are considered together. However, the exact sensitivity and specificity of skin prick testing are highly dependent on the allergens used as well as the different variables inherent in this bioassay. The accuracy of prick testing with different types of allergens is discussed below.
Positive results — 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. A positive skin test in the absence of a relevant clinical history may be indicative of subclinical sensitization (ie, may be a true positive) that may eventually become clinically apparent, or it may be a false-positive reaction. It may also occur in patients who, due to their lifestyles, are less exposed to allergens that they are sensitized to.
When standardised 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.
Inhalant extracts are highly accurate in the diagnosis of allergic rhinitis. A large body of literature has examined this issue. The following are a few representative examples:
- The performance of skin prick testing for identifying respiratory allergy was studied using 10 pollen allergens, 2 house dust mite allergens, and 1 cat allergen. Testing was performed in 50 patients with asthma or allergic rhinitis (group 1), 50 without these conditions but with a positive immediate family history of the disorders (group 2), and 100 subjects without a personal or family history of asthma or allergic rhinitis (group 3). At least one positive test reaction (as defined by a mean wheal diameter of greater than or equal to 3 mm) was observed in 90, 46, and 29 percent of those in groups 1, 2, and 3, respectively, with accuracy based in part on the allergen. As an example, a positive reaction to the cat allergen was associated with a sensitivity, specificity, and diagnostic accuracy of 90 percent, when compared with the clinical history.
- In one study of 365 consecutive patients aged 12 years or older, the predictive value of history alone for seasonal allergic rhinitis was found to be 82 to 85 percent, and that of history in combination with prick/puncture skin testing, 97 to 99 percent.
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.
Negative results — 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 discussed previously, the negative-predictive accuracy of food testing using commercial extracts is lower.
Intradermal method — Intradermal tests are more reproducible than prick/puncture skin tests and are approximately 100- to 1000-fold more sensitive. However, false-positive reactions are more common and this type of testing carries a higher risk of inducing a systemic allergic reaction.
Technique — Intradermal (or intracutaneous) 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 or 27 gauge needle, positioned at a 45 degree angle, is used to make a 2 to 3 mm “bleb” of extract intradermally. The technique is identical to that used to place the intradermal tuberculin skin test.
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 so much more sensitive than prick/puncture tests, the starting test dose of intradermal extract should range between 100- and 1000-fold more dilute than the prick/puncture test solution. Serial dilutions of progressively less dilute material may be required for the optimization of sensitivity for skin testing with insect venoms or drugs and to assess for potentially life-threatening anaphylactic sensitization.
Definition of a positive test — A positive-intradermal result is a wheal of 5 mm or larger in most instances. However, the 2008 practice parameter of the leading American allergy society states that “any reaction larger than the negative control may indicate the presence of specific IgE antibody. Given the greater sensitivity and equivocal reproducibility of intradermal testing, however, small positive reactions may not be clinically significant”. Evidence-based studies on standardized intracutaneous test grading are lacking. A survey of American board-certified allergists reported that 85 percent use the criterion of 3 mm greater than the negative control as a threshold for a positive-intradermal test result.
End-point dilution technique — The technique of end-point dilution is a variant of intradermal testing that is used to identify the concentration of an extract that produces a reaction of a defined size. The more sensitive an individual is to a given extract, the lower the concentration required to produce a reaction of a given size. End-point dilution is performed by injecting a series of dilutions of an extract into a patient’s skin, until a specific diameter wheal or flare is obtained. Uses include:
- Research studies of immunotherapy efficacy, because it provides a more precise measure of a patient’s sensitivity.
- Bioassays in commercial labs to standardize the strength of extracts.
- Evaluation of patients in whom nasal or bronchial challenges must be performed, as it can be used to assess the initial concentration for challenge.
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 with a single dilution.
Accuracy — Intradermal tests are more reproducible than prick/puncture skin tests and are more sensitive. However, false-positive reactions are more common.
- For inhalant allergens, intradermal testing has a higher rate of false positives compared with prick/puncture testing. A positive intradermal test, in the context of negative skin prick test, has a low-positive predictive value for the presence of symptoms upon allergen exposure. False positives may be related to naturally-occurring histamine, endotoxins, and other skin irritants in inhalant extracts. Intracutaneous bleeding can be falsely interpreted as a positive reaction. For some allergens, such as cat and grass allergens, intradermal testing has been shown to add little to diagnostic accuracy. The utility of intradermal testing for other inhalant allergens has not been analyzed to this extent.
- In contrast, for venom skin testing, the high sensitivity of intradermal tests is important for the optimal detection of venom sensitivity, as failure to identify a venom allergy may lead to life-threatening consequences.
- Intradermal testing is not used in the diagnosis of food or latex allergy in the United States, due to an unacceptably high rate of systemic reactions to testing.
SAFETY — Allergy skin testing is considered a safe procedure, although it occasionally causes systemic reactions in very sensitive patients. For this reason, it is recommended that full emergency equipment and medications, including epinephrine, be present whenever skin testing is performed.
Intradermal testing and high-risk allergens — Historically, higher rates of systemic reactions were reported following intradermal injection of food allergens and latex. In response to these data, intradermal testing with latex and foods is no longer recommended. In addition, prick/puncture testing is always recommended prior to intradermal testing for aeroallergens.
Fatal anaphylaxis as a result of allergen skin testing is exceedingly rare and nearly always associated with intradermal testing without prior prick/puncture testing. For this reason, intradermal testing should only be performed after negative prick/puncture testing.
Overall incidence of systemic reactions — The overall rate of systemic reactions to skin testing in a prospective study of nearly 1500 patients was 3.6 percent, although none were severe. The large majority of systemic reactions in this study were triggered by intradermal testing to aeroallergens.
Anaphylaxis with prick/puncture testing — In a prospective pediatric study of almost 6000 children, the rate of systemic reactions to skin prick testing only (no intradermal testing) was 0.001 percent. Risk factors for systemic reactions in children included age <1 year and active eczema. Two cases of anaphylaxis were reported to skin prick testing with commercial fish extracts in children, both of whom had a history of asthma. One also had a history of atopic dermatitis. Skin prick testing using fresh foods has also caused rare anaphylactic reactions in children.
A single report described a case of delayed-onset anaphylaxis in an adult with active asthma, in which the reaction developed two hours after prick testing to inhalant allergens.
The only known case of fatal anaphylaxis associated with skin prick testing occurred in an adult patient with food allergy and poorly-controlled asthma, who was skin tested with 90 food allergens (an excessive number) during a single testing session.
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- The three components in the diagnosis of an immunoglobulin E (IgE)-mediated allergy are identification of the allergen, demonstration of IgE specific to that allergen, and confirmation that symptoms occur when the patient is exposed to the allergen. Skin testing is a diagnostic tool used in the second component.
- A positive skin test, in isolation, is not sufficient to make a diagnosis of allergy, as it must also be demonstrated that the patient develops symptoms on exposure to that allergen.
- Skin testing is a bioassay that detects the presence of allergen-specific IgE on the surface of a patient’s cutaneous mast cells. Allergen is pricked into the skin and if allergen-specific IgE is present on the patient’s mast cells, the cells are activated and produce localized pruritus, swelling, and erythema. Skin testing should be performed by an allergy expert, as there is a small, but definite risk of inducing an allergic reaction.
- The allergic diseases for which skin testing is useful in diagnosis include allergic asthma, rhinitis and conjunctivitis, food allergy, some medication allergies, some venom allergies, and depending on the availability of testing reagents, latex allergy.
- Skin testing is contraindicated 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. Such patients should undergo in vitro allergy testing instead.
- The results of skin testing are influenced by medications, physiologic characteristics of the patient, the device used, and the allergen extract used. Antihistamines must be discontinued prior to testing.
- The allergens relevant to human disease are mostly proteins from other living organisms that are able to trigger the formation of IgE in genetically-susceptible individuals. Allergens for testing are obtained from natural sources and are heterogeneous. Efforts are underway to standardize their production around the world.
- There are two commonly used methods of skin testing for IgE-mediated disorders: prick/puncture and intradermal. Prick/puncture is performed first and is sensitive but not very specific. Intradermal testing is more sensitive, although false positives are common and it carries a higher risk of inducing an allergic reaction.
- Prick/puncture skin testing has low specificity if all allergens are considered together. However, sensitivity and specificity are both excellent with standardized, high potency extracts, such as pollen extracts. In addition, the negative-predictive value of skin prick testing with certain allergens (eg, inhalants and foods) is very high and skin testing can exclude allergy with relative certainty.
- Intradermal tests are usually performed following negative prick/puncture tests, and are approximately 100- to 1000-fold more sensitive. Intradermal testing is not performed in the diagnosis of food or latex allergy, due to an unacceptably high rate of both false positives and systemic reactions to testing. In contrast, intradermal testing is important in the diagnosis of drug and insect venom allergies.