The skin prick test – European standards

SPT is indicated if a type I (immediate type) allergy is suspected, based on the medical history and clinical symptoms; they can identify sensitivity to inhalant, food, drug or occupational allergens. SPTs thus provide objective confirmation of sensitivity, whereas the relevance of such sensitivity to allergens should always be carefully interpreted in the light of the clinical history so that appropriate advice concerning avoidance measures can be given and, as necessary, the correct allergen(s) prescribed for specific immunotherapy (SIT). SPT results correlate with those of nasal challenge which may also be used as a surrogate to test clinically relevant sensitization [5].

Another indication of SPT is to screen for a predisposition to develop atopic diseases, which can be done with a limited number of allergens, or to identify all sensitized subjects in a given population. SPT also can be used in epidemiologic studies to determine trends in sensitization rates or regional differences and to help standardize allergen extracts.

SPT is used to test adults and children from birth onwards. Repeated testing may be necessary in order to detect new sensitizations, especially in children, when symptoms change, or if new environmental allergens are suspected.

SPT interpretation utilizes the presence and degree of cutaneous reactivity as a surrogate marker for sensitization within target organs, i.e., eyes, nose, lung, gut and skin. When relevant allergens are introduced into the skin, specific IgE bound to the surface receptors on mast cells are cross-linked, mast cells degranulate, and histamine and other mediators are released. This produces a wheal and flare response which can be quantitated. Many different allergens can be tested simultaneously because the resultant reaction to a specific allergen is localized to the immediate area of the SPT.

The chief advantage of SPT as compared to an in vitro measurement of specific IgE antibodies is that the test can be interpreted within 15 to 20 minutes after the reagent is applied to the skin. Moreover, the test gives a visual indication of the sensitivity which can be used in order to impact the patient’s behavior. SPT can also be utilized to test less common allergens, such as certain medications, and fresh fruits and vegetables where no specific IgE antibody measurements are available.

The skin scratch test, first described by Blackley in 1873 [6], is not recommended as a test modality for inhalant or food allergens since results are more difficult to interpret and standardize. Scratch testing may result in varying quantities of allergen absorbed, mechanical irritation of the skin [7] , bleeding at the test site, and carries a higher risk of inducing a systemic allergic reaction.

Intradermal skin tests are more sensitive but less specific than SPT [42]. They are more labor-intensive and require more precise techniques. These tests have occasionally been associated with serious systemic allergic reactions and even death from anaphylaxis [43, 44]. In clinical practice, SPT tests should always be performed first since a positive test circumvents the necessity for intradermal skin testing. Extracts utilized for intradermal skin testing are less concentrated (1:10–1:1000; 0.00001 μg/ml up to 1 μg/ml [42, 45]) than those utilized for SPT and should be free of glycerine, in order to avoid false-positive reactions. In the diagnosis of pollen allergy, several studies indicate that positive intradermal skin tests do not necessarily correlate with clinical symptoms [22, 42] whereas there is a very good correlation between SPT results and clinical allergy symptoms [25]. Thus, for the most part, SPT is preferable to intradermal testing, the latter being primarily used for Hymenoptera venom sensitivity, sensitization to medications, and where an allergen is considered historically relevant and in the circumstance that the SPT is negative [46]. Titration tests for Hymenoptera venoms are usually begun at concentrations of 1:1000 or after prior negative SPT using 1:100.

Positive and negative controls should be measured first. The negative control excludes the presence of dermographism which, when present, makes the tests difficult to interpret. The histamine control should be positive to make sure that the test materials are applied correctly and to exclude negative SPT results due to potentially interfering medications taken by the test subject (Table 2). The largest diameter of the wheal of each particular test is measured, a positive being a wheal of ≥ 3 mm [62] since the longest diameter is a better estimate of wheal surface area than the mean perpendicular diameters of a skin prick test above a certain value (17 mm²). The negative control is no longer used to deduct its size from the positive tests. Including the longest diameter of pseudopods does not increase sensitivity for determining the degree of sensitisation.

Since histamine reactivity in the skin varies among individuals, independent of skin test reactivity to allergens [52], the skin test results to allergens should not be related to the size of the histamine reaction [63]. The size of the wheal is not solely due to histamine as some subjects with positive SPT reaction show no significant histamine release to these allergens as assessed by microdialysis technique [64]. Reproducibility is greater when only the diameter of the wheal, and not the associated erythema, is measured [65, 66]. In order to achieve a permanent record, the size of the wheal may be outlined with a pen, blotted onto a cellophane tape, and transcribed onto paper and/or stored electronically.

The SPT confirms sensitization to a specific allergen, however, its clinical relevance must be interpreted based on the medical history and clinical symptoms (see Additional file 1: Table S3). Sometimes, conjunctival, intranasal, oral or even bronchial challenge provocation tests are performed to support clinically relevant sensitivity. The clinical relevance of SPT results varies, depending on the allergen utilized and the population tested. For example, sensitization to house dust mite occurs in some subjects in the absence of clinical relevance [4].

Allergen extracts ideally should be standardized based on the content of the major and minor allergenic determinants since not all patients are allergic to each antigen within an individual extract. They should have batch-to-batch consistency and the skin test results should be comparable when the same extracts from different manufacturers are utilized. Since allergen extracts are biological mixtures containing a variety of different proteins, glycoproteins and polysaccharides, this is difficult to achieve. In fact, SPT results obtained with the same allergen with extracts from different manufacturers vary [67–72]. Thus, when SPT results are compared, the allergen extract utilized should be obtained from the same manufacturer. Likewise, effective allergen immunotherapy requires specified quantities of allergenic components in the extracts used for immunotherapy.

The fact that accurate standardization of extracts is of great importance for their quality has led manufacturers to implement extensive protocols for standardization. Each company uses its own in-house reference material and unique units to express potencies. Such variability among different manufacturers leads to an inability to compare different products and test results. However, since most major allergens of relevant allergens have been identified during the last several decades, the concept introduced is to quantitate the major allergens in each of the individual extracts. Such quantification will allow comparison between products by different manufacturers. In 2001, an EU funded project, the CREATE project, was introduced to encourage standardisation of allergen extracts based on their content of major allergen(s). The project evaluated the use of recombinant allergens as reference materials for major allergen measurements [73]. Another attempt to standardize extracts involved the development of recombinant allergen extracts. Even though some of these recombinant allergens show comparability to allergen extracts derived from source material [74–76], they only cover a limited number of allergens and are still under investigation.

Extracts should not contain preservatives which can cause false positive reactions, e.g., sodium merthiolate. Nor should they be mixed with other allergens, e.g. house dust mite with dog dander extract. When testing with non-commercial allergens, there is a real need to use control tests in non-allergic subjects to compare the results with subjects who are allergic. For certain plant allergens, especially for fresh foods and vegetables, the prick-to-prick method is more reliable than using manufactured extracts [61].

The tight regulation of skin test extracts has made their production and registration problematic and costly for the pharmaceutical industry. This has led to gaps in the registration of specific extracts in certain European countries.

The authors suggest that a standard SPT panel for inhalant allergens, based on the GA²LEN study [3], be utilized throughout Europe. This panel includes the following 18 allergens: hazel (Corylus avellana), alder (Alnus incana), birch (Betula alba), plane (Platanus vulgaris), cypress (Cupressus sempervirens), grass mix (smooth meadow grass/Poa pratensis, cock’s foot grass/Dactilis glomerata, perennial rye grass/Lolium perenne, timothy grass/Phleum pratense, meadow fescue/Festuca pratensis, meadow oat grass/Helictotrichon pretense), Olive (Olea europaea), mugwort (Artemisia vulgaris), ragweed (Ambrosia artemisiifolia), Alternaria alternata (tenuis), Cladosporium herbarum, Aspergillus fumigatus, Parietaria, cat, dog, Dermatophagoides pteronyssinus, Dermatophagoides farinae, and cockroach (Blatella germanica). Allergens can be supplemented as necessary for regional or for particular patient needs.

SPT results should be appropriately interpreted based on clinical symptoms, medical history, and, where necessary, other test results (specific IgE antibody measurements) in order to assess possible allergy to a specific allergen. The probability of a given sensitization to be clinically relevant depends on the type of allergen and country where the patient lives [4]. The clinical relevance of any detected sensitization should be determined by an allergologist after taking a complete history and performing a physical examination. When SPT results and the history are inconclusive, provocation tests may help to determine the clinical relevance of the SPT sensitization, e.g., before initiation of a specific immunotherapy.

SPT is highly specific and sensitive, 70-95% and 80-97%, respectively, to diagnose inhalant allergies [76]. The positive predictive value to diagnose allergic rhinitis based only on the clinical history is 77% for persistent allergy and 82-85% for intermittent seasonal allergy [17]. This increases to 97-99% if SPT is utilized [17].

The negative predictive value of a negative SPT and in vitro IgE antibody test for cat allergen are identical at 72-75% for cat allergy [42]. A negative SPT for Dermatophagoides pteronyssinus has a negative predictive value in older adults of 90%-95%. However, the positive predictive value ranges from 29% to 43% in older subjects and 77% to 100% for younger subjects [23].

Sensitivity and specificity are lower for food allergens, ranging from 30-90% and 20-60%, depending on the type of allergen and methods utilized, i.e. pricking with extracts vs. prick-to-prick techniques described earlier [77]. Double-blind placebo-controlled challenge studies in children demonstrate that SPT possesses a positive predictive value of 76% and 89% for clinical reactions to cow’s milk and hen’s egg, respectively [78].

The objective value of SPT for drug allergy depends on the tested drug. In most cases, a positive SPT makes drug allergy very probable; whereas a negative result does not necessarily indicate that the patient will not react on challenge to the drug [79]. However, for penicillin, the negative predictive value is high. In 98.5% of patients with a negative SPT, no type I allergy was observed upon challenge while the remaining 1.5% of patients had mild and self-limiting reactions, e.g., urticaria [80]. In many cases, intradermal testing is appropriate after negative SPT. Some drugs, e.g., muscle relaxants or opioids may cause SPT false-positive results. When evaluating patients for IgE-mediated drug allergy to antibiotics other than penicillin, SPT should be performed with the unadulterated pharmaceutical agent. Late readings (> 24h) of SPTs and especially intradermal skin tests are very valuable in the clarification of adverse drug reactions.

For suspected insect venom allergy, intradermal tests are the primary mode for detecting sensitization. SPT is performed prior to intradermal testing.

Sensitizations to aeroallergens, as measured by SPT, may precede symptomatic allergy. Prospective studies show that 30-60% of such subjects become allergic depending on the type of allergen tested and the time to follow-up [81, 82]. Furthermore, sensitization can exist to an allergen that is no longer clinically relevant.

Sensitization rates vary depending on the geographic region as measured in population-based and in patient-based studies like the European Community Respiratory Health Survey (ECHRS), the International Study of Asthma and Allergies in Childhood (ISAAC), and the GA²LEN Pan-European skin prick test study. Exposure rates and genetic differences can explain some of these variations [83, 84]. With increased human mobility, differences in exposure to various flora or alterations in the allergenicity of pollen, possibly caused by pollution [85, 86], and changes in sensitization occur over time [87]. Longitudinal studies investigating sensitization over time provide data on such trends [88, 89].

The same allergen extracts and ideally even the same batches of extracts should be utilized when comparing a test result from one place to another or over time. For epidemiological studies, the standard prick test panel should be utilized to ensure comparability with the GA²LEN study results (see Table 1; [3]). Distribution of standard operating procedures for both technical aspects and data collection assures the highest degree of comparability. Studies of allergic sensitization should take place over an extended period of time, ideally, a year, since (i) skin test reactivity increases during the pollen season [90] and (ii) allergic individuals tend to seek care when they have symptoms. This can skew detected prevalence of sensitization in such studies. It is also important to note the seasonal differences of various allergens based on geographic locations which can further distort sensitization rates between countries.

More than 1,800 allergenic molecules are identified (http://www.allergome.org/script/statistic.php; [91]). The use of recombinant allergen molecules for SPT should improve sensitivity and standardization of SPT by reducing non-specific reactivity due to irritant compounds contained in biologically derived extracts, in particular, food extracts [92]. New in vitro techniques using recombinant molecules and micro-technology may permit testing of hundreds of compounds simultaneously, thus improving diagnostic possibilities and potentially even eliminating the need for skin testing [93].

Further studies are needed to (1) compare extracts from different manufacturers, (2) investigate intra subject variability, and (3) determine the relevance of SPT for pan-allergens. Since new allergens are being identified in Europe, e.g., acerola (Malpighia glabra), and others are becoming more prevalent, e.g., Lepidoglyphus destructor, new studies should also investigate the relevance of these allergens.

There is general agreement that the core diagnostic test for type I immediate allergy, i.e. the SPT, should be further standardized to include standardized procedures and allergen panels. Additional allergens can be added to this “core”, when indicated. Such standards are likely to: (1) improve the quality of patient diagnosis and care, and (2) reduce variability of results and thus make test results comparable. Further studies are necessary to define worldwide standards for allergen extracts.

Lucie Heinzerling, Karl-Christian Bergmann, Megon Bresciani, Guido Burbach, Ulf Darsow, Stephen Durham, Wytske Fokkens, Mark Gjomarkaj, Tari Haahtela, Ana Todo Bom, Stefan Wöhrl: GA²LEN member.