Abstract: Although skin prick tests are a widely used tool, the methodology is not well studied. An examination was therefore performed to assess the variability of skin reactivity of different skin test sites for 40 subjects. Using standard histamine solution in a highly controlled setting, we found a great deal of variation in the the size of the wheal from site to site and from subject to subject. This variation suggests that skin prick tests require careful interpretation and that the results may need verification by more sophisticated and reproducible methods.
Skin prick tests are still a widely-used tool in diagnosis of Gell and Coombs type I allergy, even though there are currently more sophisticated techniques. The advantage of skin prick tests are: lower cost as compared to in vitro techniques and low ratio of systemic effects as well as a lower percentage of false positive results as compared to other in vivo tests. Thus, although prick tests are far from idea, the spectrum of clinical applications of this method is very broad.(1)
The "ideal" diagnostic method for performing prick tests has not been well studied. The classic interpretation of test results is based on comparison of skin response to substances tested to the "standard" histamine wheal/erythema response. This interpretation system is a canon inherited from early allergists. It is based on the assumption that the skin forms some kind of physiological and pharmacological continuum, in other words its reactivity does not depend on the site receiving the stimulus. This supposition may not be true, given the recent revelations in skin pathophysiology.(2) Given this discrepancy, it was decided to examine the variability of skin reactivity of different skin sites to prick testing. The response to a standard solution of histamine was chosen because histamine is a final common mediator of the immediate hypersensitivity immune response and the histamine response is already the criterion used in the interpretation of prick tests. Examination showed psoriatic plaques over the patient's trunk and limbs. There was confluent erythema, hyperkeratosis with scaling and fissuring on the palms of the hands and lichenification on the dorsa. The distal forearms were also involved, with parakeratotic papules scattered along the borders of the dermatitis. The patient underwent a standardized diagnostic procedure for farmers' occupational diseases (1).
Forty right-handed, healthy volunteers (24 females and 16 males aged 21 - 41 years) were tested. All subjects were informed about the course of the experiment and gave signed consent to participate. For each subject, three skin prick tests were performed at one sitting on the skin of the ventral forearm: 3 cm distal from a line determined by epicondyli of the humerus (site A), 3 cm proximal from restricta (site C), and in the middle of the distance between the above points (site B). All patients were tested between 4 p.m. and 6 p.m.. All testing was done with standard histamine solution (histamine hydrochloride 1 mg/ml, Biomed Cracow) from the same bottle. Subjects were free from any medication for at least 2 weeks. Patients taking antihistamines within the preceding three months were excluded from the study.
The testing procedure was performed in sitting position after 30 minutes adaptation to the testing room. In all patients skin reactivity was tested on the left forearm. The prick tests were performed by the same trained and experienced person using Allergopharma lancets. The skin reaction to histamine after 20 minutes was measured using following technique: the contours of the wheal area were marked using a thin-line dermatograph. They were taken using a transparent, self adhesive cellophane tape. Next, the tape was put on a millimeter paper, where the count of the marked areas followed. The intra-individual difference between sizes of the minimal and the maximal wheal areas expressed as alpha ratio computed after following rule: ratio = [(maximal area - minimal area)/maximal area]. The results were analyzed statistically using signs test.
The resulting wheal areas ranged from 3 to 126 mm2 (Table I). No relationship between the test site and wheal size was found. The differences between results for the three sites were not statistically significant at a significance level p=0.05. However, the intra-individual difference ranged from 11 to 90% (lower quartile 23%, upper quartile 51%). It was not possible to describe using simple mathematical models the dependency between wheal area size and site of test area.
| [mm2] | A | B | C |
|---|---|---|---|
| Mean | 34 | 33 | 27 |
| Median | 29 | 30.5 | 21 |
| Minimum | 3 | 3 | 3 |
| Maximum | 126 | 116 | 77 |
| Lower quartile | 14.5 | 15 | 12 |
| Upper quartile | 46 | 38 | 38 |
| Interquartile range | 31.5 | 23 | 26 |
Theoretically, the inter-individual reaction to histamine should show a minor variability as compared to reactions to allergens because some sources of variability (e.g. different grade of sensitization to a given allergen) were excluded. Moreover, numerous allergenic test substances are produced from raw materials of plant and animal origin. There is a possibility of differences between substances in different series. By using a mediator instead of an allergen, it was intended to omit this source of variability. Further known sources of variability of prick test results are circadian (3) and seasonal variations.(4) Michel and colleagues (5) have shown that a potent source of variability is technique of administration of histamine to the skin. Wise et al. (6) found correlation between cerebral lateralization and skin reactivity asymmetry in humans.
In this study patients were tested between 4 p.m. and 6 p.m. in order to avoid circadian variability and in short, 2-week period in December to avoid the seasonal variability (even though histamine and not pollen allergens were used). All patients tested were right-handed and the test was performed always on the left arm. In all patients the same skin test technique was used, moreover, the tests were performed by the same, trained and routined person. The results suggest that the intra-individual variation is enough to evoke a skepticism as regarded to reproducibility of skin prick tests. In this aspect, the trials on quantitative skin prick testing (7) are of great interest, although as far to now they are used in research rather than in practice.
The results of this study suggest that even under highly controlled conditions, there is a considerable variability of skin reactions to histamine both between individuals and for the same individual at different skin sites. Furthermore this variation was not a function of the specific site tested and it was not possible to describe the variability using simple mathematical models. Skin prick tests require careful interpretation. The physician should be aware that this method only gives tentative results which need further verification by more sophisticated, but also more specific and reproducible methods.
This article has been cited in: Zarei M, Remer CF, Kaplan MS, Staveren AM, Lin CK, Razo E, Goldberg B. Optimal skin prick wheal size for diagnosis of cat allergy. Ann Allergy Asthma Immunol 2004; 92 (6): 604-610.
© Radoslaw Spiewak
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