Not one lancet for multiple SPT.

Abstract

. SKIN prick tests (SPT) are a common diagnostic method used in allergology. The sensitivity and speci®city of this method depend on the allergen extracts and the method used to perform the tests. This study aimed to determine whether one lancet can be used for skin prick testing with several allergens in the same patient, as this method is used by many practitioners in an attempt to reduce costs. Twelve women and 24 men with (23) or without (13) positive SPT to common allergens participated in the study. Standardization of the method was in line with the position papers of the EAACI (1, 2). Tests were performed with single-use lancets, made of metal with a 1mm long point, specially designed for SPT (Pohl), and standard allergen extracts (Der p, Der f, cat, and grass) (Allergopharma). Positive controls were histamine (10 mg/ml) and codeine (10 mg/ml); the negative control was 50% glycerosaline solution. SPT were applied simultaneously by two methods: one lancet and one prick on one forearm (the ``single test'' method), and one lancet and multiple pricks on the other (the ``multiple test'' method). If one lancet was used for more than one prick, it was carefully cleaned with cotton moistened with 75% ethanol. The skin tests were applied to the volar surfaces of both forearms with a distance of 3 cm between the test sites. The methods were randomly assigned to the left and right forearms, respectively. Histamine, codeine, and allergens were applied alternatively with placebo. The tests were read after 10 min for histamine and after 15 min for the other extracts. All wheal sizes were measured as the mean of the longest diameter and the midpoint of the perpendicular diameter. A false-positive reaction was de®ned as a reaction to placebo after a positive reaction to histamine, codeine, and allergen: mean wheal of $1 mm above baseline ± ®rst variable; mean wheal of $3 mm above ± second variable. The statistical analysis was done with the Wilcoxon rank-sum test and the Spearman rank factor. In the ``multiple test'' method, falsepositive tests at the placebo site following allergen administration were recorded in 20 out of 35 pricks performed when all reactions were considered, and in 13 out of 35 pricks when a 3-mm cutoff was considered. This was observed neither in the ``single test'' method nor with histamine or codeine in both methods. The number of false-positive reactions in the ``multiple test'' method was statistically signi®cant and independent of the criteria for false-positive tests (P=0.0001 for wheal of $1 mm; P=0.0015 for wheal of $3 mm). The size of the false-positive lesion after allergen did not depend on the type of allergen but on the intensity of the reaction to the allergen applied before placebo (R=0.672, P=0.00001) (Fig. 1). The use of one lancet (0.1 euro) to perform 20 skin pricks would result in a saving of 1.9 euro. Unfortunately, our study has shown a statistically signi®cant rise in the number of false-positive tests when using one lancet for multiple skin pricks. This is due to trace amounts of allergen remaining on the lancet even after thorough wiping. False-positive results are mainly observed in subjects who have strong reactions to speci®c allergens. Therefore, the above method cannot be used to identify factors responsible for allergy, particularly in patients who have been quali®ed for immunotherapy and in scienti®c studies. For ®nancial reasons, it can be used in screening tests and epidemiologic studies of A signi®cant rise in false positives was found.