Use of Erythema as An Interpreting Index of the Skin Prick Test

ABSTRACT

PURPOSE: There are many methods to interpret the skin prick test which has been commonly used to find out the causative allergen in allergic disease. Among them two are most frequently used. One is to measure the absolute size of wheal which is caused by the allergen and the other is to compare the size of wheal for allergen with that of positive control. However the wheal is smaller than erythema at the same dosage of allergen and changes less with according to the allergen dosage. So we investigated the feasibility of the size of erythema as one of the index in the interpretation of skin prick test. METHODS: We carried out skin prick tests on various allergens, including Dermatophagoides pteronyssinus (Dp), Dermatophgoides farinae (Df), and house dust (HD) in 220 patients who visited the pediatric allergy clinic at Inha Hospital from January 1990 to January 1996. RESULTS: Among the 220 patients studied, 120 children (55%) showed positive skin test. There was significant correlation between wheal and erythema (P<0.0001 for Dp, P<0.0001 for Df, and P<0.0001 for HD). There was a significant statistical correlation (P<0.0001) between the allergen/histamine wheal ratio (A/HW ratio) and the size of erythema for Dp, P<0.0001 for Df, and P<0.0001 for HD. There was a significant statistical difference in the wheal size, A/HW ratio, and erythema size, allergen/histamine erythema ratio (A/HE ratio) for Dp, Df, HD between positve and negative group of RAST. As the RAST class increases, the erythema size and A/HE ratio for Dp, Df, and HD showed a tendency to increase. Sensitivity and specificity of skin test with cut off value of 20mm for erythema were 87.9%, 58.3% in Dp, 85.2%, 66.6% in Df, 61.4%, 71.9% in HD and 25mm were 75.9%, 83.3% in Dp, 67.6%, 91.6% in Df and 40.9%, 100% in HD. CONCLUSION: We conclude that erythema size can be used as one of the interpreting index of skin prick test. We also recommand more than 25mm as a cut off value for positive reaction for Dp, Df and more than 20mm for HD in bronchial asthma.