Optimization of skin testing. I. Choosing allergen concentrations and cutoff values by factorial design.

Standardized extracts of Phleum pratensis (grass) and Dermatophagoides pteronyssinus (house-dust mite) were used as test allergens for multiple regression in order to determine optimum concentrations and cutoff values with regard to diagnostic capacity and safety. If a RAST value of class 1 or more was taken as an indication of sensitization, it was found that the optimum concentrations for the detection of sensitization are in the range 10-100 BU/ml and 1500-10,000 BU/ml for intracutaneous tests (ICT) and skin prick tests (SPT), respectively. The skin test results were expressed as histamine ratios. Using allergen concentrations of 30 and 3,000 BU/ml, we found cutoff values of 0.87 and 0.53 and predictive values of 87.1% and 79.1% for ICT and SPT, respectively. The maximum wheal size (mean wheal size + 2 SD) to be expected in 95% of the population was 26.6 mm (ICT) and 10.9 mm (SPT), sizes regarded as safe by most clinicians. In conclusion, by using this method with a limited number of patients, one can probably improve the diagnostic precision and safety of the skin test. In the second part of this study, these hypotheses were prospectively tested in a multicenter study.