Significance of Borderline Levels of Specific IgE obtained by FAST-Plus Assay

The significance of “borderline” levels of allergen-specific IgE as measured by in vitro assays has been questioned. Patients whose specific IgE tests Patients were tested for twelve antigens using the FAST-Plus methodology. All 0/1 results were checked using skin tests at a 1:500 concentration. Positive (histamine) and negative (diluent) controls were used. The antigen-induced wheals were compared with those produced by a control wheal of 2% glycerine (the glycerine concentration in a 1:500 dilution). Positive wheals were arbitrarily considered to be those whose diameter after 10 minutes exceeded that of the glycerine control wheal by 2 mm or more. Using the limits of calibrator fluorescence for the FAST-Plus test in effect before 1990, a significant discordance between skin test results and the class 0/1 in vitro readings was evident. Using the standards in effect since 1990, marked concordance between class 0/1 results and positive skin tests was noted. This was most marked for pollens, less so for molds. Using current standards, FAST-Plus class 0/1 results are best considered positive (pending clinical confirmation), rather than negative.