Immunoassay of specific IgE: Use of a single point calibration curve in the modified radioallergosorbent test

Interest in immunoassay standardization has prompted development of specific IgE assays reporting results related to the international IgE reference. To examine the single point calibration curve employed in the modified RAST assay (MRT) to convert MRT counts to IgE units, independent dilutions of a 25 kU/L total IgE reference and nine allergic sera (three each for short ragweed, cat, and timothy) were made in horse serum and assayed. In a log-log plot, the single point curve was, by definition, linear over its entire range; the dilution curve was curvilinear because of reagent system saturation, which was at 7 kU/L. Curves were not parallel (P < .001). Allergen-specific dilution curves showed saturation points at values similar to or less than the total IgE system. The linear portions of these curves paralleled the total IgE dilution curve but not the single point curve. This lack of parallelism would have resulted in varying magnitudes of error in estimation of IgE antibody levels in the upper and lower assay ranges, and would imply a lower detection limit for IgE than that which the assay actually has. Modified RAST assay is not appropriate in research or a clinical situation in which accurate quantitative results are needed. Modified RAST assay would furthermore be an inappropriate means of assigning units to proposed reference preparations for standardization.