Underestimation of specific immunoglobulin E by microtiter plate enzyme-linked immunosorbent assays

We developed an ELISA by use of monoclonal anti-IgE to measure allergen-specific IgE. We measured perennial ryegrass (PRG) specific immunoglobulin E (sIgE) with conventional single incubations and with the recently reported transfer method to decrease interference in the assay. Paired sera in 10 patients before and during immunotherapy with PRG extract were analyzed for PRG sIgE with single serum incubations and with the sum of five serial serum incubations. A standardized reference serum and patient sera were incubated 12 hours and then transferred to a second well, incubated 12 hours more, and transferred again. The process was repeated until five pairs of wells had been incubated with each serum dilution. Color was developed, and optical density was measured. PRG sIgE was calculated by interpolation from the reference curve. Evidence of interference was inferred when less PRG sIgE was detected by single incubation than by transfer. Inhibition of 71.3% occurred in the sera drawn during immunotherapy. Inhibition of 32.9% was found before immunotherapy. Also, a late peaking of the well-to-well curve of optical density was observed in nine of 10 sera drawn during immunotherapy (the exception had not reached full strength), whereas all of the sera drawn before immunotherapy had normal curves. This suggests that sIgE determinations performed in microtiter plate format with single incubations will usually underestimate the sIgE level. This drawback needs to be kept in mind by the clinician interpreting the results of these assays.