Failure to detect extremely high levels of serum IgE with an immunoradiometric assay.

Two patients with extreme elevations in serum IgE were found to have falsely low levels of IgE when measured by a commercial immunoradiometric assay. Dilution studies performed on serum from both patients revealed the rise and subsequent fall of the dose-response curve characteristic of the high-dose "hook" or prozone effect. Specimens with unexpectedly low levels of serum IgE obtained by either commercial immunoradiometric assay or enzyme immunoassay should be reanalyzed after dilution.