Enzyme linked immunoassay for the detection of platelet associated IgG.

An enzyme linked immunoassay incorporating antihuman globulin coupled with alkaline phosphatase has been developed to measure platelet associated IgG (PAIgG). Using a method in which platelet IgG is extracted into the fluid phase after appropriate procedures, we were able to bind the 'solubilized' PAIgG to commercially obtained antihuman IgG (AHG) which had previously been coated onto polystyrene. The amount of PAIgG thus bound was subsequently measured by the addition of the enzyme reagent using p-nitro phenyl phosphate as substrate. With this technique platelets from normal donors were found to have 2.6-17.4 ng/10(6) platelets (mean +/-2 SD). These values are higher than those obtained when assay systems using intact platelets are employed. Platelets from patients with immune thrombocytopenia had PAIgG values of 8.2-98.0 ng/10(6) platelets. In a few patients with disorders other than autoimmune thrombocytopenia (AITP) increased levels of PAIgG were also demonstrated. The assumption that increased PAIgG always represents platelet autoantibody may not be valid. The relevance of PAIgG as a parameter in the diagnosis and clinical management of patients with AITP is discussed.