The Direct Assay for Platelet-Associated IgG (PAIgG): Lack of Association Between Antibody Level and Platelet Size

Elevated levels of platelet-associated IgG (PAIgG) found by direct assay in patients with immune thrombocytopenia, may in part reflect passive absorption of IgG to megathrombocytes present in increased proportions in thrombocytopenia caused by increased platelet destruction. Using a direct assay for PAIgG based on the inhibition of lysis of IgG-coated sheep red blood cells, we correlated these levels with two platelet volume parameters, median platelet volume and percentage megathrombocytes, using a Coulter Channelyzer. The mean PAIgG was 4.7 fg IgG/platelet in 30 controls, 6.6 fg IgG/platelet in 15 patients with nonimmune thrombocytopenia, and 41.6 fg IgG/platelet in 32 patients with immune thrombocytopenia. Only in the immune thrombocytopenic patients was there a correlation between platelet count and PAIgG (r = 0.66, p < 0.001); however, there was no correlation between the PAIgG and either of the two parameters of platelet volume. Furthermore, all 4 patients with nonimmune thrombocytopenia who had an increase percentage megathrombocytes (20%-36%) had normal levels of PAIgG. These results indicate that the elevated levels of PAIgG in patients with immune thrombocytopenia is specific for this disorder and not due to passive absorption of IgG to megathrombocytes, which are present in increased proportions in these conditions.