Relationship of raised platelet IgG in thrombocytopenia to total platelet protein content.

Platelet-associated IgG (PA IgG) of washed platelets of 20 normal controls and 45 thrombocytopenic patients was measured by radioimmunoassay and related to total platelet protein content. Mean values (+/- 2 SD) for 20 normals were 4.4 +/- 3.5 fg IgG per platelet (fg IgG/pl), 2.1 +/= 1.0 pg protein per platelet (pg protein/pl) and a ratio of IgG per g platelet protein of 2.1 +/- 1.8 mg IgG/g protein. Patients could be divided into two groups: Group I (31) with normal IgG/g protein ratios and either normal (16) or raised (15) fg IgG/pl, and Group II (14) with both fg IgG/pl and IgG/g protein raised. Of the 21 patients with idiopathic thrombocytopenia (ITP), 13 were in Group I (seven with raised and six with normal fg IgG/pl), and eight in Group II. There was significant correlation between PA IgG (fg IgG/pl) and platelet protein for the entire Group I (r = 0.95, P less than 0.001). The ITP patients in this group showed a similar correlation (r = 0.77, P less than 0.001), suggesting that raised PA IgG levels in Group I patients may be a result of protein-rich platelets. 60-90% of PA IgG was found in the soluble fraction of lysates of platelets from both normals and patients. Thus, it is either non-specifically associated with platelets or is antibody directed towards easily solubilized platelet antigens. Nine thrombocytopenic patients without immune disorders had raised PA IgG. These results question the assumption that raised PA IgG always indicates specific antiplatelet antibody.