Prospective evaluation of a new platelet glycoprotein (GP)-specific assay (PakAuto) in the diagnosis of autoimmune thrombocytopenia (AITP).

Assays measuring platelet-associated immunoglobulin G (PAIgG), while highly sensitive, lack specificity in diagnosing autoimmune thrombocytopenia (AITP). We prospectively evaluated a new commercially available glycoprotein (GP)-specific assay, the PakAuto (GTI, Brookfield, WI), for its clinical usefulness in distinguishing immune from nonimmune thrombocytopenia (TP), in 216 patients with autoimmune TP (both primary "idiopathic" and "secondary") and 46 patients with TP due to other causes. This assay is designed to detect both platelet-associated (direct assay) and plasma (indirect assay) antiplatelet antibodies specific for GPs IIb/IIIa, Ib/IX, and Ia/IIa. The mean platelet counts of the immune (79 +/- 7 x 10(9)/L) and nonimmune groups (78 +/- 7 x 10(9)/L), were similar (P=0.95). The direct assay was positive in 114/216 patients with AITP (53%), and 13/46 with nonimmune TP (28%). Among the AITP group, the majority (61%) of patients with positive test results had autoantibodies reactive against all three GP targets. The sensitivity, specificity, positive, and negative predictive values for the direct PakAuto were 53%, 72%, 90%, and 24%, respectively, comparable to previously published experience of GP-specific assays. However, in some cases of TP due to nonimmune cause, the PakAuto was highly specific. Only 3 of 22 patients with gestational and 1 of 8 with familial/congenital TP had a positive direct assay, indicating that the test may be particularly useful for excluding an immune etiology for TP in certain patient subgroups.