This invention relates to recombinant GPIIIa molecules that lack the ligand binding βA domain and bind to HPA-1 antibodies in monomeric form. These GPIIIa molecules can be recombinantly expressed at high levels and may be useful, for example, in the detection of HPA-1 antibodies. Such detection may be useful, for example, in the diagnosis and therapy of HPA antibody disorders such as Neonatal Alloimmune Immune Thrombocytpenia (NAIT), Post Transfusion Purpura (PTP) and Platelet Refractoriness (PR).