PURPOSE: The aim of the study was to investigate immunologic causes of thrombocytopenia in critically ill patients, especially causes that were related to platelet-associated IgG antibodies.

SUBJECTS AND METHODS: All patients admitted to two intensive care units between May 1 and October 30, 1997, who developed thrombocytopenia (less than 100 × 10⁹ platelets/L) were studied prospectively. We measured platelet-associated IgG with a radioimmunoassay using I¹²⁵-labeled polyclonal antihuman IgG. Characterization of platelet-associated IgG was assessed with a monoclonal antibody immobilization of platelet antigen. Circulating immune complexes were also assayed.

RESULTS: Of the 61 patients with thrombocytopenia, elevated platelet-associated IgG was found in 18 (30%). Associated antiplatelet autoantibodies (glycoprotein IIb/IIIa) were detected in 4 patients, circulating autoantibodies (glycoprotein Ib/IX) were detected in sera from 2 patients, and circulating immune complexes were detected in 3 patients. The nature of the platelet-associated IgG could not be determined in 10 patients. Elevated platelet-associated IgG was associated with sepsis and previous cardiopulmonary bypass. Thrombocytopenic patients with elevated platelet-associated IgG had a lower nadir platelet count (58 ± 27 × 10⁹/L vs 74 ± 24 × 10⁹/L, P = 0.03).

CONCLUSION: Elevated platelet-associated IgG, some of which are platelet autoantibodies, is frequent in thrombocytopenic patients with sepsis or after cardiopulmonary bypass.