Determination of Antibody Activity by Platelet Aggregation.

Platelets play an important role in hemostasis by forming clots and stopping bleeding. In immune thrombotic conditions, platelets and leukocytes are aberrantly activated by pathogenic antibodies resulting in platelet aggregates and NETosis, leading to thrombosis and thrombocytopenia. A simple assay that assesses platelet function and antibody activity is light transmission aggregometry. This assay can be used to determine antibody activity in patients with disorders such as heparin-induced thrombocytopenia (HIT) and vaccine-induced thrombotic thrombocytopenia (VITT). Briefly, for detection of pathogenic antibody, platelet-rich plasma (PRP) is treated with a specific agent (e.g., patient sera or purified patient antibodies) with constant stirring. Upon activation, platelets undergo a shape change and adhere to each other forming aggregates. This causes a reduction in opacity allowing more light to pass through PRP. Light transmission through the cuvette is proportional to the degree of platelet aggregation and is measured by the photocell over time. The advantage of this protocol is that it is a simple, reliable assay that can be applied to assess antibody activity in thrombotic conditions. Light transmission aggregometry does not require the use of radioactive reagents and is technically less demanding compared with ¹⁴C-serotonin release assay, another common assay for detecting antibody activity. Key features • This protocol can be used to assess platelet function and to detect platelet activating antibodies in diseases such as HIT and VITT. • Does not require radioactive reagents, requires an aggregometer; based on the light transmission aggregometry protocol, adapted for detection of VITT and other platelet-activating antibodies. • Two positive controls are required for reliable detection of antibodies in diseases such as HIT/VITT, namely a weak HIT/VITT antibody and a physiological agonist. • For detection of HIT/VITT antibodies, it is essential to use donors known to have platelets reactive to these antibodies to avoid false negative results.