It is important to differentiate betwteen extravascular (autoimmune thrombocytopenia, ATP) and intravascular (thrombotic thrombocytopenia, TTP) platelet destruction in thrombocytopenia. Betathromboglobulin (BTG), a platelet-specific protein with a plasma half life of 20 minutes is released in-vivo from platelets by various stimuli and may reflect platelet activation or destruction. BTG concentration can be measured in plasma usin a radioimmunoassay to a sensitivity of 1 ng/ml., (nomal 28.0 ± 8.0 ng/ml., n = 70). Plasma BTG was measured in 3 patients with ATP (platelet counts: 17, 20, 16 x 109/L) and 2 patients with TTP (platelet counts: 20, 40 x 109/L). In ATP, BTG was normal (22, 11, 17 ng/ml.) and in TTP, BTG was elevated (80, 72 ng/ml.). Plasma BTG remained normal in ATP after treatment. BTG remained elevated in TTP (120 ng/ml.) even when the platelet count became normal (220 x 109/L) but while fragmented RBC were still present and became normal (21 ng/ml.) on complete recovery. These data suggest that plasma BTG may be useful in differenfiating extravascu1ar from intravascular platelet destruction by detecting increased concentrations of BTG in plasma.
