Thyroxine binding globulin radioimmunoassay in dried blood spotted on filter paper

A method is described for the determination of thyroxine binding globulin (TBG) in dried blood spotted on filter paper using reagents from a test kit for the measurement of TBG in plasma. By minor modifications of the recommended procedure it was possible to improve precision, sensitivity and tracer displacement. Appropriate TBG standard samples were prepared in 'artificial blood' consisting of a suspension of erythrocytes in buffer with bovine serum albumin (50 milligrams). There is a good correlation between plasma TBG RIA results and blood spot TBG RIA results (r = 0.93). Attention must be paid to the stability of the TBG in blood: our experiments show a decrease of TBG content if filter paper cards with dried blood are stored longer than one month.