The accuracy and standardization of TBG kits

Three kits for measurement of serum thyroxine-binding globulin (TBG) were compared with a reference radioimmunoassay in forty-three normal euthyroid subjects. The Corning Immophase kit gave similar results to those with the reference method with satisfactory precision and accuracy. The Behringwerke AG RIA-gnost kit gave results with adequate but lower correlation with the reference method (r = 0.80) than that of the Corning kit trial (r = 0.92). The mean value was significantly greater than the reference method mean, indicating that the Behringwerke kit was inaccurately standardized. The CEA-Sorin TBG kit (Eurotope services) gave results with a poor correlation against the reference method (r = 0.59). The mean value was also significantly greater than the reference mean indicating that this kit is both inaccurately standardized and less precise than the other two. It is stressed that TBG-kits need to be periodically standardized against reference methods in the absence of properly validated primary standards in all batches.