A sensitive and specific method for determining testosterone estradiol binding globulin (TeBG)

A simple, rapid and highly sensitive method for measuring TeBG and how it can be used for obtaining a Free Testosterone Index (FTI) is described. Dihydrotestosterone (DHT) is used instead of testosterone owing to its higher affinity for this globulin. Non-specific binding (NSB) is estimated by incubating the plasma samples in parallel, with and without a 100-fold excess of cold DHT. To determine the level at which TeBG becomes saturated and to work far from this region of the curve, where sensitivity diminishes, hot-only experiments were performed. Saturation was found to occur beyond 1000 pg DHT. Control normal sera were compared with samples from hypogonadic patients and Scatchard plots were done to find a parallel between the two groups and a point of maximal discrimination among them at about 500 pg DHT. Since an excellent reproducibility between experiments was found, Scatchard plots were not routinely done and only the binding capacity of a control normal pool at the point of maximal discrimination was compared to that of the unknown sample and calculated as follows: TeBG binding capacity = DHT bound by the normal pool/DHT bound by the unknown. The FTI is obtained by multiplying the total testosterone by the TeBG binding capacity. Total testosterone (Tt), TeBG and FTI were analysed in 38 normal volunteers classified according to age in four groups: 20-30 (n=13), 40-50 (n=7), 50-60 (n=8), 60-70 (n=10). Although a slight difference was observed when Tt was compared between the different age groups (P<0.05), there was a much higher significant difference for TeBG and FTI (P<0.0001). (author)