Verification of a method for sexual hormone-binding globulin analysis and estimation of free testosterone

Abstract: Introduction: Sexual hormone-binding globulin (SHBG) is a protein that binds to androgens and oestrogens, especially testosterone. The fraction of testosterone that is not bound to SHBG is the biologically active fraction which makes its determination more relevant than determining the total amount of circulating testosterone. It is difficult to measure the plasma concentration of free testosterone; therefore calculations using the concentrations of testosterone and SHBG are used to estimate the amount of free testosterone. A few calculations include the concentration of albumin because testosterone also binds to albumin. The main aims of this study were to verify a method for the determination of SHBG and to calculate a reference interval for free androgen index (FAI, testosterone/SHBG) in women. Other calculations for determination of the free testosterone fraction were compared. Methods: Testosterone, SHBG and albumin were measured in serum from 20 men and 100 women. Testosterone and SHBG was measured using immunoassays on a Roche Modular E instrument (ECLIA). Albumin was measured with a c8000 Architect instrument. Four calculations, two with only testosterone and SHBG and two with testosterone, SHBG and albumin were compared.  Results/Conclusion: The verification of the SHBG method was successful which means that the method can be taken into routine use. A reference interval for FAI was constructed. It was difficult to show if other estimation of free testosterone would work better than FAI in clinical practice. This is discussed.