Impact of recent biochemical findings on the determination of free and bioavailable testosterone: Evaluation and proposal for clinical use

Background: The mass action law-based calculation methods for free testosterone (FT) and "bioavailable" testosterone (BAT) - routinely used for assessing androgen disorders - rely on the supposition that the sex hormone binding globulin (SHBG) molecule contains one steroid binding domain (SBD). However, recent biochemical investigations revealed that this molecule actually comprises two SBDs. This necessitates new equations (Calc2) for FT and BAT calculation. Methods and results: Calc2 was deduced using the recently published SHBG-testosterone association constant and was compared to other FT and/or BAT determination methods (e.g., a conventional calculation version [CalcV], ammonium sulfate precipitation [for measuring BAT] relying on total testosterone [TT] by mass spectrometry, and a FT radioimmunoassay [RIA] proving satisfactory). In contrast to CalcV BAT, Calc2 BAT fitted ammonium sulfate precipitation BAT. An analogous result was found by means of a comparison of FT ratios determined by RIA, Calc2, and CalcV, respectively. Additionally, Calc2 data mostly appear to meet literature data (measured by methods such as equilibrium dialysis) better than CalcV data do. Conclusions: There is evidence that Calc2 affords reliable FT and BAT derivation from assayed TT, SHBG, and albumin making direct FT and BAT measurements unnecessary in most cases. Finally, Calc2 was formatted as a preliminary nomogram for convenient clinical use.