Testosterone, Free, Mass Spectrometry/Equilibrium Dialysis (Endocrine Sciences) LabCorp

Free testosterone is the fraction of total testosterone that is not bound to circulating proteins. The free fraction is available to diffuse into target cells to interact with receptors or enzymes. The concentration of free testosterone is low relative to total testosterone, typically <3% of the total testosterone concentration. Total circulating testosterone is bound to sex hormone-binding globulin (SHBG) or albumin. Measurement of total and free testosterone is key in diagnosis of androgen excess or deficiency. Free testosterone is best measured by equilibrium dialysis, while total testosterone is measured by HPLC with tandem mass spectrometry.1-3 Free testosterone is estimated in this profile using these recommended techniques.

Tritiated testosterone is added to the sample and allowed to dialyze across a membrane. Once equilibrium has been achieved, the solutions on both sides of the membrane are sampled to determine the percent free testosterone. Total testosterone is measured by HPLC with tandem mass spectrometry. Serum free testosterone is then calculated by multiplying the percent free testosterone by the total testosterone concentration with unit correction.