The concentration of free testosterone is extremely low, typically <2% of the total testosterone concentration. In most males and females, >50% of total circulating testosterone is bound to sex hormone-binding globulin, SHBG, and most of the rest is bound to albumin. Routinely available assay methods that used to measure total testosterone are not sensitive enough to accurately quantitate the free testosterone fraction directly. Free testosterone is estimated in this particular test by a direct, analogue radioimmunoassay method. This assay uses a labeled testosterone analogue that has a low binding affinity for both albumin and SHBG but is bound by antitestosterone antibody used in the assay.