Replaces Direct Free Testosterone and FAI For most clinical purposes total testosterone measurements are satisfactory. However, where the level of binding protein (SHBG) may be unusually high or low e.g. obesity then calculating the free testosterone from total testosterone and SHBG may be helpful. Free testosterone comprises only 1-2 % of total serum testosterone.
Free testosterone can be measured directly or calculated from the total testosterone and SHBG. Assays measuring free testosterone directly have poor reliability are not now recommended. Launceston Pathology will now determine free testosterone by the Vermuelen calculation which uses total testosterone, sex hormone binding globulin (SHBG) together with known protein binding constants.
The calculated free testosterone replaces the direct measurement of free testosterone and will also replace the free androgen index (FAI).
Requests for free testosterone will now result in a calculated free testosterone, total testosterone and SHBG.
Requests for testosterone and SHBG will automatically generate a calculated free testosterone.
Under Medicare rules, doctors should request both testosterone and SHBG in order to obtain the free testosterone level.
References
Vermuelen A et al. 1999 A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocinol Metab. 84:3666-72
Rosner W et al. 2007 Utility, limitations and pitfalls in measuring testosterone: An Endocrine Society Position Statement. J Clin Endocrinol Metab. 92:405-13