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Aging Male. 2004 Dec;7(4):280-9.

Reflections concerning biochemical parameters of androgenicity.

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Section of Endocrinology, Medical Clinic, University Hospital Gent, Belgium.


Due to the highly variable concentration of sex hormone binding globulin (SHBG) and the many factors affecting it, the evaluation of the androgen status may require the measurement of a parameter of bioactive plasma testosterone. As, however, no practical, clinical useful direct method for measurement of plasma androgen bioactivity is available, indirect biochemical parameters are used. All have their limitations and pitfalls. In this paper are discussed some of the factors influencing the values obtained with different methods (direct measurement of free testosterone by analog radioimmunoassay, dialysis, ammonium sulfate precipitation, free testosterone index, calculated free and bioavailable testosterone), all of which may explain the variability of data reported in the literature. It is concluded that, whereas determination of bioavailable testosterone by dialysis or ammonium sulfate precipitation of SHBG-bound testosterone is work-intensive and not really suitable for clinical routine, while direct measurement of free testosterone by analog immunoassay yields unreliable results, only the free testosterone index and calculated bioavailable testosterone are adapted for clinical routine. The limitations of the free testosterone index, a dimensionless parameter which does not reflect a defined bioavailable testosterone concentration, are discussed. As the same measurements of testosterone and SHBG required for determination of the free testosterone index permit the calculation of bioavailable testosterone, which yields a defined androgen concentration, it is advisable to prefer the latter over the free testosterone index, which should no longer be used.

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