More on the Analog Free-Testosterone Assay

Abstract

In their recent article, Winters et al. (1) reported free and bioavailable testosterone in 28 purportedly healthy men with widely ranging body mass indexes. The authors showed that total and free, but not bioavailable, testosterone correlated positively with sex hormone-binding globulin (SHBG) and concluded that SHBG concentrations influenced free-testosterone concentrations as measured by the analog assay. We have also measured free testosterone, total testosterone, and SHBG, using the same kits from Diagnostic Products Corp. in 40 healthy volunteers (Fig. 1⇓ ). Like Winters et al. (1), we found a correlation ( r = 0.54) between total testosterone and SHBG, with total testosterone (nmol/L) = 0.17 SHBG (nmol/L) + 11.3. However, we found no statistically significant correlation ( r = 0.05) between free testosterone and SHBG, with free testosterone (pmol/L) = −0.05 SHBG (nmol/L) + 63.3. We noted an apparent error in the y -axis scale for free testosterone in their Figs. 1⇓ and 3; the values ranged between 0 and 3 pmol/L, not in keeping with the range of free testosterone in their subjects as stated in Table 1. Figure 1. Correlation between serum SHBG and total ( left ) and analog free ( right ) testosterone in 40 healthy males. Their additional studies do not appear to support SHBG affecting the analog assay. First, their finding of lower than expected free-testosterone … bAuthor for correspondence. Fax 412-692-4019; e-mail winters{at}med1.dept-med.pitt.edu