[Evaluation of an immunoassay kit to measure free testosterone]

The best indicators to the diagnosis of hypogonadism are free and bioavailable testosterone circulating levels. Free and bioavailable testosterone measurements are complex. However, simple kits for direct measurement of free testosterone by analog immunoassay are available. We examined the utility of an enzymoimmunoassay kit for free testosterone measurement.

One hundred thirty-three healthy males were included. Total testosterone, SHBG, albumin and free testosterone was measured. We used two different methods to free testosterone estimation: direct measurement by enzymoimmunoassay and mathematical calculation with Vermeulen's formula, which uses albumin concentration, total testosterone and SHBG to calculate free testosterone (method recommended by the International Society for the Study of the Aging Male). We compared the two methods means values and a linear regression study was performed.

Mean age was 37 +/- 11 years. Mean serum concentration for total testosterone was 21.43 +/- 6.8 nm ol/L. The mean value for free testosterone measured by direct and mathematical method was 0.0508 +/- 0.0118 nmol/L and 0.474 +/- 0.123 nmol/L respectively. In linear regression study exists a positive correlation between both methods (p< 0.05), although correlation coefficient is very low (r = 0.25).

There are significant statistical differences between the measurements of free testosterone by direct and mathematical methods. Although certain correlation is observed, this is very low. In conclusion, free testosterone measurement by enzymoimmunoassay is not reliable.