Free testosterone by direct and calculated measurement versus equilibrium dialysis in a clinical population

The value of clinically available free testosterone (FT) assays remains controversial. Here, we evaluate the agreement between the radioimmunoassay (RIA) and calculated FT (cFT) versus equilibrium dialysis (EqD), considered the gold standard.

Fifty-six consecutive men (aged 26-77) had blood samples assessed for FT, including men with treated and untreated testosterone deficiency (TD) and men without TD. Samples were split and tested by the two methodologies at a Quest Diagnostics national reference laboratory. cFT was calculated by the Vermeulen method.

A robust correlation was noted for RIA and EqD (r = 0.966) and for cFT and EqD (r = 0.986). Strong correlations were observed for men receiving testosterone therapy and for men in the lowest and highest quartiles for total and FT. The correlation of total testosterone with FT was similar for cFT (r = 0.843), RIA (r = 0.806), and EqD (r = 0.809). Sex-hormone binding globulin (SHBG) was not correlated with any measure of FT. Bland-Altman analysis demonstrated similar bias for both cFT and RIA, although cFT consistently overestimated FT. Numerical values for RIA were approximately one seventh of EqD values.

These results support the clinical use of both RIA and cFT as measures of FT. Due to numerical differences, each test requires its own set of reference values.