Correlation between Calculated Bioavailable Testosterone and Free Testosterone by Direct Immunoassay

Hong, Bumsik; Hong, Jun Hyuk; Song, Seunghun; Ahn, Tai Young

Abstract

PURPOSE
We investigated whether free testosterone measured by direct immunoassay(aFT) reflects aging as well as bioavailable testosterone(free testosterone index; cBT) calculated from serum testosterone(T) and sex hormone-binding globulin(SHBG).
MATERIALS AND METHODS
Serum T, SHBG, and aFT were measured in sera of 414 patients who presented with erectile dysfunction and partial androgen deficiency symptoms but no serious medical comorbidities. Their mean age was 52.5 years(<50: 152, >or=50: 262). The cBT was obtained by calculation from T and SHBG. The mean values of T, aFT, SHBG, and cBT were compared according to the age range. We analyzed the correlation between cBT and aFT and calculated the correlation coefficient.
RESULTS
The mean values of T(ng/mL), aFT(pg/mL), SHBG(nmol/L), and cBT(nmol/L) were 3.39, 13.08, 24.3, and 6.04, respectively, in the 3rd and 4th decades; 4.34, 14.72, 23.36, and 7.74 in the 5th and 6th decades; and 4.05, 13.83, 27.32, and 5.96 in the 7th and 8th decades. The SHBG increased and T/aFT/cBT declined as age increased from the 5th and 6th to the 7th and 8th decades. The change was statistically insignificant only for aFT. The correlation between cBT and aFT was weak, with a correlation coefficient of 0.391.
CONCLUSIONS
Because free testosterone measured by direct immunoassay did not reflect the age-related changes of SHBG, a single measurement is not a reliable index of bioavailable testosterone.