Measurement-specific bioavailable testosterone using concanavalin A precipitation: comparison of calculated and assayed bioavailable testosterone.

Objective: To assess the value of calculated bioavailable testosterone (cBT) and assayed BT (aBT) for the diagnosis of late-onset hypogonadism (LOH) in middle-aged and elderly subjects.

Methods: In order to assay serum BT, sex hormone-binding globulin was precipitated with concanavalin-A and then testosterone was measured using liquid chromatography-tandem mass spectrometry. To validate the non-sex-hormone-binding-globulin-bound testosterone, gel filtration chromatography and concanavalin-A sepharose were used. Following this validation, the usefulness between aBT and cBT was evaluated in clinical samples.

Results: Eighty-eight healthy male volunteers (mean age 65.6 years, range: 50-86) were recruited for this study. A significant correlation was found between cBT and aBT (R(2) = 0.53, P < 0.01). Mean value ratio (cBT/aBT) was 2.48. Both cBT (R(2) = 0.122) and aBT (R(2) = 0.251) decreased with age. Variations in aBT were less marked than those for cBT, suggesting that aBT can be used to determine age-related reduced testosterone levels.

Conclusion: aBT levels are more reliable than cBT levels for the diagnosis of LOH in middle-aged and elderly subjects.