Age-associated discrepancy between measured and calculated bioavailable testosterone in men.

BACKGROUND: Bioavailable testosterone (BT) concentration is considered the best marker for evaluating testicular function in men. The decrease of BT in older men is more pronounced than the decrease in total testosterone because of the parallel increase in sex hormone-binding globulin (SHBG) concentrations. Measurement of BT is therefore crucial for the diagnosis of hypoandrogenism in the aging male population.
METHODS: We compared BT concentrations measured by a specific RIA after ammonium sulfate precipitation (BT(meas)) with those obtained by theoretical calculations (BT(cal)) in plasma samples from 694 young men (14 to 49 years old) and 51 older men (50 to 81 years old). We based theoretical calculations on Vermeulen's simplified mass equation using total testosterone and SHBG concentrations.
RESULTS: BT(cal) and BT(meas) correlated significantly in young (Pearson r = 0.87) and aging (r = 0.89) men, but the BT(cal):BT(meas) ratio differed markedly between the 2 groups (2.28 vs 3.48; P <0.001).
CONCLUSIONS: In men, there is an age-associated discrepancy between calculated and measured BT concentrations. We suggest some hypotheses for the discrepancy, but additional studies will be performed to finally elucidate this difference in results and to determine the most appropriate method for BT measurements in older men.