Summary: In men, measurement of serum testosterone and estradiol levels with immunoassays correlated with mass spectroscopic measurements, and correlations of sex steroids with volumetric bone mineral density were similar. Introduction: While immunoassays have been used extensively for measurement of serum testosterone (T) and estradiol (E2) levels, there is concern about their specificity, particularly at low E2 levels as present in men. Methods: We compared T and E2 measured by mass spectroscopy to levels measured by immunoassay in men (n=313, age 22 to 91 years) and related these to volumetric bone mineral density (vBMD) at various skeletal sites. Results: Serum T and non-SHBG bound (or bioavailable) T levels by immunoassay correlated well with the corresponding mass spectroscopy measurements (R=0.90 and 0.95, respectively, P<0.001); the correlations for serum E2 measured using the two techniques were less robust (R=0.63 for total E 2 and 0.84 for bioavailable E2, P<0.001). Overall relationships between serum bioavailable T and E2 levels with vBMD at various skeletal sites were similar for the immunoassay and mass spectroscopic measures. Conclusions: Although E2 levels with immunoassay correlate less well with the mass spectroscopic measurements than do the T measurements in men, our findings indicate that the fundamental relationships observed previously between vBMD and the sex steroids by immunoassay are also present with the mass spectroscopic measurements.
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