Comparison of sex steroid measurements in men by immunoassay versus mass spectroscopy and relationships with cortical and trabecular volumetric bone mineral density

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.