To what extent are free testosterone (FT) values reproducible between the two Washingtons, and can calculated FT be used in lieu of expensive direct measurements?

Free testosterone (FT) measurement by equilibrium dialysis and liquid chromatography-tandem mass spectroscopy (LCMS/MS) is the "gold standard." We hypothesized that calculated FT values could substitute for measured values; compared FT results reported by Walter Reed Army Medical Center (WRAMC), Washington, DC, with results reported by the Seattle Veterans Affairs Health Care System, Seattle, WA, for 3 patient groups; and evaluated the calculated FT values by gold-standard measurements. Groups 1 and 2 included samples from 54 patients evaluated in Seattle and 94 evaluated at a primary care clinic in Alaska whose samples were analyzed in Seattle, respectively, whose care resulted in ordering an FT measurement. Group 3 included samples from 64 patients evaluated in endocrine WRAMC clinics. Calculated FT values between the 2 facilities demonstrated a strong correlation (R² = 0.98) for all 212 patients. In a comparison of calculated FT values with measured levels, group 3 had an R² = 0.93; however, samples with FT values less than 50 pg/mL had a poorer correlation (R² = 0.45). Calculated FT values may accurately reflect and be substituted in the clinical setting for gold-standard values when levels are more than 50 pg/mL.