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ORIGINAL RESEARCH—ENDOCRINOLOGY: Comparison of Free Testosterone Results by Analog Radioimmunoassay and Calculated Free Testosterone in an Ambulatory Clinical Population


Abraham Morgentaler, MD, One Brookline Place, #624, Brookline, MA 02445, USA. Tel: 617-277-5000; Fax: 617-277-5444; E-mail:


Introduction.  The most widely used method for measuring free testosterone (FT) is by analog immunoassay (aFT); however, this assay has been criticized as unreliable based on laboratory studies in small groups of men. Calculated FT (cFT), derived from total testosterone (TT) and sex-hormone binding globulin (SHBG) values has been recommended in its place. There are limited data comparing aFT and cFT in clinical populations.

Aim.  The purpose of this study was to compare aFT with cFT in a population of ambulatory men in a clinical setting.

Methods.  Medical records were reviewed for 100 randomly selected men in a urology practice, yielding 140 test results complete for TT, aFT, and SHBG. Calculated FT was determined via an online calculator. Comparisons were made with Pearson rank coefficients.

Main Outcome Measures.  Pearson rank correlation between aFT and cFT.

Results.  Mean patient age was 52.3 ± 14.3 years (range 24–80). Mean TT was 443.0 ± 208.3 ng/dL (range 110–1276). Mean aFT was 1.22 ± 0.54 ng/dL (range 0.24–3.8) and mean cFT 9.4 ± 4.5 ng/dL (range 1.8–27.8). Mean SHBG was 34.2 ± 19.5 nmol/L (range 9–127). A strong correlation was observed for aFT and cFT (r = 0.88, P < 0.0001), particularly at low concentrations. Significant correlations were also noted between aFT and TT (r = 0.73, P < 0.0001), and between cFT and TT (r = 0.82, P < 0.0001). Numerical values for aFT were approximately one-eighth of the values obtained for cFT. Neither aFT nor cFT correlated with SHBG.

Conclusions.  A strong correlation was observed between aFT and cFT in this clinical population of ambulatory men. Different sets of reference values must be applied for each of these tests. Moreno SA, Shyam A, and Morgentaler A. Comparison of free testosterone results by analog radioimmunoassay and calculated free testosterone in an ambulatory clinical population. J Sex Med 2010;7:1948–1953.