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

Abstract

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.