Comparison of free testosterone results by analog radioimmunoassay and calculated free testosterone in an ambulatory clinical population

J Sex Med. 2010 May;7(5):1948-53. doi: 10.1111/j.1743-6109.2009.01473.x. Epub 2009 Sep 1.


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.

Publication types

  • Comparative Study

MeSH terms

  • Adult
  • Aged
  • Ambulatory Care
  • Androgen-Binding Protein / blood*
  • Comorbidity
  • Computers, Analog*
  • Erectile Dysfunction / blood
  • Humans
  • Hypogonadism / blood
  • Hypogonadism / diagnosis
  • Male
  • Middle Aged
  • Predictive Value of Tests
  • Radioimmunoassay / methods*
  • Reference Values
  • Retrospective Studies
  • Testosterone / blood*


  • Androgen-Binding Protein
  • Testosterone