Sex hormone binding globulin (SHBG) is a glycoprotein which binds hormones such as testosterone. Around 97% of circulating testosterone is bound to SHBG and is therefore biologically unavailable; approximately 23% of testosterone is free or loosely bound to proteins such as albumin, and is biologically active, or bioavailable. Free testosterone is very technically challenging to quantify; in order to circumvent this problem, equations are used to estimate free testosterone, as recommended by the British Society for Sexual Medicine. Much work has been done to standardise testosterone analysis, including production of certified reference materials and reference measurement procedures, but such attention has not been paid to the other analytes used in calculated free testosterone equations such as SHBG. We decided to compare SHBG results obtained from the 5 most common SHBG assays found in clinical laboratories in the UK (Abbott Architect, Roche, Beckman, Siemens Immulite and Siemens Centaur) to investigate the difference in results obtained from the different methods and assess how these differences could impact on calculated free testosterone by the Vermeulen equation and also free androgen index (FAI) in females.
Anonymised surplus serum samples (40 male, 40 female) were analysed for SHBG by the five different methods. Sample aliquots were frozen at −80°C and kept frozen until analysis. Paired t-tests were carried out for all permutations of assays and results were found to be significantly different (P<0.05). The comparison between the Roche and Siemens Centaur assay showed a greater difference in the male results (P<0.001) than the female samples (P<0.05). Differences in SHBG had more of an effect on FAI than on the Vermeulen equation. These differences found in SHBG analysis, combined with any variations in testosterone and albumin measurement, could impact on clinical decisions.