AFTC, that is testosterone determined by equilibrium dialysis, is arguably the method of choice for measuring FT in vitro. Vermeulen et al.18
determined the correlation of AFTC with other accepted clinical measures of testosterone. They found the correlation coefficient was 0.987 with calculated FT, 0.937 with immunoassayed free testosterone (aFT), and 0.848 with FAI. Calculated FT, therefore, approaches the accuracy of measuring testosterone by dialysis. Conditions that alter SHBG may alter the results of not only total testosterone but also FT. Conditions such as obesity, hypothyroidism and acromegaly can lead to lowered levels of SHBG, and as such confound the results of FT. Otherwise, calculated FT may be a practical means for the clinician to measure FT, as it is less time-consuming and expensive than measuring testosterone by equilibrium dialysis (AFTC).