Fritz and coworkers (1) have provided data arguing against the validity of a popular analog-based direct assay for free testosterone (T). Their rigorous dissec-tion of characteristics of the assay and use of carefully defined solutions demonstrates that the assay corre-lates generally with total T but does not measure dia-lyzable T; this observation is consistent with prior res-ervations about this class of assay by reported by the Endocrine Society (2, 3) and of other investigators (4–6). WHY WORRY ABOUT FREE T? Testosterone circulates in the blood of men and women in several forms. It is bound tightly to sex-hor-mone binding globulin (SHBG), loosely to albumin, and unbound to proteins (free) (7). In most, but not
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Swerdloff et al. (2008) studied this question.