Free testosterone measurement by the analog displacement direct assay: old concerns and new evidence.

Fritz and coworkers (1 ) have provided data arguing against the validity of a popular analog-based direct assay for free testosterone (T). Their rigorous dissection of characteristics of the assay and use of carefully defined solutions demonstrates that the assay correlates generally with total T but does not measure dialyzable T; this observation is consistent with prior reservations about this class of assay by reported by the Endocrine Society (2, 3 ) and of other investigators (4 – 6 ).