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Free Testosterone Measurement by the Analog Displacement Direct Assay: Old Concerns and New Evidence

Division of Endocrinology, Department of Medicine, Harbor-UCLA Medical Center and Los Angeles Biomedical Research Institute, Torrance CA

^aAddress correspondence to this author at:, Chief, Division of Endocrinology, Harbor-UCLA Medical Center, Los Angeles Biomedical Research Institute, 1000 West Carson Street Box 446, Torrance, CA 90509-2910, Fax (310) 533-0627, E-mail swerdloff@labiomed.org

The first 20% of the full text of this article appears below.

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)(5)(6).

why worry about free t?

Testosterone circulates in the blood of men and women in several forms. It is bound tightly to sex-hormone binding globulin (SHBG), loosely to albumin, and unbound to proteins (free) (7). In most, but not all, clinical conditions, a measurement of total T is adequate for the evaluation of a patient. It is widely believed that the SHBG-bound T is not readily available to most tissues, whereas albumin-bound and free T are bioavailable. Because SHBG concentrations can be influenced by many factors (e.g., decreased by obesity, testosterone treatment, and hyperandrogenic female conditions such as polycystic ovary syndrome and increased by aging, pregnancy, and estrogen therapy), there are clinical situations in which measured concentrations of total T may not reflect the bioavailable concentrations or the clinical status of the patient (3)(7)(8). In these circumstances a supplemental test assessing bioavailable or free T will be helpful in clinical decision-making.

how is free t or bioavailable t determined?

Free T has been estimated for some time by dialyzing serum across a . . . [Full Text of this Article]

what did fritz and coworkers find in their studies?

what does the analog assay measure?

do the findings of fritz and coworkers apply to other branded analog-based free t assays?

why are these analog-based free t assays so widely used?

should the analog-based free t assay be used in clinical practice?

are the current methods of total t assays adequate?

The following articles in journals at HighWire Press have cited this article:

				C. Wang, E. Nieschlag, R. Swerdloff, H. M. Behre, W. J. Hellstrom, L. J. Gooren, J. M. Kaufman, J.-J. Legros, B. Lunenfeld, A. Morales, et al. Investigation, Treatment, and Monitoring of Late-Onset Hypogonadism in Males: ISA, ISSAM, EAU, EAA, and ASA Recommendations J Androl, January 1, 2009; 30(1): 1 - 9. [Full Text] [PDF]

				C Wang, E Nieschlag, R Swerdloff, H M Behre, W J Hellstrom, L J Gooren, J M Kaufman, J-J Legros, B Lunenfeld, A Morales, et al. Investigation, treatment and monitoring of late-onset hypogonadism in males: ISA, ISSAM, EAU, EAA and ASA recommendations Eur. J. Endocrinol., November 1, 2008; 159(5): 507 - 514. [Full Text] [PDF]