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Clinical Chemistry 52: 474-481, 2006. First published December 29, 2005; 10.1373/clinchem.2005.052126
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Right arrow Endocrinology and Metabolism
(Clinical Chemistry. 2006;52:474-481.)
© 2006 American Association for Clinical Chemistry, Inc.

Endocrinology and Metabolism

Serum Bioavailable Testosterone: Assayed or Calculated?

Frank Giton1, Jean Fiet1,2,a, Jérôme Guéchot3, Fidaa Ibrahim4, Françoise Bronsard5, Dominique Chopin1 and Jean-Pierre Raynaud6

1 Emi INSERM 03-37, Centre de Recherches Chirurgicales, CHU Henri Mondor, Faculté de Médecine, Créteil, France.
2 Laboratoire de Biochimie, Faculté de Pharmacie, Paris, France.
3 Laboratoire d’hormonologie, Hôpital St. Antoine, Paris, France.
4 Laboratoire d’hormonologie, Hôpital St. Louis, Paris, France.
5 Institut Universitaire de Technologie de Cachan, Cachan, France.
6 Université Pierre et Marie, Paris, France.

aAddress correspondence to this author at: Centre de Recherches Chirurgicales, Faculté de Médecine, 8 rue du Général Sarrail, 94010 Créteil Cedex, France. Fax 33-1-49-81-35-52; e-mail fiet{at}univ-paris12.fr.

Background: Bioavailable testosterone (BT), circulating testosterone not bound to sex hormone–binding globulin (SHBG), is thought to easily penetrate cells. We compared BT measurements obtained by assays with those obtained by calculation with different testosterone association constants.

Methods: We obtained sera from 2 groups of hypogonadal men [group 1 (G1), 1421 samples; group 2 (G2), 170 samples] and a group of healthy men [group 3 (G3), 109 samples]. We added minute doses of [3H]testosterone to the sera, precipitated the SHBG-bound fraction of testosterone with ammonium sulfate (50% saturation), and then assayed serum BT (ABT) as %BT x total. Calculated BT (CBT) was determined with theoretical association constants of testosterone for SHBG (Ks = 1 x 109 L/mol) and albumin (Ka = 3.6 x 104 L/mol) and paired optimal Ks and Ka values obtained by use of Microsoft Excel software.

Results: CBT calculated with theoretical constants differed from ABT by >30% in 85.7% (G1), 84.1% (G2), and 77.9% (G3) of samples, and the mean CBT/ABT ratios were 1.57 (G1), 1.85 (G2), and 1.50 (G3) in spite of fairly good correlations. CBT calculated with paired optimal Ks and Ka differed from ABT by <30% in 87.4% (G1), 87.5% (G2), and 97.5% (G3) of samples, and mean CBT/ABT ratios were 0.95–1.04.

Conclusions: To obtain CBT values as close as possible to ABT, optimal paired association constants determined for each studied population must be used instead of the theoretical association constants. Considering the uncertainty of calculating BT, however, use of the ammonium sulfate precipitation method for determining BT is advisable.




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Age Attenuates Testosterone Secretion Driven by Amplitude-Varying Pulses of Recombinant Human Luteinizing Hormone during Acute Gonadotrope Inhibition in Healthy Men
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H. Dechaud, A. Denuziere, S. Rinaldi, J. Bocquet, H. Lejeune, and M. Pugeat
Age-Associated Discrepancy between Measured and Calculated Bioavailable Testosterone in Men
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