Serum bioavailable testosterone: assayed or calculated?
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 10(9) L/mol) and albumin (Ka = 3.6 x 10(4) 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 K(s) and K(a) 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.
Giton
Frank
F
Emi INSERM 03-37, Centre de Recherches Chirurgicales, CHU Henri Mondor, Faculté de Médecine, Créteil, France.
Fiet
Jean
J
Guéchot
Jérôme
J
Ibrahim
Fidaa
F
Bronsard
Françoise
F
Chopin
Dominique
D
Raynaud
Jean-Pierre
JP
D000728:Androgens / Q000627:therapeutic use; D015415:Biomarkers / Q000097:blood; D003205:Computing Methodologies; D006801:Humans; D007006:Hypogonadism / Q000097:blood / Q000188:drug therapy; D008297:Male; D011485:Protein Binding; D012016:Reference Values; D044967:Serum; D012738:Sex Hormone-Binding Globulin / Q000378:metabolism; D013739:Testosterone / Q000097:blood* / Q000627:therapeutic use
D000728:Androgens; D015415:Biomarkers; D012738:Sex Hormone-Binding Globulin; D013739:Testosterone