Predictive accuracy and sources of variability in calculated free testosterone estimates

Ann Clin Biochem 2009;46:137-143
doi:10.1258/acb.2008.008171
© 2009 Association for Clinical Biochemistry

 

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Original Articles


Gideon Sartorius1,2,
Lam P Ly1,2,
Ken Sikaris3,
Robert McLachlan4 and
David J Handelsman1,2


1 Andrology Department, Concord Hospital;
2 ANZAC Research Institute, University of Sydney, Sydney, NSW 2139;
3 Melbourne Pathology, University of Melbourne, Melbourne;
4 Prince Henry’s Institute of Medical Research, Clayton, Victoria, Australia


Corresponding author: Prof David J Handelsman. Email: djh{at}anzac.edu.au


Background: Serum free testosterone (FT) concentrations are commonly requested,but because reference FT methods are too laborious various calculationalalgorithms for FT based on total testosterone (TT) and sex hormone-bindingglobulin (SHBG) are frequently used. This study provides thefirst large-scale evaluation of the predictive accuracy andsources of variability for different FT formulae compared withdirect laboratory measurements.

Methods: Using a large data-set of direct FT measurements by centrifugalultrafiltration, the predictive accuracy of five different formulasfor cFT (four existing plus a new formula) is evaluated in 3975consecutive blood samples. In a second data-set of 124 samplesfrom a reference panel of healthy eugonadal young men, we estimatethe relative influence of the five algorithms and eight differentTT and two SHBG assays including all available commercial totalTT and SHBG assays together with a gas chromatography/mass spectrometryT reference method.

Results: cFT formulae show wide discrepancies with equilibrium-bindingalgorithms showing systematic overestimation relative to directFT measurements, whereas two empirical cFT methods were moreconcordant. Variations between commercially available TT immunoassayshave a strong impact on calculation of FT with TT assays contributing82.2% of overall variance compared with 13.7% for the cFT algorithmsand 4.1% for the SHBG assays.

Conclusions: If FT measurements are requested and direct measurement impractical,cFT formulae using TT and SHBG immunoassays provide an approximationto direct FT measurement that is strongly dependent on the TT,cFT formula used and, to a lesser extent, SHBG immunoassays.


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