A SIMPLE AND RAPID METHOD TO MEASURE NON-PROTEIN-BOUND FRACTIONS OF CORTISOL, TESTOSTERONE AND OESTRADIOL BY EQUILIBRIUM DIALYSIS: COMPARISON WITH CENTRIFUGAL FILTRATION

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Abstract
A simple method for measuring the free, non-protein-bound steroid fraction in plasma by equilibrium dialysis is described. The alteration occurring in the volume of the inner phase (undiluted plasma) is corrected by the difference in weight before and after dialysis. Total cortisol was determined by radioimmunoassay. Compared with normal values for healthy women and men (9.3 .+-. 1.4 ng/ml), women with increased estrogenic activity showed slight elevations of free cortisol. Compared to cortisol, the percentage of dialyzable testosterone and estradiol was lower in women than in men (1.57 vs. 2.08% for testosterone and 1.68 vs. 2.15% for estradiol). In healthy men the concentration of free steroid was 13.1 .+-. 1.0 ng free testosterone/100 ml plasma (0.44 .+-. 0.045 pg free estradiol/ml plasma), in healthy women 0.64 .+-. 0.07 ng/100 ml (0.98 .+-. 0.10 pg/ml) and in women receiving estrogens 0.37 .+-. 0.04 ng/100 ml (0.01 .+-. 0.015 pg/ml). When the method described here for determining the free fractions of cortisol (n: 45), estradiol (n: 18) and testosterone (n: 18) at 37.degree. C is compared with the method of centrifugal filtration, the correlation was r: 0.80, r: 0.86 and r: 0.91, respectively. In practice, equilibrium dialysis with undiluted plasma is simple, fast and can be applied to all steroid hormones. It allows direct measurements of non-protein-bound steroids under nearly physiological conditions.

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