A simple ultrafiltration method for determining unbound estradiol in serum

The Amicon MPS-1 centrifugal ultrafiltration device and the YMT membrane were used to measure free estradiol in serum. Two independent assays were required. The first measured total estradiol-17 beta and the second ultrafiltrable fraction. The unbound fraction was determined by counting [3H]estradiol-17 beta in 0.15-0.3 mL ultrafiltrates of 0.5 to 1 mL of serum that had been equilibrated with [3H]estradiol-17 beta at 37 degrees C. The assay is rapid (1 h), practicable (requires 0.6 mL serum) and reproducible (CV 4.6% within assay and 6.5% between assays). Samples from apparently healthy men and women and from pregnant women gave results that agreed with those obtained by equilibrium dialysis. The percentages of free serum estradiol-17 beta were similar during both early and late follicular phases of the menstrual cycle. Samples from women with hirsutism gave values for total and free estradiol-17 beta which were not significantly different from those of normal females when no consideration was given to the phase of the menstrual cycle. The means for both of these female groups were significantly different from those of males.