Sensitive high-performance liquid chromatographic method using coulometric electrode array detection for measurement of phytoestrogens in dried blood spots

As the epidemiological and physiological investigation of isoflavones and lignans expands, the need for sensitive methods for analyzing large numbers of samples intensifies. We have developed a method using high-performance liquid chromatography (HPLC) equipped with a coulometric electrode array detector for separation and sensitive detection of daidzein (Da), equol (Eq), genistein (Ge) and enterolactone (Enl) in dried blood spots (DBS). Detection limits ranged from 4.5 pg or 0.09 ng/mL (Eq) to 19 pg or 0.38 ng/mL (Ge) on column. Signal linearities ranged from detection limits to 200 ng/mL (Eq, Enl) and 600 ng/mL (Da, Ge) sample concentration. Correlations between DBS and serum concentrations were 0.66 (Enl), 0.88 (Eq), 0.98 (Ge) and 0.99 (Da). Intra-assay coefficients of variation (CVs) were less than 8% and inter-assay CVs ranged from 2.4 to 20.2% for Da, Eq and Ge for three levels of controls. Enl intra-assay CV was 13.6% for the low pooled control. Analytic recovery ranged from 87% (inter-assay Ge) to 98% (inter-assay Enl). DBS concentrations of Da, Ge and Eq were stable for at least 8 weeks at 4 and 25 °C, and at 37 °C for at least 5 weeks, with Enl showing greater variability at all temperatures but relative stability for 7 weeks. Measurement of samples from 135 perimenopausal Japanese women consuming habitual diets in Kyoto and Fukushima prefectures showed the former to have the expected lower concentrations of Da and Eq (416 and 87 nM) as well as Enl (49 nM) compared to the latter locale (566, 145 and 72 nM, respectively). This method could be useful in large epidemiological research or detailed physiological studies.