Inter-laboratory comparison of a yeast bioassay for the determination of estrogenic activity in biological samples

An inter-laboratory exercise was performed with a yeast estrogen bioassay, based on the expression of yeast enhanced green fluorescent protein (yEGFP), for the determination of estrogenic activity in extracts of calf urine samples. Urine samples were spiked with 1 and 5 ng mL−1 17β-estradiol and 17α-ethynylestradiol, 10 and 50 ng mL−1 mestranol, and 100 ng mL−1 testosterone and progesterone. Sample extracts of blank and spiked urine samples were prepared at our laboratory and sent to seven laboratories together with a reagent blank, a DMSO blank, and eight 17β-estradiol stock solutions in DMSO ranging in concentration from 0 to 545 ng mL−1. Sample extracts and standards were coded and tested blindly. A decision limit (CCα) was determined based on the response of seven blank urine samples. Signals of the negative controls, e.g. urine samples spiked with 100 ng mL−1 testosterone or progesterone, were all below the determined CCα and were thus screened as compliant. Positive controls, i.e. the urine samples spiked at two levels with 17β-estradiol, 17α-ethynylestradiol and mestranol, were almost all screened as suspect, i.e. gave signals above the determined CCα. Determined EC50 values calculated from the 17β-estradiol dose–response curves obtained by the seven laboratories ranged from 0.59 to 0.95 nM.