Corticosterone in bird eggs: The importance of analytical validation

It was recently found that high concentrations of chicken yolk gestagens and gestagen metabolites hamper corticosterone quantification via immunoassays. However, the situation in chicken albumen is still unresolved. In addition, the ratio of steroid hormone in the yolk of wild birds might differ. To investigate these matters, corticosterone and gestagens were measured in individual fractions of high-performance liquid-chromatographic separations of chicken albumen and yolk of red jungle fowl. Similarly, yolk extracts of hens with corticosterone-releasing implants or placebos were analysed to assess the impact of elevated plasma corticosterone concentrations on authentic yolk corticosterone levels. We also compared the results of a previously used corticosterone enzyme immunoassay (EIA) to those from a commercial radioimmunoassay (RIA) kit. The analytical validations of chicken albumen, bankiva yolk and yolks from hens with or without artificially elevated plasma corticosterone levels indicated that the main share of the immunoreactivity measured via corticosterone immunoassays was caused by substances other than authentic corticosterone. In albumen, the concentration of authentic corticosterone was below the detection limit. Analysis of bankiva yolk revealed three major gestagen peaks with concentrations of up to 2000 ng per fraction and a corticosterone peak of about 0.8 ng per fraction. Both corticosterone assays found a slightly higher corticosterone peak in a corticosterone-implanted hen's yolk (EIA: 0.7 ng; RIA: 0.5 ng per fraction) compared to the sham-treated female (EIA: 0.5 ng; RIA: 0.2 ng per fraction) but both antibodies also bound to several other substances, presumably gestagens. Although a certain amount of circulating corticosterone might pass into the yolk, direct quantification of corticosterone in non-homogenized avian egg samples via immunoassays is not advisable