Analysis of steroids in songbird plasma and brain by coupling solid phase extraction to radioimmunoassay

It is a common practice to extract steroids from plasma, serum, or tissue samples prior to steroid measurement by radioimmunoassay (RIA) or enzyme immunoassay (EIA). Steroid extraction is critical because it can remove substances that interfere with the RIA or EIA. Steroid extraction is commonly achieved using organic solvents, such as diethyl ether or dichloromethane. However, organic solvent extractions can suffer from low recovery, imprecise recovery, or incomplete removal of assay interference. Here, we describe validations of a simple protocol to extract steroids (e.g., dehydroepiandrosterone, corticosterone, and estradiol) from avian plasma, serum, and brain tissue using solid phase extraction (SPE) with commercially available C18 columns. We compare various methods for (1) eluting steroids from columns, (2) drying eluates, and (3) resuspending dried eluates prior to RIA. The SPE method yields high and consistent recoveries. The SPE method also effectively separates steroids from interfering substances, even when extracting steroids from lipid-rich plasma and brain tissue. These data indicate that SPE is superior to organic solvent extraction on several measures. SPE should be broadly useful for extracting steroids from plasma or tissue samples.