A sensitive and convenient method for measurement of corticosterone in rat serum.

Although competitive protein-binding assays for corticosterone have been in use for several years, most employ cumbersome methods for the separation of bound and free steroid. This paper describes conditions under which dextran-coated charcoal can be successfully used for this purpose. When the time of exposure to suspended charcoal was carefully controlled, approximately 90% of free steroid was adsorbed and negligible dissociation of bound steroid occurred. The assay was highly sensitive, having a standard curve ranging from 0.05 to 1.0 ng corticosterone. When known amounts of corticosterone were added to serum, 89--95% recoveries were achieved. Other steroids showed negligible cross-reactivity.