Effect of oral prednisone on the measurement ofplasma steroid concentrations by the competitive protein-binding radioassay

Plasma steroid concentrations were measured in asthmatic children before and after therapeutic administrationof prednisone or 6-methylprednisolone (Medrol). Before therapy, results of a competitive protein-binding radioassay were significantly lower than those obtained with a fluorometric method of analysis. After administration of prednisone, fluorometric assay revealed a consistent decrease in plasma steroid concentrations. However, the protein-binding method demonstrated marked increases in plasma steroid concentration which rose progressively as the prednisone dose increased. The plasma steroid concentrations reached peak levels within one hour after administration of prednisone and remained elevated for four to eight hours. Radioassay also revealed small increases in plasma steroid concentrations after administration of large doses of Medrol. The results strongly suggest that orally administered prednisone (and to a lesser extent Medrol) causes significant interference with the estimation of plasma cortisol concentrations by the competitive protein-binding method.