Are serum cortisol measurements by immunoassays reliable?

Routinely used automated immunoassays have been found to give unrealiable measurements of thyroid hormones in the presence of either high or low levels of thyroxine-binding globulin. Thyroid hormones are not the only analytes bound to specific binding proteins that are measured by immunoassays. Preliminary data from a series of cases, comparing IA measurements to those obtained by liquid chromatography-tandem mass spectrometry, reveal for the first time that IA measurements report falsely low (by an average of 27%) serum cortisol concentrations. Initial findings suggest that IA measurements of serum cortisol are affected by high concentrations of corticosteroid binding globulin.