Spurious overestimation of plasma cortisol in patients with chronic renal failure.

Plasma cortisol concentrations in patients with chronic renal failure were measured by five different RIAs, four commercial methods, and an in-house method after paper chromatography. It was observed that cortisol concentrations of the same samples measured by the various methods did not agree. The difference was attributed to cross-reactivity of the antisera with steroids and glucuronide conjugates which circulate at high concentrations in patients with chronic renal failure. With two of the methods, values were higher in unextracted plasma (P less than 0.001) than after paper chromatography (in-house procedure). The values obtained with the other two methods were not significantly different from those obtained by the postpaper chromatography method for baseline and postdexamethasone samples. Plasma cortisol values after methyrapone administration were much higher when measured by any of the four commercial assays than after paper chromatography. A simple dichloromethane extraction improved the results from one of the methods. Laboratories are encouraged to assess carefully the behavior of cortisol antisera before using them in the assay of plasma cortisol concentrations in patients with chronic renal failure.