Cortisol measurement in patients receiving metyrapone therapy

Current guidance recommends titrating the dose of metyrapone against serum cortisol concentration, in patients under medical management of Cushing's syndrome. In the UK, this almost always involves measuring serum cortisol concentration by immunoassay, the performance of which is questionable in the presence of altered steroid metabolism. Sera from two patients receiving metyrapone were analysed using a liquid chromatography tandem mass spectrometry (MS) steroid assay to identify which steroids, if any, were elevated in these patients. In addition, control serum was spiked with a series of steroids to identify any potential positive interferences in a cortisol immunoassay. Serum 11-deoxycortisol concentration was elevated in both of the patients studied. One patient also had an elevated serum 17-hydroxyprogesterone concentration and the other an elevated androstenedione. In addition, the results of the interference studies indicated that the cortisol immunoassay was susceptible to interference from 11-deoxycortisol, 17-hydroxyprogesterone and 21-deoxycortisol. However, the magnitude of interference, in the serum cortisol immunoassay, due to these three steroids could not account for the discrepancy between the cortisol concentrations measured by immunoassay and those measured by MS. Both clinicians and laboratory staff should be aware of these interferences when monitoring patients undergoing treatment with metyrapone, and consequently serum should be measured in these patients by MS, not by immunoassay.