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Background: The accurate measurement of cortisol by immunoassay is compromised by the potential for cross-reactivity of reagent antibodies with structurally related steroids present in serum. This susceptibility is potentiated when normal steroid metabolism is altered pharmaceutically by antisteroidogenic drugs utilized in the management of Cushing's syndrome to moderate cortisol production. The clinical implications of falsely elevated cortisol results include over-treatment and unrecognized hypoadrenalism. To investigate the effect of the 11b-hydroxylase inhibitor metyrapone on serum cortisol assay, a comparison of measurement by immunoassay versus liquid chromatography-tandem mass spectrometry (LC-MS/MS) was conducted. Methods: Cortisol was measured in serum from three patient groups: (1) patients receiving metyrapone (n ¼ 112 samples from 10 patients); (2) control group of patients diagnosed with Cushing's syndrome currently receiving no treatment (n ¼ 31); and (3) control group of patients with no adrenal pathology and not receiving medication known to interfere in cortisol immunoassay (n ¼ 67). Conclusions: These data show that liability of immunoassay measurement of serum cortisol to interference in patients receiving metyrapone may lead to erroneous clinical decisions concerning dose titration.

The measurement of both serum and plasma 5-hydroxyindoleacetic acid can be used for the diagnosis and monitoring of patients with neuroendocrine tumours. Provided renal function is taken into consideration, either of these tests should be incorporated into standard practice as an alternative assay to urinary 5-hydroxyindoleacetic acid.