Abstract
The respondents 1 raise a number of valid points for discussion in response to our study. 2 The critical issue is that the degree of positive interference caused by metyrapone therapy in the cortisol immunoassay cannot be explained solely by cross-reactivity of 11-deoxycortisol (11DOC). 2–4 As requested by the respondents, we have performed spiking studies with exogenous 11DOC to assess cross-reactivity in the Centaur XP assay (Siemens, Surrey, UK). Linear regression analysis of increasing 11DOC against measured serum cortisol gave a slope value of 0.13. The immunoassay kit insert reports cross-reactivity of 7.3% for 11DOC when spiked into a supraphysiological concentration of 27,590 nmol/L. We therefore replicated this experiment and observed a cross-reactivity of 13.6%, a level that falls short of accounting for the interference documented in the cohort of metyrapone-treated patients in our study. We speculated that the accumulation of steroid precursors, additional to 11DOC, may have a cumulative effect on the observed cross-reactivity in cortisol immunoassays. 2 However, it is possible that a particular and as yet elusive precursor steroid may have a high degree of cross-reactivity with the Centaur XP cortisol reagent antibodies. This question requires future investigation and may be addressed by performing urine steroid profile analyses in patients during metyrapone treatment.
Does adrenocorticotropic hormone (ACTH)-drive correlate with the extent of interference observed? Unfortunately, in the cohort of patients described in our study, paired ACTH results are not available. However, the majority of metyrapone-treated patients in our study had a single ACTH result on our laboratory database or an ACTH result near the time of cortisol day curve sampling. Analysis of the mean delta cortisol against ACTH concentration gave a modest Pearson correlation coefficient of 0.62 (n= 12) and it was evident that the highest ACTH result (465 μg/L) corresponded to the highest delta cortisol value (157 nmol/L). The daily metyrapone dose for these patients ranged from 500 mg to 3 g/d and this variation confounds an accurate assessment concerning the role of ACTH-drive on the observed interference. A robust quantitative assessment in this regard would require a standardized metyrapone dosing regimen and a cortisol day curve sampling protocol to include the collection of a paired plasma sample for baseline ACTH measurement.
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