Endocrine Abstracts

Metyrapone interference in serum cortisol immunoassay

David Halsall¹, Laura Owen³, Adam Viljoen⁴, Kevin Taylor¹, Helen Simpson¹, Krishna Chatterjee² & Mark Gurnell²

¹Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK; ²University of Cambridge Metabolic Research Laboratories, Cambridge, UK; ³University Hospital of South Manchester NHS Foundation Trust, Manchester, UK; ⁴East and North Hertfordshire NHS Trust, Stevenage, UK.

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Metyrapone (MT) is used in the medical management of Cushing’s syndrome as it decreases serum cortisol (CT) levels by inhibiting adrenal β-hydroxylation of 11-deoxycortisol, the final step in CT synthesis. CT precursors, in particular 11-deoxycortisol (DOC), increase following MT therapy. Monitoring glucocorticoid replacement in patients taking MT could therefore be confounded as DOC cross-reacts in commonly used immunoassays (IA) for serum CT. Serum CT results from two patients taking MT and hydrocortisone replacement are presented (Table 1). In both patients, insufficient glucocorticoid replacement was suspected on clinical criteria. In each case serum CT was measured by three commonly used IA; all showed positive bias compared to a liquid chromatography tandem mass spectrometric method (LCMSMS). 11-DOC, the CT precursor, was elevated in both cases.

Table 1

Patient	A	B	Cross reactivity
Metyrapone dose	1 g qds	1 g qds
Hydrocortisone dose	1.5 mg/h iv	5 mg tds
ACTH (ng/l)	68	2166
CT (nmol/l) LCMSMS	86	32
IA Siemens centaur	272	263	0.151* (DOC)
IA Beckman	195	204	0.181* (DOC)
IA Roche	173	169	0.035* (DOC)
11-DOC (nmol/l) LCMSMS	235	700

Cross reactivity of DOC in each immunoassay was determined by spiking cortisol-free serum with DOC. The observed discrepancy between CT levels measured by IA versus LCMSMS could not be explained by the degree of 11DOC cross reactivity in any of the CT IAs. However the observed positive bias of IAs was proportional to serum (DOC) and (ACTH) levels. This suggests that another ACTH-regulated steroid precursor might cross-react in CT IAs, contributing to their positive bias. Immunoassay methods cannot be relied upon to provide reliable estimates of serum CT in MT treated patients. The precise basis for the positive bias of IAs remains to be elucidated.

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Endocrine Abstracts (2010) 21 P329