Endocrine Abstracts

Measurement of cortisol is used in the differential diagnosis of Addisons’ disease (deficiency), Cushing’s disease (excess), hypopituitarism, adrenal hyperplasia and carcinoma. Cortisol is routinely measured by immunoassay, which has wide inter-assay variability with manufacturer dependent bias. Despite this, universal cut-offs are used as criteria for interpretation of dynamic function tests of the pituitary adrenal axis.

Using a fully validated LC–MS/MS method, comparison between the Roche Elecsys immunoassay has been performed. Additionally, synacthen stimulation tests (SST) and overnight dexamethasone suppression tests (DST) have been analysed on both methods. Cortisol levels were on average 25% lower by LC–MS/MS, compared with the immunoassay. Five out of 62 SST (~8%) which satisfied criteria (>550 nmol/l at 30 min) by immunoassay, failed on the LC–MS/MS method and did not reach 550 nmol/l at 60 min. An additional four SST which reached >550 nmol/l at 30 min by immunoassay, did not measure >550 nmol/l until 60 min on LC–MS/MS. 11% of SSTs failed to response to synacthen on both methods. Preliminary data has shown no difference between assays for the DST. However, with the proven positive bias of the immunoassay, some DST that fail to suppress may show adequate suppression when analysed by LC–MS/MS.

In conclusion, interpretation of cortisol dynamic function tests should consider the method of analysis and local guidance should be provided.