Endocrine Abstracts

A 56-year-old woman presented with progressive swelling of her face and fatigability. Investigating for Cushing’s, her 24-hour Urine Free Cortisol was negative at 43 and 70nmol/24hr (Reference interval or R.I. < 146 nmol/24hr) with overnight dexamethasone suppression test showing full suppression of cortisol to 34nmol/l. Her short synacthen test was normal at 30-minute cortisol of 753nmol/l (R.I.>420); on Zumenon. Other pituitary function tests were unremarkable but plasma ACTH was >1000ng/l on 3 occasions (Siemens® Immulite assay; R.I. 7.1-56.3ng/l) and did not suppress with dexamethasone. Given the discordance between the ACTH and cortisol results the ACTH sample was reanalyzed using a different method (Roche®). The results were strikingly different with values of 8 and < 3ng/l (R.I. < 50 ng/l) pre and post dexamethasone compared to 1214 and 1075ng/l using the Siemens assay. ACTH recovery following polyethylene glycol (PEG) precipitation (Smith et al J Clin Endocrinol Metab. 2002;87(12):5410–5415) was low (< 1%) compared to two control patients with plasma [ACTH] of 1044 and 377ng/l (66.5% and 67.7% PEG recoveries). These findings are consistent with immunoglobulin interference affecting the Immulite assay. Notably the apparent ACTH concentration increased following 1:1 dilution of sample with saline (164% compared to 96 and 85% in control samples) which is typical, although not diagnostic of a ‘macro-hormone’ interference (Imunoglobulin:ACTH complex). Gel filtration chromatography of patient’s plasma confirmed the presence of a high molecular mass ACTH immunoreactive species co-eluting with the immunoglobulin fraction, that was absent from a control plasma. Given the reliance on ACTH assays to provide a differential diagnosis of Cushing’s syndrome, awareness of this type of assay interference and use of relatively simple laboratory methods investigations such as method comparison and PEG precipitation to confirm, will prevent misdiagnosis and unnecessary investigations.