Background: Liquid Chromatography Tandem Mass Spectroscopy (LCMS) is increasingly replacing traditional immunoassays in endocrine testing by virtue of its specificity and lack of cross-reactivity. We investigated whether the newer, more specific urinary free cortisol (UFC) assays have comparable sensitivity in patients with new or relapsed Cushing’s disease.
Method: 69 consecutive UFC samples from two tertiary hospitals were analysed on LCMS (user defined cut off < 150nmol/day), Roche extracted immunoassay (manufacturer cut-off < 380 nmol/day), and Abbott unextracted immunoassay (manufacturer cut-off < 487 nmol/day, user defined cut-off < 280 nmol/day). Samples were classified as A) Cushing’s (new diagnosis on histology, or known Cushing’s /clinical features with at least one positive midnight salivary cortisol/dexamethasone suppression test), or B) Unlikely Cushing’s by two independent Endocrinologists.
Results: 31 UFCs were positive on at least one method, of which 12 were true positives using clinical classification as the gold standard. Roche had 26 positive UFC, followed by LCMS (n = 20) and Abbott (n = 13 user defined cut-off, n = 5 manufacturer cut-off). All UFCs from confirmed Cushing subjects were positive on the Roche, false positives included patients on prednisolone and acutely unwell patients. The more specific methods (LCMS, Abbott) missed 2 relapsed Cushing’s disease. Area under the curve was 0.99 for Roche (C.I: 0.98 – 1.0), 0.85 for Abbott (C.I: 0.71 – 0.99), and 0.80 for LCMS (C.I: 0.62 – 0.98). Using Abbott user defined cut-off had the strongest agreement with clinical classification (Kappa = 0.56), using manufacturer’s recommended cut-off missed 8 samples of Cushing’s disease.
Conclusions: Although Roche assay cross-reacts with prednisolone, it detected two patients with relapsed Cushing’s disease that were missed by more specific assays. ACTH increases upstream steroid metabolites which cross react with less specific UFC methods, this cross-reactivity might play a role in early detection.