Diagnostic utility of the glucagon stimulation test in comparison to the insulin tolerance test in patients following pituitary surgery

OBJECTIVE: The glucagon stimulation test (GST) like the insulin tolerance test (ITT) stimulates both ACTH and GH secretion. However, there are limited data with modern assays on sensitivity and specificity for GST in comparison to ITT. The aim of this study was to evaluate the diagnostic utility of the GST for GH deficiency (GHD) and adrenal insufficiency (AI) in patients following pituitary surgery. DESIGN AND PATIENTS: ITT and GST were performed within 7 days in 49 patients at least 3 months after transsphenoidal surgery. Serum GH and cortisol were measured by Immulite 2000 assay (Siemens AG). Receiver-operating characteristic (ROC) analysis was performed to identify the thresholds for GST. RESULTS: In ITT, 18/49 cases were classified as AI. ROC analysis revealed a peak cortisol value >599 nmol/l in GST for adrenal sufficiency with 100% specificity and 32% sensitivity, and a peak cortisol <277 nmol/l with >95% specificity and 72% sensitivity for AI. Of the 49 subjects, 25 (51%) demonstrated levels between these cut-offs and could not be diagnosed by GST alone with sufficient accuracy. Regarding GHD, 21/49 cases were classified as insufficient by ITT. ROC analysis revealed a cut-off of 2.5 ng/ml with 95% sensitivity and 79% specificity. Of the 49 cases, seven (14%) were discordant in terms of defining GHD, with six subjects being treated for GHD according to GST although being sufficient in ITT. CONCLUSION: In our prospective series of patients with pituitary disease, GST is a potential alternative test for the assessment of GH reserve, but is a poor test for ACTH reserve. Test-specific cut-offs should be applied to avoid misinterpretation.