Endocrine Abstracts (2008) 16 P476

Prospective comparison of the glucagon stimulation test (GST) with the insulin tolerance test (ITT) in patients following pituitary surgery

Christian Berg, Timo Meinel, Ali Yuece, Harald Lahner, Klaus Mann & Stephan Petersenn

University of Duisburg-Essen, Essen, Germany.

Objective: The ITT is the gold-standard for assessment of GH and ACTH reserve but has certain contraindications. GH and cortisol responses of <3 ng/ml and <500 nmol/l, respectively, have been defined as evidence of severe deficiency. The GST like the ITT stimulates both the ACTH and GH secretion and is suggested to be a good alternative in terms of efficiacy. However, there are limited prospective data with modern assays on sensitivity and specificity for the GST in comparison to the ITT. Aim of this study was an evaluation of the diagnostic utility of the GST in patients with hypothalamo–hypopituitary disease following pituitary surgery.

Design and patients: ITT and GST were performed within 7 days in 22 patients (14 men, age 28–61) at least 3 months after transsphenoidal surgery. Serum GH and cortisol were measured by Immulite 2000 assay (Siemens AG). ROC analysis was performed to identify optimal thresholds for GST; for cortisol deficiency analysis was adjusted to achieve a sensitivity ≥95%.

Results: Regarding GHD, 11/22 cases were classified as insufficient by ITT. For GST, ROC analysis revealed a cut-off of 3.1 ng/ml with 100% sensitivity and 81.9% specificity. Only 2/22 (9%) cases showed conflicting results compared to ITT in relation to this cut-off and were discordant in terms of defining GHD. Regarding cortisol deficiency, 6/22 cases were classified by ITT as cortisol insufficient. For GST, ROC analysis revealed a cut-off of 654 nmol/l with 100% sensitivity and poor 33.3% specificity. Using this cut-off, 10/22 (45%) cases showed conflicting results compared to ITT and were discordant in defining cortisol deficiency.

Conclusion: In our prospective series of patients with pituitary disease, the GST is a good alternative test for assessment of GH reserve, but poor for ACTH reserve, as demonstrated by comparison with the ITT. Test-specific cut-offs should be applied to avoid misinterpretation.

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