Growth hormone response during oral glucose tolerance test:: The impact of assay method on the estimation of reference values in patients with acromegaly and in healthy controls, and the role of gender, age, and body mass index

Context: Besides the measurement of IGF-I, GH suppression during an oral glucose tolerance test is recommended to assess the biochemical status in acromegaly. However, the development of highly sensitive and specific GH assays necessitates a critical reevaluation of criteria for diagnosis and follow-up of disease activity. Objective: Our objective was to evaluate the between-method discrepancies in GH determinations by different immunoassays considering further confounders like age, gender, and body mass index (BMI). Design, Subjects, and Methods: We measured GH during a 75-g oral glucose tolerance test in 46 acromegaly patients (18 controlled, 28 uncontrolled; 19 men; 31-63 yr; BMI 26.4 +/- 0.4 kg/m(2)) and 213 healthy subjects (66 men; 20-76 yr; BMI 30 +/- 0.5 kg/m(2)), using three different commercially available assays [Immulite (Diagnostic Products Corp., Los Angeles, CA), Nichols (Nichols Institute Diagnostika GmbH, Bad Vilbel, Germany), and Diagnostic Systems Laboratories (Sinsheim, Germany)] that were calibrated against the recently recommended GH standards. Results: Results from all assays strongly correlated(r = 0.8-0.996; P < 0.0001). However, the results obtained with the Immulite assay were, on average, 2.3-fold higher than those obtained with Nichols and 6-fold higher than those obtained with Diagnostic Systems Laboratories. Using cutoff limits of 1 mu g/liter(Immulite) and 0.5 mu g/liter (Nichols) identified 95% of patients with active disease and 78-80% of patients in remission. Basal and nadir GH levels were significantly higher in females than in males (Immulite 2.2 +/- 0.28 mu g/liter vs. 0.73 +/- 0.15 mu g/liter and 0.16 +/- 0.01 mu g/liter vs. 0.08 +/- 0.01 mu g/liter; P < 0.001, respectively). In multiple regression analysis, age, BMI, and gender were predictors for basal and nadir GH levels. Conclusion: Postglucose GH-nadir values are assay, gender, age, and BMI specific, indicating the need of individual cutoff limits for each assay.