Context: Besides the measurement of IGF-I, GH suppression during OGTT is recommended to assess the biochemical status in acromegaly. The development of highly sensitive and specific GH assays, however, necessitates a critical reevaluation of criteria for diagnosis and follow-up of disease activity.

Objective: To evaluate the between-method discrepancies in GH determinations by different immunoassays considering further confounders like age, gender, and BMI.

Design, Subjects, and Methods: We measured GH during 75-g OGTT in 46 acromegaly patients (18 controlled, 28 uncontrolled; 19 men; 31–63 yr; BMI 26.4 ± 0.4 kg/m²) and in 213 healthy subjects (66 men; 20–76; BMI 30 ± 0.5), using three different commercially available assays (Immulite, Nichols, and DSL) 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 DSL. Using cutoff limits of 1 µg/liter (Immulite) and 0.5 µg/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 (µg/liter): 2.2 ± 0.28 vs. 0.73 ± 0.15, and 0.16 ± 0.01 vs. 0.08 ± 0.01, P < 0.001, respectively]. In multiple regression analysis, age, BMI, and gender were predictors for basal and nadir GH levels.

Conclusions: Post-glucose GH-nadir values are assay-, gender-, age-, and BMI-specific, indicating the need of individual cutoff limits for each assay.