A comparison between three automated chemiluminescence assays for growth hormone: on the use of recombinant hGH as a primary calibrant

Abstract

Objectives: GH measurements during OGTT have profound effects on therapy and follow-up management of acromegaly. To minimize the discordance between immunoassays, it is recommended to calibrate them using 22kDa-GH-preparation instead of the use of pituitary-derived calibrants. The aim of our study was to evaluate the between-method discrepancies in GH determinations by assays using different calibrants considering further confounders like age, gender, and BMI. Methods: GH was measured during a 75-g OGTT in 21 acromegaly patients (9 controlled; 6 men; age 34–63 years; BMI 25.8±0.7kg/m2) and in 207 apparently healthy subjects (64 men; age 20–76; BMI 30±0.5) using three Chemiluminescence-assays. Immulite2000 and Nichols were calibrated against NIBSC2ndIS98/574 for hGH, whereas LIAISON used recombinant-hGH as a primary calibrant. The Nichols-assay was re-calibrated using rhGH-preparation. Results: The results obtained with all assays were strongly correlated in both acromegaly patients and controls (r=0.96 and 0.99, respectively; p<0.001). The results obtained with Immulite2000 were 2-fold higher than those obtained with LIAISON and Nichols; the readings of the two latter methods were in good agreement. In controls, nadir GH concentrations were (95% percentile) 0.25, 0.41 and 0.2 for LIAISON, Immulite 2000 and Nichols, respectively. Using cut-off limits of 0.5µg/l (LIAISON) and 1µg/l (Immulite2000) identified 92–100% of patients with active disease and 67% of patients in remission. Both basal and nadir GH levels were significantly higher in females than in males (LIAISON: 1.3±0.17 vs. 0.46±0.1µg/L and 0.13±0.01 vs. 0.1±0.02µg/L, p<0.001, respectively). In multiple regression analysis age, BMI and gender were independent predictors for both the basal and the nadir GH levels (standardized β: –0.2, -0.3 and 0.2, respectively). Conclusions: Post-glucose GH-nadir values are assay-, gender-, age-, and BMI-specific. The use of rhGH as a primary calibrant is advisable to reduce the between-assay differences in GH measurements.