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.
BACKGROUND
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 microg/liter (Immulite) and 0.5 microg/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 microg/liter vs. 0.73 +/- 0.15 microg/liter and 0.16 +/- 0.01 microg/liter vs. 0.08 +/- 0.01 microg/liter; P < 0.001, respectively). In multiple regression analysis, age, BMI, and gender were predictors for basal and nadir GH levels.


CONCLUSIONS
Postglucose GH-nadir values are assay, gender, age, and BMI specific, indicating the need of individual cutoff limits for each assay.
Arafat
Ayman M
AM
Department of Endocrinology, Diabetes and Nutrition, Charité-University Medicine Berlin, Campus Benjamin Franklin, Hindenburgdamm 30, 12200 Berlin, Germany. ayman.arafat@charite.de
Möhlig
Matthias
M
Weickert
Martin O
MO
Perschel
Frank H
FH
Purschwitz
Johannes
J
Spranger
Joachim
J
Strasburger
Christian J
CJ
Schöfl
Christof
C
Pfeiffer
Andreas F H
AF
D000172:Acromegaly / Q000188:drug therapy* / Q000378:metabolism; D000328:Adult; D000367:Age Factors; D000368:Aged; D015992:Body Mass Index; D002138:Calibration; D005260:Female; D005951:Glucose Tolerance Test; D019382:Human Growth Hormone / Q000097:blood / Q000627:therapeutic use*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012737:Sex Factors
D019382:Human Growth Hormone