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OGTT ineffective for assessing biochemical control in certain patients
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In patients treated with somatostatin analog therapy, using an oral glucose tolerance test provided no benefit for assessing disease status, according to study findings.
Researchers conducted a trial to determine the association among basal growth hormone and insulin-like growth factor I levels and GH responses to OGTT. From 1985 to 2008, 482 OGTT and IGF-I measurements were observed in this retrospective database review.
Testing of diagnosis, postoperative assessment without medication, testing during somatostatin analog therapy and testing during dopamine agonist therapy were performed in 166 patients with acromegaly.
Researchers observed discordant results for OGTT in 33% of postoperative assessment without medication, in 48% of somatostatin analog therapy and in 18% of dopamine agonist therapy.
In the dopamine agonist therapy group, there was 4% discordance with normal IGF-I and nonsuppressed GH vs. the somatostatin analog therapy group (42%). For basal GH, there was no statistically significant difference in discordance.
“The OGTT exhibits a high degree of discordance when performed to assess medical therapy outcomes,” the researchers wrote. – by Christen Haigh
J Clin Endocrinol Metab. 2009;94:523-527.
The degree of GH suppression during an OGTT, IGF-1 levels, single fasting and GH day curves have been investigated to establish the diagnosis, predict mortality and assess both cure and the adequacy of medical therapy with variable results in acromegaly. Increasingly, it has been recognized that the results of an OGTT and IGF-1 testing may be discordant in establishing cure after surgery. Timing of samples, assay issues and other factors have complicated the issue. The paper by Carmichael et al. demonstrated the discordance observed in OGTT and IGF-1 measurements from 483 tests over a 23-year period in patients with acromegaly. Although discordance was found in a large percentage of patients overall, 48% discordance was observed in somatostatin analogue-treated patients. Although GH suppression during an OGTT is one of the gold standards in the diagnosis of acromegaly, this study calls into question the common practice of obtaining an OGTT for GH suppression in somatostatin analogue-treated patients to assess therapeutic efficacy, and highlights the limitations of our commonly used tests.
– Anne Klibanski, MD
Professor of Medicine, Harvard Medical School