Aims/Introduction: The effectiveness of incretin-based therapies in Asian type 2 diabetes requires investigation of the secretion and metabolism of glucose-dependent insulinotropic polypepide (GIP) and glucagon-like peptide 1 (GLP-1). Plasma extractions have been suggested to reduce variability in intact GLP-1 levels among individuals by removing interference that affects immunoassays, although no direct demonstration of this method has been reported. We have evaluated the effects of ethanol and solid-phase extractions on incretin immunoassays. We determined incretin levels during meal tolerance tests in Japanese patients with type 2 diabetes and characterized predictors for incretin secretion.
Materials and methods: Japanese patients with type 2 diabetes (23 anti-diabetic drug-naïve and 18 treated with sulfonylurea [SU] alone) were subjected to meal tolerance tests, and incretin levels were determined by immunoassays with or without extraction.
Results: Intact GLP-1 levels determined by an intact GLP-1 immunoassay with ethanol and solid-phase extractions were lower than those determined without extraction. Intact GLP-1 levels determined by the extractions were highly correlated with each other, much more so than the levels with and without extraction. Total GLP-1 was unaffected by extractions, showing that extractions remove interference only in the case of intact GLP-1. Incretin secretion after meal ingestion was similar between drug-naïve and SU-treated patients. Fasting and postprandial GLP-1 levels were correlated positively with fasting free fatty acids and negatively with dipeptidyl peptidase-4 activity.
Conclusions: Ethanol and solid-phase extractions remove interference for intact GLP-1 immunoassay. SU showed little effect on incretin secretion. GLP-1 and GIP secretion were predicted by different factors. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2011.00141.x, 2012).
Keywords: Extraction; Immunoassay; Incretin.