An improved enzymatic assay for glycated serum protein

Abstract

Recent studies have shown that blood HbA1c levels alone may not accurately reflect serum glucose concentrations in all diabetic patients. For certain diabetic patients, there exists a glycation gap. It was reported that the glycation gap information obtained by measuring HbA1c and glycated serum protein (GSP) or glycated albumin (GA) together may improve evaluation of diabetic patients by more reliably predicting complications of diabetes than HbA1c alone. Therefore, a new GSP assay for clinical use was developed and its performance was evaluated. Diazyme's enzymatic GSP assay (trademarked GlycoGap^®) is formulated with a 2-part liquid stable reagent system with a shelf-life of >15 months when stored at 2–8 °C. The assay was highly reproducible with within-run and total imprecisions of ≤1.3% CV. Method comparison studies showed good correlations with a previous powder version GSP assay (r² = 0.9966) and with Lucica^® GA-L assay (r² = 0.9746). The assay was linear within the range of 21–1354 μmol L⁻¹ with a reference range of 151–300 μmol L⁻¹ and was not affected by substances commonly found in human specimens such as ascorbic acid, bilirubin, hemoglobin, glucose, triglycerides, or uric acid. Diazyme's GSP assay was highly accurate with no interferences from endogenous reducing substances which interfere strongly with the traditional NBT based fructosamine assay. A conversion equation was developed to allow conversions of GSP values (μmol L⁻¹) into % of GA values, and a reference range for %GA was established for the US population. The relationship between %GA and %HbA1c was also investigated by measuring both %GA and %HbA1c values of blood samples from both diabetic and non-diabetic donors.