Effect of nitroblue tetrazolium concentration on the fructosamine assay for quantifying glycated protein

Experiments with nitroblue tetrazolium (NBT) at 0.25, 0.50, and 0.75 mmol/L confirmed that reactivity of 1-deoxy-1-morpholino-D-fructose, the standard, was more sensitive to reagent concentration than was protein ketoamine and demonstrated why an increase in NBT or a decrease in sample volume yields lower values for serum fructosamine. Analyses of clinical samples from diabetic and nondiabetic patients and from healthy subjects indicated that, with NBT at 0.25 mmol/L, discrimination between diabetics and other subjects was improved by changing the sample:reagent volume ratio from the usual 1:10 to 1:25 (9- to 12-min reading time). Discrimination improved further with NBT at 0.75 mmol/L and a 1:25 ratio but a later reading time (18-21 min); these conditions may minimize the effects of some but not all interferents.