<P> The fasting blood glucose level, which is measured after a fast of 8 hours, is the most commonly used indication of overall glucose homeostasis, largely because disturbing events such as food intake are avoided . Conditions affecting glucose levels are shown in the table below . Abnormalities in these test results are due to problems in the multiple control mechanism of glucose regulation . </P> <P> The metabolic response to a carbohydrate challenge is conveniently assessed by a postprandial glucose level drawn 2 hours after a meal or a glucose load . In addition, the glucose tolerance test, consisting of several timed measurements after a standardized amount of oral glucose intake, is used to aid in the diagnosis of diabetes . </P> <P> Error rates for blood glucose measurements systems vary, depending on laboratories, and on the methods used . Colorimetry techniques can be biased by color changes in test strips (from airborne or finger borne contamination, perhaps) or interference (e.g., tinting contaminants) with light source or the light sensor . Electrical techniques are less susceptible to these errors, though not to others . In home use, the most important issue is not accuracy, but trend . Thus if a meter / test strip system is consistently wrong by 10%, there will be little consequence, as long as changes (e.g., due to exercise or medication adjustments) are properly tracked . In the US, home use blood test meters must be approved by the federal Food and Drug Administration before they can be sold . </P> <P> Finally, there are several influences on blood glucose level aside from food intake . Infection, for instance, tends to change blood glucose levels, as does stress either physical or psychological . Exercise, especially if prolonged or long after the most recent meal, will have an effect as well . In the typical person, maintenance of blood glucose at near constant levels will nevertheless be quite effective . </P>

Which of the following is the reference method for measuring serum glucose