Glucose can be measured in whole blood, serum, or plasma, but plasma is recommended for diagnosis. The molality of glucose (i.e., amount of glucose per unit water mass) in whole blood and plasma is identical. Although red blood cells are essentially freely permeable to glucose (glucose is taken up by facilitated transport), the concentration of water (kg/L) in plasma is ~11% higher than that of whole blood. Therefore, glucose concentrations in plasma are ~11% higher than whole blood if the hematocrit is normal. Glucose concentrations in heparinized plasma are reported to be 5% lower than in serum. The reasons for the latter difference are not apparent, but may be attributable to the shift in fluid from erythrocytes to plasma caused by anticoagulants. The glucose concentrations during an OGTT in capillary blood are significantly higher than those in venous blood [mean of 1.7 mmol/L (30 mg/dL), equivalent to 20% to 25%], but the mean difference in fasting samples is only 0.1 mmol/L (2 mg/dL).

Although methods for glucose analysis exhibit low imprecision at the diagnostic decision limits of 7.0 mmol/L [(126 mg/dL), fasting] and 11.1 mmol/L [(200 mg/dL), post glucose load], the relatively large intraindividual biological variablilty (CVs of ~5% to 7%) may produce classification errors. On the basis of biological variation, glucose analysis should have analytical imprecision <3.4%, bias <2.6% and total error <8.0%.[1,2]