Fasting plasma glucose is the preferred screening test recommended by the American Diabetes Association for the diagnosis of diabetes in children and non pregnant adults (1). If abnormal, the test should be repeated on at least one additional day to confirm the diagnosis of diabetes (4). The test should be performed after an 8 hour fast. For routine clinical practice, fasting plasma glucose is preferred over the oral glucose tolerance test for the diagnosis of diabetes because it is rapid, easier to administer, is more convenient for patients and providers, and has a lower cost (1). The use of the oral glucose tolerance test, however, may be warranted in high risk individuals with normal fasting plasma glucose levels.

A casual plasma glucose level, which is obtained at any time of the day regardless of the time of the last meal, can be used in individuals with symptoms of hyperglycemia. A casual plasma glucose level of >200 mg/dl (11.1 mmol/l) is diagnostic of diabetes. In the absence of acute metabolic decompensation, the diagnosis should be confirmed by repeating the casual plasma glucose or by obtaining a fasting plasma glucose level on at least one additional occasion.

For the diagnosis of diabetes, standard venous plasma glucose specimens should be obtained. Specimens should be processed promptly, since glucose is metabolized at room temperature and the process is influenced by storage temperature and storage time. This breakdown is accelerated in the presence of bacteria or leukocytosis. If serum is in contact with cells for a prolonged length of time (>30 min), the addition of a preservative such as sodium fluoride is recommended.

Whole blood glucose specimens obtained with point-of-care devices should not be used for the diagnosis of diabetes because of the inaccuracies associated with these methods. Capillary and venous whole blood glucose concentrations are approximately 15% lower than plasma glucose levels in fasting specimens.