Glucose, Plasma

According to the ADA, a fasting glucose >125 mg/dL on more than one occasion is adequate for the diagnosis of diabetes mellitus.

The ADA further classifies an intermediate group of individuals whose glucose levels do not meet criteria for diabetes yet are higher than those considered normal. These individuals were defined as having impaired fasting glucose (IFG) in plasma between 100-125 mg/dL.¹

The ADA defines three levels of hypoglycemia, with Level 1 cut-point being the most sensitive for detection.² Level 1 hypoglycemia is defined as a measurable glucose concentration <70 mg/dL but >53 mg/dL. A blood glucose concentration of 70 mg/dL has been recognized as a threshold for neuroendocrine responses to failing glucose in people with diabetes. Because many people with diabetes demonstrate impaired counterregulatory responses to hypoglycemia and/or experience hypoglycemia unawareness, a measured glucose level <70 mg/dL is considered clinically important.

Criteria for diagnosis of diabetes include²:

A1C >6.4% (utilizing NGSP certified and standardized to the DCCT assay);* or

Fasting glucose level >125 mg/dL;* or

A two-hour plasma glucose >199 mg/dL during an OGTT;* or

Classic symptoms of hyperglycemia or hyperglycemic crisis, with random plasma glucose >199 mg/dL.

*In the absence of unequivocal hyperglycemia, criteria 1-3 should be confirmed by repeat testing.

Recent evidence revealed a diurnal variation in FPG, with mean FPG higher in the morning than in the afternoon, indicating that many cases of undiagnosed diabetes would be missed in patients seen in the afternoon. Glucose concentrations decrease ex vivo with time in whole blood because of glycolysis. The rate of glycolysis, reported to average 5% to 7% [~0.6 mmol/L (10 mg/dL)] per hour, varies with the glucose concentration, temperature, white blood cell count and other factors. Glycolysis can be attenuated by inhibition of enolase with sodium fluoride (2.5 mg fluoride/mL of blood) or, less commonly, lithium iodacetate (0.5 mg/mL of blood). These reagents can be used alone or, more commonly, with anticoagulants such as potassium oxalate, EDTA, citrate or lithium heparin. Although fluoride maintains long-term glucose stability, the rate of decline of glucose in the first hour after sample collection in tubes with and without fluoride is virtually identical. (Note that leukocytosis will increase glycolysis even in the presence of fluoride if the white cell count is very high). After four hours, the glucose concentration is stable in whole blood for 72 hours at room temperature in the presence of fluoride. In separated, nonhemolyzed, sterile serum without fluoride, the glucose concentration is stable for 14 days at 25°C and 4°C.