Reference ranges are as follows:
Fasting plasma glucose: 70-99 mg/dL
Postprandial plasma glucose at 2 hours: Less than 140 mg/dL
Random plasma glucose: Less than 140 mg/dL
Serum glucose values are 1.15% lower than plasma glucose values. 
Values for diabetes mellitus are as follows:
Fasting plasma glucose: Greater than 125 mg/dL
Random plasma glucose: Greater than 200 mg/dL
Postprandial glucose at 2 hours: Greater than 200 mg/dL
Impaired fasting glucose: Fasting glucose of 100-125 mg/dL
Impaired glucose tolerance testing: Postprandial glucose at 2 hours of 140-200 mg/dL
The value for hypoglycemia is as follows:
Value less than 60 mg/dL
Collection and Panels
Collect 0.5-1 ml of blood in gray-top tube containing sodium fluoride. For serum glucose, a red-top tube can be used. Serum is separated within 45 minutes of collection.
For fasting glucose testing, collect the blood sample in the morning after an overnight or 8-hour fast.
For postprandial glucose testing, collect the blood sample 2 hours after a regular meal.
For oral glucose tolerance testing, after oral intake of 75 g of glucose, collect blood samples at 1 hour and 2 hours.
For gestational diabetes testing, parameters are 1 hour after 50 g of glucose and 2 hours after 100 g of glucose.
Glucose is a monosaccharide and is a primary metabolite for energy production in the body. Glucose enters via GLUT receptors. Of the 10 GLUT receptors, GLUT-4 receptors are present in muscle and adipose tissues and require insulin for glucose transport. Glucose is initially used by glycolysis and is converted to pyruvate.
During this process, 4 adenosine triphosphates (ATPs) and 2 NADHs are generated and 2 ATPs are used, resulting in net production of 2 ATPs. This process does not use oxygen (anaerobic metabolism). In the presence of oxygen, pyruvate undergoes metabolism by the Krebs cycle in the mitochondria. During the Krebs cycle, each molecule of pyruvate generates 2 ATPs, 8 NADHs, and 2 FADH2s. Each NADH is converted to 3 ATPs and each FADH2 is converted to 2 ATPs. NADHs generated during glycolysis require 1 ATP each to transport across the mitochondrial membrane to be converted to ATPs. Thus, during aerobic metabolism, each molecule of glucose can generate 36 ATPs. Glucose is primarily stored as glycogen in muscles and liver.
Glucose is measured by enzymatic methods. The following 3 enzymatic methods are available:
Hexokinase method: D glucose+ ATP hexokinase glucose 6-phosphate G-6PD +NADP 6 phosphogluconate + NADPH; absorbance of UV light by NADPH is measured
Glucose oxidase method: In the presence of oxygen and water, glucose oxidase converts glucose to D-glucono, 1,5 lactone, and hydrogen peroxide; hydrogen peroxide in the presence of a peroxidase enzyme oxidizes a colorless chromogenic substance to a colored substance
Glucose dehydrogenase method: D-glucose + NAD glucose dehydrogenase D-gluconolactone + NADH; absorbance of UV light by NADH is measured
Glucose testing is used for the following:
Screening/diagnosis of hyperglycemia
Monitoring control of diabetes mellitus
Screening/diagnosis of hypoglycemia
The American Diabetes Association has defined the following criteria for the diagnosis of diabetes mellitus  :
Impaired fasting glucose: 100-125 mg/dL
Diabetes mellitus: Fasting glucose value of at least 126 mg/dL confirmed by repeat testing
Impaired glucose tolerance: Blood glucose value of 140-200 mg/dL after 2 hours of a glucose load
Diabetes mellitus: Blood glucose value of greater than 200 mg/dL after 2 hours of a glucose load confirmed by repeat testing or other testing
Gestational diabetes mellitus: Blood glucose value of 95, 180, 155, or 140 mg/dL on fasting, 1 hour, 2 hours, or 3 hours, respectively, after 100-g glucose loading
Plasma is the preferred method for diagnoses of diabetes. 
To diagnose diabetes mellitus, testing should be repeated on a different day.
A very high WBC count can lead to a false low glucose level.
Morning fasting plasma glucose values are higher than afternoon glucose levels. 
In heparinized plasma, glucose concentrations are 5% lower than plasma.