How can we trust Blood Glucose Test?

Glucose concentrations decrease in the test tube by 5–7% per hour due to glycolysis. Therefore, a sample with a true blood glucose value of 126 mg/dL would have a glucose concentration of ~110 mg/dL after 2 h at room temperature. Samples with increased concentrations of erythrocytes, white blood cells, or platelets have even greater rates of glycolysis. A common misconception is that sodium fluoride, an inhibitor of glycolysis, prevents glucose consumption. While fluoride does attenuate in vitro glycolysis, it has no effect on the rate of decline in glucose concentrations in the first 1 to 2 h after blood is collected, and glycolysis continues for up to 4 h in samples containing fluoride. The delay in the glucose stabilizing effect of fluoride is most likely the result of glucose metabolism proximal to the fluoride target enolase. After 4 h, fluoride maintains a stable glucose concentration for 72 h at room temperature. A recent publication showed that acidification of the blood sample inhibits glycolysis in the first 2 h after phlebotomy, but the collection tubes used in that study are not commercially available. Placing tubes in ice water immediately after collection may be the best method to stabilize glucose initially but this is not a practical solution in most clinical situations. Separating cells from plasma within minutes is also effective, but impractical.
The nature of the specimen analyzed can have a large influence on the glucose concentration. Glucose can be measured in whole blood, serum, or plasma, but plasma is recommended by both the ADA and World Health Organization (WHO) for diagnosis. However, many laboratories measure glucose in serum, and these values may differ from those in plasma. There is a lack of consensus in the published literature, with glucose concentrations in plasma reported to be lower than, higher than, or the same as those in serum. Importantly, glucose concentrations in whole blood are 11% lower than those in plasma because erythrocytes have a lower water content than plasma. The magnitude of the difference in glucose between whole blood and plasma changes with hematocrit. Most devices that measure glucose in capillary blood use whole blood. While the majority of these report a plasma equivalent glucose value this result is not accurate in patients with anemia (unless the meter measures hematocrit).
The source of the blood is another variable. Although not a substantial problem in the fasting state, capillary glucose concentrations can be 20–25% higher (mean of 30 mg/dL) than venous glucose during an OGTT. This finding has practical implications for the OGTT, particularly because the WHO deems capillary blood samples acceptable for the diagnosis of diabetes.
Fasting glucose concentrations vary considerably both in a single person from day to day and also between different subjects. Intraindividual variation in a healthy person is reported to be 5.7–8.3%, whereas interindividual variation of up to 12.5% has been observed. FPG can range from 112–140 mg/dL in an individual with an FPG of 126 mg/dL.
Numerous factors that occur before a sample is measured can influence results of blood tests. Examples include medications, venous stasis, posture, and sample handling. The concentration of glucose in the blood can be altered by food ingestion, prolonged fasting, or exercise. It is also important that measurements are performed in subjects in the absence of intercurrent illness, which frequently produces transient hyperglycemia. Similarly, acute stress (e.g., not being able to find parking or having to wait) can alter blood glucose concentrations.