Fasting Blood Glucose
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p401-402
2025-10-29
65
The measurement of fasting blood glucose is a crucial test for the diagnosis of diabetes mellitus and for the screening of subjects at high risk of developing this disease. This measurement must be carried out in a laboratory, and great care must be taken in the appropriate collection and storage of the sample (preanalytical phase), in the choice of the method (analytical phase), and in the reporting and communication of results (post-analytical phase).
Preanalytical Aspects
Several preanalytical variables can affect blood glucose measurement.
As reported in the most recent recommendations on screening and diagnosis of diabetes mellitus, glycemia must be measured in venous plasma, respecting a fasting time of not less than 8 and not more than 12 h. The choice of the biological matrix to be used (whole blood, plasma, or serum) is essential because the results obtained on different matrices present a significant variation: glycemia measured in whole blood is lower than that measured in plasma since red blood cells contain less water than plasma and glucose dissolves in free water. The molality of glucose (i.e., the amount of glucose per unit mass of water) is identical in whole blood and plasma. Although red blood cells are freely permeable to glucose, the water content is about 93% in plasma and 73% in erythrocytes. Consequently, if the hematocrit is normal, plasma glucose is about 10–12% higher than whole blood glucose. Serum blood glucose is also about 5% higher than plasma.
Another critical aspect is the choice of glycolysis inhibitor, which must be contained in the tube if the plasma cannot be separated from the corpuscular elements within 30 min of collection.
To minimize glycolysis, guidelines recommend placing the tube in water with melting ice immediately after collection and separating the plasma from the cells by centrifugation.
The addition of sodium fluoride, an enolase inhibitor, produces an antiglycolytic action. However, a study has shown that this inhibition is lost within 4 h of collection. In this regard, SIBioC issued a formal recommendation in 2018 that the ternary mixture with citrate, sodium fluoride, and ethylenediaminetetraacetic acid (EDTA) should be used for blood glucose measurement for diagnostic purposes. Lithium heparin with sodium fluoride may still be used to monitor the ward’s glycemic control.
The glucose consumption rate in vitro, at room temperature, averages 5–7% per hour, which can be quantified as approximately 0.6 mmol/L (10 mg/dL). However, some variables, such as glucose concentration, storage temperature, the number of white blood cells, and hematocrit, can increase or decrease the rate of glycolysis in whole blood. Proper storage of the sample allows blood glucose to remain stable for a more extended period. Glucose concentration stability in separated, unhemolyzed, sterile serum without glycolysis inhibitors has been demonstrated for 8 h at 25 °C and 72 h if the sample is stored at 4 °C.
Intraindividual (CVi) and interindividual (CVg) biological variabilities were 4.5% and 5.8% on plasma and 5.6% and 7.5% on serum, respectively.
There is, moreover, a circadian variability of glycemia, determined by the secretion of hormones that influence glucosidic metabolism (growth hormone, cortisol, and corticotropin); the average values of fasting glycemia are therefore higher in the morning than in the afternoon. For this reason, it is recommended to have blood glucose measurements between 7 and 10 a.m.
Analytical Aspects
Blood glucose measurement is performed in most laboratories using well-standardized enzymatic methods. Although enzymatic methods based on glucose oxidase and glucose dehydrogenase are also commercially available, guidelines recommend using methods based on hexokinase. The latter, using ATP, catalyzes the phosphorylation of glucose into glucose-6-phosphate. Glucose-6-phosphate dehydrogenase, in the presence of nicotinamide adenine dinucleotide phosphate (NADP), causes the oxidation of glucose-6-phosphate to gluconate-6-phosphate. The rate of NADPH formation during the reaction is directly proportional to glucose con centration and is measured photometrically.
The methods based instead on glucose oxidase are more subject to interference from various types of substances (e.g., uric acid, bilirubin), which reduce hydrogen peroxide (H2O2) with the risk of underestimating glycemia. The influence of hemolysis and lipemia is negligible.
Based on biological variability, the glucose determination should have an analytical imprecision of <6.9%.
Postanalytical Aspects
Blood glucose values should be expressed in mmol/L, the unit of measure adopted by the international system and, as such, the international reference standard. However, blood glucose is still expressed in mg/dL in many laboratories.
Fasting plasma glucose concentration increases with increasing age between the third and sixth decades and then remains constant.
Extreme blood glucose values, that is, below 2.2 mmol/L (40 mg/dL) or above 27.7 mmol/L (500 mg/dL), should be reported as quickly as possible so that the clinician can implement measures to resolve the acute critical event (severe hypo or hyperglycemia).
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