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Excerpt
Blood glucose measurement is one of the most frequently performed laboratory tests in hospitals. Many instruments utilizing different technologies are available for glucose measurement. In the past, glucose measurement was based on reduction reactions (i.e., copper, ferricyanide, o-tolidine). Today, most methods of analysis use enzymes as reagents (i.e., hexokinase, glucose oxidase, glucose dehydrogenase). These enzymatic assays are highly specific for glucose.
The most common methods for measuring bedside blood glucose (including those that utilize dry chemistry reagent strips) use glucose oxidase. Glucose plus oxygen and water are converted to gluconic acid plus hydrogen peroxide. The enzyme, peroxidase, and a chromogenic oxygen acceptor (such as o-dianisidine, 4-amino-phenazone, o-tolidine) produce a color which can be measured and used to quantitate glucose concentration. However, the peroxidase reaction can be inhibited by uric acid (>150 mg/L), ascorbic acid (>250 mg/L), bilirubin, glutathione, creatinine, L-cysteine (>1.5 g/L), L-dopa (>100 mg/L), dopamine, methyldopa, and citric acid (>15 g/L). Normal therapeutic concentrations of these substances in blood do not interfere with the assay. However, high concentrations of these substances, as found in the urine, can compete with the chromogen and result in a lower glucose concentration. Others have reported the glucose oxidase method to be affected by hematocrit (lower blood glucose at higher hematocrits), plasma protein concentration, temperature of the sample (enzymatic reaction decreased by lower temperature), sample pH, and acetaminophen concentration. Strong oxidizing substances such as hypochlorite and chlorine bleach (which may be used to disinfect containers) can produce a false-positive reaction.
In this issue of Critical Care Medicine, Dr. Kurahashi and colleagues [1] report on the use of a bedside analyzer which uses an enzyme-electrode method for measuring glucose. This method utilizes glucose oxidase to convert beta-D-glucose to D-gluconic acid. In the process, yellow ferricyanide is converted to colorless ferrocyanide plus hydrogen ions. The ferrocyanide is oxidized at the anode while hydrogen ion is reduced at the cathode, generating an electric current. The current is measured by a glucometer and converted to a glucose concentration. However, the oxygen tension of the blood sample can alter the glucose measurement. In the presence of oxygen, beta-D-glucose is also converted to D-gluconic acid and hydrogen peroxide by glucose oxidase. This reaction reduces the amount of beta-D-glucose available to react with ferricyanide. The result is a lower perceived glucose concentration. Although ambient oxygen concentrations have little influence on this reaction, high PO2 values can increase the reaction. These investigators [1] report that their bedside blood glucose analyzer underestimated blood glucose concentration (when compared with an oxygen independent laboratory analyzer) and that the underestimate was related to a sample oxygen tension. The degree of underestimation at high PO sub 2 (>250 torr; >33.3 kPa) was clinically significant and could lead to unnecessary treatment of the patient with glucose or decrease in insulin dosage. Dr. Kurahashi and colleagues [1] did not find that their glucose measurements were altered by hematocrit, plasma protein concentration, creatinine, uric acid, or bilirubin concentrations.
Some instruments use a polarographic oxygen electrode which measures the rate of oxygen consumption after addition of the sample to a solution of glucose. The rate of decrease in PO2 is a measure of glucose concentration. Glucose can be quickly determined without requiring the reaction to go to completion. This reaction does not use peroxidase, and interferences with the peroxidase step are eliminated. However, the reaction is affected by whole blood since blood cells consume oxygen. The reaction is best used with plasma, serum, urine, and cerebrospinal fluid.
The hexose kinase method involves the phosphorylation of glucose with adenosine triphosphate.
