Definition

Glucose is the most important carbohydrate fuel in the body. In the fed state, the majority of circulating glucose comes from the diet; in the fasting state, gluconeogenesis and glycogenolysis maintain glucose concentrations. Very little glucose is found in the diet as glucose; most is found in more complex carbohydrates that are broken down to monosaccharides though the digestive process. About half of the total carbohydrates in the diet are in the form of polysaccharides and the remainder as simpler sugars. About two-thirds of the sugar in the diet is sucrose, which is a disaccharide of glucose and fructose. Glucose is classified as a monosaccharide because it cannot be broken down further by hydrolysis. It is further classified as a hexose because of its six-carbon skeleton and as an aldose, because of the presence of an aldehyde group on carbon 1. The aldehyde group condenses with a hydroxyl group so that glucose exists as a hemiacetal ring structure. This ring structure explains many of the reactions of glucose.

Ordinarily the concentration of glucose in the blood is maintained at a relatively stable concentration from 80 to 120 mg/dl. The strong reducing properties of glucose made it relatively easy to measure and thus the clinical estimation of circulating glucose was one of the earliest tests available to the clinician. The recent introduction of microglucose oxidase technology has now made it possible for the patient to measure his or her own blood glucose concentration and undoubtedly makes the estimation of blood glucose the most widely used test of blood chemistry. An understanding of the methods of blood glucose measurement will help the clinician to interpret values and avoid the pitfalls of inaccurate testing.

Technique

The concentration of glucose is highest in the arterial circulation. Laboratory determinations are usually done on venous samples. If the venous circulation is delayed, such as by leaving a tourniquet on for a prolonged period of time, the concentration falls even further. Thus, samples should be obtained after releasing the tourniquet. Studies have shown that blood glucose concentration may fall as much as 25 mg/dl when a tourniquet has been left in place for 6 minutes. The concentration of glucose in capillary samples is intermediate between venous and arterial. Warming the extremity increases the capillary flow and "arterializes" the sample, while cooling or a tourniquet decreases the flow and lowers the concentration of glucose.

Both red cells and leukocytes contain glycolytic enzymes. Therefore glucose will be consumed and the concentration of glucose in a sample of whole blood will decline with time. The rate of loss is generally said to be approximately 5% per hour, but may be as rapid as 40% in 3 hours. Consumption of glucose in whole blood samples can be prevented by adding sodium fluoride to the specimen to inhibit the glycolytic enzymes. This approach is the generally applied method in the clinical laboratory. It is effective except in situations where the system is overwhelmed, such as in specimens from patients with leukemia, which contain large numbers of leukocytes. Sodium fluoride has a major disadvantage in that its use makes the sample unacceptable for other determinations such as sodium and uric acid.

Rapid separation of the sample or cooling will also prevent glycolysis and will allow the sample to be used for other determinations. Unhemolyzed samples that have been separated within 30 minutes of drawing are generally considered adequate. Rapid cooling of the sample followed by centrifugation is even more effective in preventing glycolysis. These methods require more attention to detail and are therefore not suitable for routine laboratory use. On occasion, circumstances will require that the glucose concentration be determined on an ordinary serum sample. If the specimen has been promptly centrifuged, it is reasonable to ask the laboratory to measure the glucose concentration even though a sodium fluoride (green top) tube was not used.

Glucose concentration may be determined in whole blood, plasma, or serum samples. If whole blood is used, the concentration will be lower than if plasma or serum is used. This is due to the greater water content of the cellular fraction. Under usual circumstances, the concentration of glucose in whole blood is about 15% lower than in plasma or serum, but the difference will be less in patients with low hematocrits.

Blood glucose cannot be determined accurately on postmortem specimens because both glycogenolysis and glycolysis continue after death. A reasonable estimate of the antemortem blood glucose concentration can be obtained by measuring the glucose concentration of the vitreous of the eye, which does not contain glycolytic enzymes.

Glucose Tolerance Testing

In 1979, an international work group sponsored by the National Diabetes Data Group of the National Institutes of Health developed a classification framework and criteria for the diagnosis of diabetes mellitus. The recommendations are endorsed by the American Diabetes Association and other groups. The classification and criteria are summarized in Table 141.1. Frequently, the diagnosis of diabetes mellitus can be made based on elevated random or fasting blood sugars. In special circumstances, a glucose tolerance test is needed to establish the diagnosis.

Table 141.1

Classification of Glucose Intolerance.

The test should be done in the morning, after a 10- to 14-hour fast in which the patient is permitted only water. Patients are not allowed to smoke during the test. A fasting blood sugar is obtained. Adults are then given a drink containing 75 g of glucose (the 75 g dose is actually a compromise between the European custom of giving 50 g and the United States custom of giving 100 g). Children are given 1.75 g/kg ideal body weight up to 75 g, and pregnant patients are given 100 g of glucose (the older 100 g dose is retained for pregnant patients because all information on pregnancy outcome is based on tests using the 100 g load). Timing is begun when the patient begins to drink the glucose solution. Blood samples are obtained every 30 minutes for 2 hours in adults and children. For pregnant patients, blood samples are obtained every hour for 3 hours. Criteria for interpretation are given in Table 141.1.

Basic Science

There are three basic approaches to the laboratory measurement of blood glucose concentration: reducing methods, condensation methods, and enzymatic methods. Reducing methods are the oldest and take advantage of the reducing properties of glucose to change the state of a metal ion while glucose is being oxidized. Reducing methods are nonspecific, and any strong reducing agent can cross react to yield spuriously elevated values. While steps can be added to remove most cross-reacting reducing agents, this approach has largely been abandoned in the clinical laboratory.

The aldehyde group of glucose can undergo condensation with aromatic compounds to yield a colored product. In the most commonly used condensation reaction, o-toluidine reacts with glucose to form a glucosamine that has an intense green color. The color is then measured spectrophotometrically to estimate the glucose concentration. The reaction is rapid, and the intense color allows a high degree of sensitivity. Other aldoses can cross react, but only mannose and galactose give a highly colored product. These sugars are not found in great concentrations in the blood and their cross reactivity is ordinarily not significant. o-Toluidirie has the drawback of being highly corrosive and toxic. For this reason, this method is rapidly being phased out of the clinical laboratory.

The enzyme glucose oxidase reacts with glucose, water, and oxygen to form gluconic acid and hydrogen peroxide. The hydrogen peroxide can then be used to oxidize a chromogen or the consumption of oxygen measured to estimate the amount of glucose present. Glucose oxidase is specific for β-d-glucose, so cross reaction with other sugars is not a problem. In aqueous solution, approximately 66% of d-glucose is in the β state and 34% exists as α- d-glucose. The rate of interconversion is pH and temperature dependent. Some methods add a glucomutarostase to the reagents to speed up the conversion to the beta anomere, but this does not seem to alter the clinical results. The measurement of generated hydrogen peroxide is not as specific as the first glucose oxidase reaction. Numerous reducing substances can potentially inhibit the oxidation of the chromogen. Although uric acid and creatinine, even in uremic patients, seem to have little effect on the results, ascorbic acid will yield spuriously low blood glucose measurements. The high concentration of uric acid found in urine will affect the result and so glucose oxidase methods are not directly applicable to urine samples. The measurement of oxygen consumption using an oxygen-specific electrode avoids the problem of interfering reducing agents. In general, the glucose oxidase method is relatively inexpensive and specific.

Clinical Significance

References

1. Aziz S, Hsiang YH. Comparative study of home blood glucose monitoring devices: Visidex, Chemstrip bH, Glucometer, and Accu-Chek bG. Diabetes Care. 1983;6:529–32. [PubMed: 6653308]
2. Bergman M, Felig P. Self-monitoring of blood glucose levels in diabetes. Principals and practice. Arch Intern Med. 1984;144:2029–34. [PubMed: 6385899]
3. Blaedel WJ, Uhl JM. Nature of materials in serum that interfere in the glucose-oxidase–peroxidase-0-dianisidine method for glucose, and their mode of action. Clin Chim. 1975;21:119–24. [PubMed: 1116263]
4. Coe JI. Postmortem chemistry of blood, cerebrospinal fluid, and vitreous humor. Leg Med Annu, 1976. Wecht CH, ed. New York: Appleton-Century-Crofts, 1977;55–92. [PubMed: 325316]
5. Dietzler DN, Smith CH. Carbohydrates. In: Sonnenwirth AC,Jarett L, ed. Gradwohl's clinical laboratory methods and diagnosis. St. Louis: CV Mosby, 1980;210–49.
6. Drucker RF, Williams DR, Price CP. Quality assessment of blood glucose monitors in use outside the hospital laboratory. J Clin Pathol. 1984;36:948–53. [PMC free article: PMC498428] [PubMed: 6875021]
7. Goldstein DE, Parker KM, England JD. et al. Clinical application of glycosylated hemoglobin measurements. Diabetes. 1982;31(suppl3):70–78.
8. Hanrahan JB, Sax SM, Cillo A. Factitious hypoglycemia in patients with leukemia. Am J Clin Pathol. 1963;40:43–45. [PubMed: 13952575]
9. Hotkamp HC, Verhoef NJ, Leijnse B. The difference between the glucose concentration in plasma and whole blood. Clin Chim Acta. 1975;59:41–49. [PubMed: 1122647]
10. Howanitz PJ, Howanitz JH. Carbohydrates. In: Henry JB, ed. Clinical diagnosis and management by laboratory methods. Philadelphia: W.B. Saunders, 1984; 168–69.
11. Lombrail P, Cathelineau G, Gervais P. et al. Abnormal color vision and reliable self-monitoring of blood glucose. Diabetes Care. 1984;7:318–21. [PubMed: 6331996]
12. Loughlin WC, Mosenthal HO, Halpern R. Effect of tourniquets on venous blood sugar values. J Lab Clin Med. 1943;28:1165–70.
13. National Diabetes Data Group. Classification and diagnosis of diabetes mellitus and other categories of glucose intolerance. Diabetes. 1979;28:1039–57. [PubMed: 510803]
14. Overfield CV, Savory J, Heintges MG. Glycolysis: a re-evaluation of the effect on blood glucose. Clin Chim Acta. 1972;39:35–40. [PubMed: 5038762]
15. Yatscoff RD. et al. Interference of fetal hemoglobin and labile glycosylated hemoglobin with measurements of glycosylated hemoglobin. Clin Chim. 1983;29:543–49. [PubMed: 6186416]