Blood glucose testing
Venous plasma or serum has the advantage over whole blood of providing values for glucose that are independent of hematocrit and reflect levels in the interstitial spaces. For these reasons and because plasma and serum lend themselves to automated analytic procedures they are used in most laboratories. The glucose concentration is 10-15 % higher in plasma or serum than in whole blood because structural components of blood cells are absent. Whole blood glucose determinations are seldom used in clinical laboratories but have been used by diabetic patients during self-monitoring of capillary blood glucose, a technique widely accepted and recommended now the management of diabetes mellitus. Recently, however, many new reflectance meters have been modified to directly record serum glucose rather than to calculate whole blood glucose concentrations.
Venous blood samples
The laboratory methods regularly used for determining plasma glucose utilize enzymatic methods (such as
glucose oxidase or hexokinase), colorimetric methods (such as a toluidine), or automated methods. The automated methods utilize reduction of copper or iron compounds by reducing sugars in dialyzed serum. They are convenient but are not specific for glucose, since they react with other reducing substances (which are elevated in azotemia or with high ascorbic acid intake). Samples should be collected in tubes containing sodium fluoride, which prevents glycolysis in the blood sample that would artifactually lower the measured glucose level. If such tubes are not available, samples must be centrifuged within 30 minutes of collection and the plasma or serum stored at 4 °C.
Capillary blood samples
Several strip (glucose oxidase) based methods for use with portable, battery operated reflectance meters that give a digital readout are now available. First-generation reflectance meters required exact timing by the operator as well as careful removal of all traces of blood from the strip prior to reading of the colour. Second-generation devices have eliminated these two potential sources of technical error by providing automatic timing and allowing quantitation without removal of the blood. The timing required for glucose measurements by these meters varies from 12 seconds to 45 seconds and as little as 2-5 mL of blood are needed for analysis by most meters. To monitor their own blood glucose levels, patients must prick their fingers with a 21-gauge lancet, a procedure that can be facilitated by a small plastic trigger device. Third-generation devices are presently in the developmental stage and represent a noninvasive method relying on infrared absorption spectra, which allow quantitation of glycemia flowing through capillary beds of the finger or earlobe. Present pilot models are relatively large and expensive, but they appear to be accurate and have the great advantage of eliminating painful pricks to the fingers (8,9).
Testing for Ketonuria / Ketonemia
Commercial products are available to test for the presence of ketones in the urine. Most strips utilize a nitroprusside reaction that measures only acetone and acetoacetate. Although these tests do not detect b-hydroxybutyric acid, which lacks a ketone group, the semi quantitative estimation of the other ketone bodies is nonetheless usually adequate for clinical assessment of ketonuria. Other conditions besides diabetic ketoacidosis may cause ketone bodies to appear in the urine; these include starvation, high-fat diets, alcoholic ketoacidosis, fever, and other conditions in which metabolic requirements are increased. Serum ketone measurements, using strip-based technology has also now become available.
Glycosylated hemoglobin
Glycohemoglobin (GHb) is produced by a keto-amine reaction between glucose and the amino terminal amino acid of both beta chains of the hemoglobin molecule. The major form of glycohemoglobin (Hb A1C), which normally comprises only 4-6% of total hemoglobin, is abnormally elevated in diabetics. The glycosylation of hemoglobin is dependent on the concentration of blood glucose. The reaction is not reversible, so that the half-life of glycosylated hemoglobin relates to the life span of red cells (which normally circulate for up to 120 days). Glycohemoglobin generally reflects the state of glycemia over the preceding 8-12 weeks, thereby providing a method of assessing chronic diabetic control (10).
Venous plasma or serum has the advantage over whole blood of providing values for glucose that are independent of hematocrit and reflect levels in the interstitial spaces. For these reasons and because plasma and serum lend themselves to automated analytic procedures they are used in most laboratories. The glucose concentration is 10-15 % higher in plasma or serum than in whole blood because structural components of blood cells are absent. Whole blood glucose determinations are seldom used in clinical laboratories but have been used by diabetic patients during self-monitoring of capillary blood glucose, a technique widely accepted and recommended now the management of diabetes mellitus. Recently, however, many new reflectance meters have been modified to directly record serum glucose rather than to calculate whole blood glucose concentrations.
Venous blood samples
The laboratory methods regularly used for determining plasma glucose utilize enzymatic methods (such as glucose oxidase or hexokinase), colorimetric methods (such as a toluidine), or automated methods. The automated methods utilize reduction of copper or iron compounds by reducing sugars in dialyzed serum. They are convenient but are not specific for glucose, since they react with other reducing substances (which are elevated in azotemia or with high ascorbic acid intake). Samples should be collected in tubes containing sodium fluoride, which prevents glycolysis in the blood sample that would artifactually lower the measured glucose level. If such tubes are not available, samples must be centrifuged within 30 minutes of collection and the plasma or serum stored at 4 °C.
Capillary blood samples
Several strip (glucose oxidase) based methods for use with portable, battery operated reflectance meters that give a digital readout are now available. First-generation reflectance meters required exact timing by the operator as well as careful removal of all traces of blood from the strip prior to reading of the colour. Second-generation devices have eliminated these two potential sources of technical error by providing automatic timing and allowing quantitation without removal of the blood. The timing required for glucose measurements by these meters varies from 12 seconds to 45 seconds and as little as 2-5 mL of blood are needed for analysis by most meters. To monitor their own blood glucose levels, patients must prick their fingers with a 21-gauge lancet, a procedure that can be facilitated by a small plastic trigger device. Third-generation devices are presently in the developmental stage and represent a noninvasive method relying on infrared absorption spectra, which allow quantitation of glycemia flowing through capillary beds of the finger or earlobe. Present pilot models are relatively large and expensive, but they appear to be accurate and have the great advantage of eliminating painful pricks to the fingers (8,9).
Testing for Ketonuria / Ketonemia
Commercial products are available to test for the presence of ketones in the urine. Most strips utilize a nitroprusside reaction that measures only acetone and acetoacetate. Although these tests do not detect b-hydroxybutyric acid, which lacks a ketone group, the semi quantitative estimation of the other ketone bodies is nonetheless usually adequate for clinical assessment of ketonuria. Other conditions besides diabetic ketoacidosis may cause ketone bodies to appear in the urine; these include starvation, high-fat diets, alcoholic ketoacidosis, fever, and other conditions in which metabolic requirements are increased. Serum ketone measurements, using strip-based technology has also now become available.
Glycosylated hemoglobin
Glycohemoglobin (GHb) is produced by a keto-amine reaction between glucose and the amino terminal amino acid of both beta chains of the hemoglobin molecule. The major form of glycohemoglobin (Hb A1C), which normally comprises only 4-6% of total hemoglobin, is abnormally elevated in diabetics. The glycosylation of hemoglobin is dependent on the concentration of blood glucose. The reaction is not reversible, so that the half-life of glycosylated hemoglobin relates to the life span of red cells (which normally circulate for up to 120 days). Glycohemoglobin generally reflects the state of glycemia over the preceding 8-12 weeks, thereby providing a method of assessing chronic diabetic control (10).
