According to the most recent position statement of the American Diabetes Association, self-monitoring of blood glucose using POCT is considered an essential component in the management of the diabetic patient since it helps monitor glycemic control, detects hypoglycemia, assists in adjusting therapy, helps in understanding glycemic fluctuations, and can reduce or delay the onset of vascular, neurological, renal, and retinal complications.
Self-monitoring of blood glucose is carried out by sophisticated electronic devices that are easy to use and take up little space. There are different types of instruments on the market based on different principles.
A small blood sample is taken from the diabetic patient by superficial skin puncture, usually on the outside of the fingers or, if preferred, on the fingertip, the sites with the best blood supply. Alternative sampling sites (forearm or ear lobe) may also be used, especially in children. The drop of capillary blood (a few microliters) is then deposited on a strip inserted into the machine’s slot.
The principle on which the most common glucose meters are based is the quantification of an enzymatic reaction, the oxidation of glucose, which is proportional to the concentration in the blood. The reaction is carried out by placing a blood sample on a strip containing glucose oxidase (GOD) as the enzyme. During the oxidation reaction, gluconic acid and H2O2 are produced. Subsequent quantification of the reaction can be obtained either reflectometrically (measurement of light reflected from the colored test strip as a result of the chromogenic reaction catalyzed by glucose oxidase and peroxidase and subsequent conversion of the signal to glucose concentration) or impedance (electrical conductivity of blood induced by the electrical current generated by glucose oxidation). The electrons produced by the reaction form a current that is specifically calibrated to reflect the glucose concentration in whole blood.
In addition to devices based on the GOD reaction, there are others commercially available that use the reaction of glucose-dehydrogenase (GDH), an enzyme that catalyzes the formation of glucono-δ-lactone d NADH. Three different types of enzymes can be used in GDH-based glucose meters: glucose 1-dehydrogenase (1-GDH), glucose-6-phosphate 1-dehydrogenase (G6PDH), and quinoprotein glucose dehydrogenase (PQQ-GDH). As with all enzymes belonging to the class of dehydrogenases, they can have the natural cofactor (NAD or NADP) unbound or weakly bound to the enzyme (1-GDH and G6PDH) and therefore must necessarily be added to the glucose meter test strip or strongly bound to the enzyme as in the case of PQQ-GDH.
The heterogeneity of the devices, the modalities of use, and the related analytical techniques make necessary the training of the patient in the use of the glucometer (with particular attention to the preanalytical, analytical, and postanalytical aspects) and the periodic verification of the training. The choice of the device must be made based on a series of considerations, of which the economic aspect is only one of the points to be considered.
Preanalytical Aspects
Many preanalytical factors can affect the accuracy of blood glucose measurement, including patient preparation, storage, and use of strips. Preanalytical procedures are elementary but can be a source of important measurement errors, so, even if trivial, they must be explained very well to the patient. Before proceeding to the puncture site, a thorough cleansing of the skin is necessary, especially if foods with a high sugar content have been handled. The skin can be disinfected with alcohol, taking care that no residue remains that could hemolyze or dilute the drop of blood obtained. The choice of the type of lancing device (with a larger or smaller needle gauge) must be made by taking into account the skin thickness of the subject, who must undergo self-monitoring.
Other patient-related preanalytical factors, such as dis eases restricting circulation, excessive hydration, or extreme hematocrit levels, may make the blood glucose measurement unreliable.
Test strips should be stored correctly in their original containers and carefully resealed after each use. They should not be exposed to high temperatures, humidity, altitude, or other environmental factors that can interfere with chemical reactions. In addition to the strips, the instrument should be maintained in good condition and serviced periodically.
Analytical Aspects
Although self-monitoring of blood glucose (SMBG) devices have traditionally been held to less stringent analytical standards than laboratory tests, there are guidelines that define analytical goals. To meet analytical accuracy requirements, 95% of capillary measurements must be within ±15 mg/dL (±0.83 mmol/L) of the mean of the values determined by the reference procedure for glucose concentrations below 100 mg/dL (<5.5 mmol/L) or ± 15% for values above 100 mg/dL. However, not all available instrumentation achieved the same analytical standard.
The patient undergoing self-monitoring must implement specific procedures to ensure proper meter operation and quality of results. It is essential that calibration be performed for those meters that do not have a self-calibration feature. The calibration is a procedure by which you put the signal provided by the instrument (instrumental reading) with the quantity to be measured and correct the differences between different batches of strips. It can be done by inserting a code, chip, or calibrating strip, which is different from instrument to instrument. It improves accuracy, so it is a process that must be repeated over time. Calibration errors are a persistent cause of error.
Control materials, consisting of control strips or solutions of known concentration simulating the analytical signal detected by the system reader, shall be used to verify the quality of the data produced. This must be carried out when the instrument is first used, and then a new lot of strips are opened if the device is damaged or if there is any doubt about the reliability of the data.
It is expected that a periodic check for correlation or inter changeability with the central laboratory results will also be implemented on the POCT analyzers, which serves both to check that the defined comparability requirements between the two systems are maintained and to transfer the documented quality of the laboratory analyzer to the POCT system.
There are many substances (e.g., drugs) that can interfere with the chemical reactions taking place on the test strip. The most important of these are ascorbic acid, paracetamol, and maltose.
Very high plasma triglyceride concentrations, typically above 2000 mg/dL, can lead to an underestimation of blood glucose by a glucometer because they decrease the amount of glucose present in the capillary volume. High bilirubin concentrations (usually greater than 500 mg/dL), as seen in jaundiced infants, patients with hepatopathy, hepatitis, or some forms of anemia, can create interference, especially when using glucose meters based on the GDH method.
Ascorbic acid, commonly known as vitamin C, has the potential to interfere with glucometer measurements, although levels of 1–2 mg/dL do not generally result in significant interference. However, overuse of vitamin C can cause this type of interference.
Finally, the type of interference may depend on the chemistry of each test strip, so it may also vary from one meter to another.
As new glucose meters continue to appear on the market, it is helpful to continually consult the literature to see comparative assessments.
Postanalytical Aspects
Two main postanalytical issues need to be known: the international unit of measurement (mmol/L) choice and the use of plasma-calibrated strips. In 2001, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) recommended using plasma-calibrated strips, which transform the value obtained from whole blood into the value obtained from plasma. This allows the data obtained by the capillary blood glucose meter to be compared with the data obtained by measuring blood glucose in venous plasma.
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