Four generations of blood glucose meter, c. 1993-2005. Sample sizes vary from 30 to 0.3 μl. Test times vary from 5 seconds to 2 minutes (modern meters are typically below 15 seconds).
Glucose meters
A glucose meter (or glucometer) is a medical device for determining the approximate concentration of glucose in the blood. It is a key element of home blood glucose monitoring (HBGM) by people with diabetes or with proneness to hypoglycemia. A small drop of blood obtained by pricking the skin with a lancet is placed on a disposable test strip, which the meter reads and uses to calculate the blood glucose level. The meter then displays the level in mg/dl or mmol/l.[1]
Since approximately 1980, a primary goal of the management of type 1 diabetes has been the achievement of closer-to-normal levels of glucose in the blood for as much of the time as possible, guided by HBGM several times a day. The benefits include a reduction in the occurrence rate and severity of long-term complications from hyperglycemia as well as a reduction in the short-term, potentially life-threatening complications of hypoglycemia.
Characteristics
There are now dozens of models of glucose meters. There are several key characteristics that may differ from model to model:
- Size: The average size is now approximately the size of the palm of the hand, though some are smaller or larger. They are battery-powered.
- Test strips: A consumable element containing chemicals that react with glucose in the drop of blood is used for each measurement. For most models this element is a plastic test strip with a small spot impregnated with glucose oxidase and other components. Each strip can only be used once and is then discarded. Instead of strips, some models use discs that may be used for several readings.
- Coding: Since test strips may vary from batch to batch, some models require the user to enter in a code that may be found on the vial of test strips. By entering the code into the glucose meter, the meter will be calibrated to the batch of test strips. Some models may not require coding.
- Volume of blood sample: The size of the drop of blood needed by different models varies from 0.3 to 10 μl. (Older models required larger blood samples, usually defined as a "hanging drop" from the fingertip.) Smaller volume requirements reduce the frequency of unproductive pricks.
- Alternative site testing: Smaller drop volumes have enabled "alternate site testing" — pricking the forearms or other less sensitive areas instead of the fingertips. Although less uncomfortable, readings obtained from forearm blood lag behind fingertip blood in reflecting rapidly changing glucose levels in the rest of the body.
- Testing times: The times it takes to read a test strip may range from 5 to 60 seconds for different models.
- Display: The glucose value in mg/dl or mmol/l is displayed in a small window. The preferred measurement unit varies by country. mg/dl are preferred in the US, mmol/l in the rest of the world. To convert mmol/l of glucose to mg/dl, multiply by 18. To convert mg/dl of glucose to mmol/l, divide by 18 or multiply by 0.055. Many machines can toggle between both types of measurements and there have been a couple of published instances in which someone with diabetes has been misled into the wrong action by assuming that a reading in mmol/l was really a very low reading in mg/dl, or the converse. Other machines are pre-set at the factory and cannot be changed.
- Whole blood glucose vs. plasma glucose: Glucose levels in plasma (one of the components of blood) are generally 10%–15% higher than glucose measurements in whole blood (and even more after eating). This is important because home blood glucose meters measure the glucose in whole blood while most lab tests measure the glucose in plasma. Currently, there are many meters on the market that give results as "plasma equivalent," even though they are measuring whole blood glucose. The plasma equivalent is calculated from the whole blood glucose reading using an equation built into the glucose meter. This allows patients to easily compare their glucose measurements in a lab test and at home. It is important for you and your healthcare provider to know whether your meter gives its results as "whole blood equivalent" or "plasma equivalent."
- Clock/memory: All meters now include a clock that is set for date and time, and a memory for past test results. The memory is an important aspect of diabetes care, as it enables the person with diabetes to keep a record of management and look for trends and patterns in blood glucose levels over days. Most memory chips can display an average of recent glucose readings.
- Data transfer: Many meters now have more sophisticated data handling capabilities. Many can be downloaded by a cable or infrared to a computer that has diabetes management software to display the test results. Some meters allow entry of additional data throughout the day, such as insulin dose, amounts of carbohydrates eaten, or exercise. A number of meters have been combined with other devices, such as insulin injection devices, PDAs, and even Game Boys.[1] A radio link to an insulin pump allows automatic transfer of glucose readings to a calculator that assists the wearer in deciding on an appropriate insulin dose. One model also measures beta-hydroxybutyrate in the blood to detect ketoacidosis (ketosis).
- Hospital glucose meters: Special glucose meters for multi-patient hospital use are now used. These provide more elaborate quality control records, and the data handling capabilities are designed to transfer glucoses into electronic medical records and the laboratory computer systems for billing purposes.
Cost
The cost of home blood glucose monitoring is substantial due to the cost of the test strips. In 2006, the consumer cost of each glucose strip ranged from about $0.35 to $1.00, so that testing four times a day costs about $1.40–$4 a day. Manufacturers often provide meters at no cost to induce use of the profitable test strips (the so-called razor and blades business model, wherein the durable unit is free or cheap, but the consumables are extremely expensive). Type 1 diabetics test as often as 10–12 times a day due to the dynamics of insulin adjustment, whereas type 2 test less frequently, especially when insulin is not part of treatment.
Some batches of counterfeit test strips for some meters have been identified, and these have been shown to produce inaccurate results. They should not be used and should be reported to the supposed manufacturer.
Accuracy
Accuracy of glucose meters is a common topic of clinical concern. Nearly all of the meters have similar accuracy (±10-15%) when used optimally. However, a variety of factors can affect the accuracy of a test. Factors affecting accuracy of various meters have included calibration of meter, ambient temperature, pressure use to wipe off strip, size of blood sample, high levels of certain drugs in blood, hematocrit, dirt on meter, humidity, and aging of test strips. Models vary in their susceptibility to these factors, and in their ability to prevent or warn of inaccurate results with error messages. The Clarke error grid is a common way of analyzing and displaying accuracy of readings related to management consequences. It is also important to remember that if you have sugar on your finger the reading can be much too high. For example, if you are low, eat something sweet and take a test without cleaning your finger. This error is much higher than a badly calibrated meter.
Future
Development of noninvasive devices may enable continuous monitoring. Research is being done on noninvasive methods for measuring blood glucose, such as using electric currents and ultrasound.
There is one noninvasive glucose meter that has been approved by the FDA: The GlucoWatch G2 Biographer is designed to be worn on the wrist, and it uses electric fields to draw out body fluid for testing. The device does not replace conventional blood glucose monitoring. One limitation is that the GlucoWatch system is not able to cope with perspiration at the measurement site. The sweat must be allowed to dry before measurement can resume. Due to these limitations and others, the product is no longer on the market.
It is speculated that within the next decade, meters may be replaced with continuous glucose sensors for many people with diabetes. This will likely decrease complications found in people with diabetes by limiting problems associated with hyperglycemia and hypoglycemia.
There are currently 2 CGMS (continuous glucose monitoring system) available. The first is Medtronic's Minimed Paradigm RTS with a sub-cutaneous probe attached to a small transmitter (roughly the size of a quarter) that sends interstitial glucose levels to a small pager sized receiver every 5 minutes. As well, the DexCom™ STS® System is available (2Q 2006). It is a hypodermic probe with a small transmitter. The receiver is about the size of a cell phone and can operate up to five feet from the transmitter. Aside from a two hour calibration period, monitoring is logged at five-minute intervals for up to 72 hours. High and low glucose alarms are user-settable.
There is currently an effort to develop an integrated treatment system with a glucose meter, insulin pump and wristop controller.
Technology
So far all glucose meters have in some way employed the oxidation of glucose to gluconolactone catalyzed by glucose oxidase.
The first-generation devices relied on the same colorimetric reaction that is still used nowadays in glucose test strips for urine. Besides glucose oxidase, the test kit contains a benzidine derivative, which is oxidized to a blue polymer by the hydrogen peroxide formed in the oxidation reaction. The disadvantage of this method was that the test strip had to be developed after a precise interval (the blood had to be washed away), and the meter needed to be calibrated frequently.
Today's glucometers use a coulometric method. Test strips contain a capillary that sucks up a reproducible amount of blood and an enzyme electrode containing glucose oxidase. The enzyme is reoxidized with an excess of ferrocyanide ion. The total charge passing through the electrode is measured and is proportional to the concentration of glucose in the blood.
Meter use for hypoglycemia
Although the apparent value of immediate measurement of blood glucose might seem to be higher for hypoglycemia than hyperglycemia, meters have been less useful. The primary problems are precision and ratio of false positive and negative results. An imprecision of ±15% is less of a problem for high glucose levels than low. There is little difference in the management of a glucose of 200 mg/dl compared with 260 (i.e., a "true" glucose of 230±15%), but the difference between 70 mg/dl and 55 (i.e., 67±15%) represents a more unsatisfactory uncertainty.
The imprecision is compounded by the relative likelihoods of false positives and negatives in populations with diabetes and those without. People with type 1 diabetes usually have glucose levels above normal, often ranging from 40 to 500 mg/dl (2.2 to 28 mmol/l), and when a meter reading of 50 or 70 (2.8 or 3.9 mmol/l) is accompanied by their usual hypoglycemic symptoms, there is little uncertainty about the reading representing a "true positive" and little harm done if it is a "false positive."
In contrast, people who do not have diabetes but periodically have hypoglycemic symptoms will have a much higher rate of false positives to true, and a meter is not accurate enough to base a diagnosis of hypoglycemia upon. A meter can occasionally be useful in the monitoring of severe types of hypoglycemia (e.g., congenital hyperinsulinism), to ensure that the average glucoses when fasting remain above 70 mg/dl (3.9 mmol/l).