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the following article is from a website called "Uptodate" an online resource healthcare professions often use to stay current and this is what they have to say about monitoring blood glucose. If you send me your e-mail over the private messaging system, I can also have it e-mailed to you from the site as well (the site is a subscription only site that I get access to from my med school's dorm computers).
I hope that the following information is useful to you. The specific information that you are looking for is under the heading "self-monitoring of blood glucose"
Blood glucose monitoring in management of diabetes mellitus
David K McCulloch, MD
UpToDate performs a continuous review of over 330 journals and other resources. Updates are added as important new information is published. The literature review for version 13.1 is current through December 2004; this topic was last changed on October 27, 2004. The next version of UpToDate (13.2) will be released in June 2005.
INTRODUCTION — Most patients with diabetes mellitus who use glucose lowering medications (especially insulin) and want to maintain good glycemic control need to measure their blood glucose concentrations often. This requires intermittent capillary blood sampling and the use of a glucose meter. In addition to self-monitoring of blood glucose (SMBG), periodic measurement of glycosylated hemoglobin (HbA1c) permits estimation of chronic glycemic control. Several practical points about blood glucose monitoring will be reviewed here, including the accuracy of glucose meters and glucose sticks, the accuracy of the operator, and how to use the information that is obtained. The use of HbA1c measurements to estimate mean blood glucose is reviewed elsewhere. (See "Estimation of blood glucose control in diabetes mellitus").
SELF-MONITORING OF BLOOD GLUCOSE (SMBG) — The American Diabetes Association (ADA) recommends that patients with type 1 diabetes monitor blood glucose at least three times daily, and patients with type 2 diabetes who are treated with insulin or oral hypoglycemic drugs monitor blood glucose daily <1,2>. Self-monitoring of blood glucose is especially important for glycemic control in patients with type 1 diabetes, because their blood glucose concentrations are less stable from day to day than are those in patients with type 2 diabetes.
In type 2 diabetes, the fasting blood glucose concentration is often used to monitor progress since it correlates well with HbA1c values <3,4>, although some authors have argued that nonfasting blood glucose measurements are a better marker of glycemic control than fasting values <4>. In practice, the actual frequency of monitoring is individualized. In some patients, testing before and at intervals after meals and occasionally during the night provides useful additional information.
The effectiveness of SMBG in terms of improving glycemic control in patients with type 2 diabetes is less clear than for type 1 diabetes <5,6>:
Data from the third National Health and Nutrition Examination Survey (NHANES III) reported no correlation between the frequency of monitoring in patients with type 2 diabetes and their HbA1c values <7>.
In contrast, a cohort study of 24,312 patients with diabetes who were members of a group model health maintenance organization found that more frequent self-monitoring of blood glucose was associated with improved glycemic control regardless of diabetes type <8>. For patients with type 2 diabetes, adherence to ADA monitoring guidelines resulted in an improvement in HbA1c of approximately 0.6 percent compared with nonadherent patients.
Although randomized trials of SMBG in type 2 diabetes have been conducted, none have conclusively demonstrated a benefit <9>.
Monitoring blood glucose is a tool, not a therapeutic intervention. It provides important information with which motivated patients can modify their behavior and improve their HbA1c values safely. Fasting blood glucose concentrations are fairly stable in patients with type 2 diabetes, but can vary by about 15 percent from day to day, and therefore changes in therapy should be based on an average over several days <10>.
URINE TESTING — Although measuring urine glucose may be much easier than measuring blood glucose, it has potential errors that limit its accuracy as a reflection of glycemic control and is rarely used <11>.
Testing for ketonuria — Measurement of urinary ketones is less subject to error because any positive value suggests the presence of ketonemia. The urine should be tested for ketones if the blood glucose concentration is above 240 mg/dL (13.3 mmol/L), during periods of illness or stress, or if there are symptoms compatible with ketoacidosis such as nausea, vomiting, and abdominal pain <11>.
SOURCES OF ERROR
Blood glucose meters — In response to complaints by consumers about the performance of some blood glucose meters, the Food and Drug Administration (FDA) performed a series of studies to evaluate the accuracy of many of the available meters <12>. Their conclusion was that most glucose meters are reasonably accurate, and require only a small drop of blood. Even with newer meters, however, accuracy during episodes of hypoglycemia may be less than optimal <13,14>.
In the past, glucose meters reported whole blood glucose values, which made it difficult to compare finger stick results with results from a laboratory, which are always plasma. However, the majority of available glucose meters now provide plasma rather than whole blood glucose values (by providing direct plasma readings or by multiplying the whole blood value by 1.12). Thus, results from most available glucose meters and commercial laboratories should now be comparable.
Several blood glucose meters are now available that use sites other than the finger to obtain blood samples in an effort to reduce the discomfort involved with fingersticks. A study of one of these devices that obtains samples from the arm found that it provided accurate results and was less painful than fingerstick testing <15>. Monitoring from alternate sites, such as the skin of the forearm, may give results which are somewhat lower than those taken at the fingertips, since they may sample venous blood rather than capillary blood. While this should not be a problem if the patient uses one or other site exclusively, the between-test variability will increase if multiple sites are used. In addition, during times when the blood glucose concentration is either rising rapidly (such as immediately after a meal) or falling rapidly (in response to rapidly acting insulin or exercise) then blood glucose results from alternate sites give significantly delayed results compared with fingerstick readings <16,17>.
Glucose strips — Some glucose strips have considerable batch to batch variation, and require recalibration to a meter every time a new batch is used. Many strips are packaged in groups of 25 inside a can containing a preservative. Common errors include leaving the lid off for long periods of time and putting several lots of strips into one can for convenience. Strips that are individually wrapped are more reliable, but more expensive.
Operator — We recommend use of one of the newer glucose meters in which the patient adds a drop of blood to a strip already inserted into the meter. Patients who are motivated and test often usually get much more reliable results than those who are less interested or who test less often (such as hospital-based nursing staff or non-expert physicians) <18,19>.
We also recommend the following steps to increase the accuracy of glucose monitoring:
The glucose meter and strips should be brought in for clinic visits. The patient's method of testing should be observed periodically and any technical mistakes corrected.
The results obtained with the glucose meter should be checked against meters of known accuracy or with a laboratory reference standard every few months.
CONTINUOUS GLUCOSE MONITORING — The current methods of SMBG are invasive, uncomfortable, and only allow periodic measurements. Several companies are developing an automatic approach that could significantly improve the quality of life of diabetic patients.
GlucoWatch — One such noninvasive device, the GlucoWatch G2 Biographer (Cygnus, Redwood City, CA), has received approval from the United States Food and Drug Administration (FDA) for both adults and adolescents <20>. It is worn as a band around the arm and extracts interstitial fluid through the skin using an applied potential (iontophoresis), measuring the glucose in the extracted sample with an electrochemical enzymatic sensor.
The most recent blood glucose is displayed on the "watch" along with an arrow that indicates if this reading is higher or lower than the last one. The patient can set glucose concentrations above and below which the device will sound an alarm. It will also sound an alarm if the rate of fall of blood glucose is rapid.
In two comparative studies of the GlucoWatch device and the HemoCue blood glucose analyzer in a total of 120 patients with diabetes, there was close agreement between the readings with both methods <21,22>, and in one of the studies <22> there was also close agreement between readings using the device and those using a glucose meter at home. There was an 18 minute delay between the GlucoWatch and blood glucose readings. These preliminary results are promising, but further refinement and simplification of the device are needed. Just as with alternate-site blood glucose testing, measurements with the Glucowatch are lower or higher, respectively, than fingerstick values when blood glucose concentrations are rapidly rising or falling.
There are concerns about the ability of glucose sensors to reliably detect hypoglycemia. In a study of 91 children and adolescents who simultaneously wore two Glucowatch devices during a twenty-four hour stay in a clinical research center, the median absolute difference between the glucose pairs was 26 mg/dL (1.4 mmol/L) with only 31 percent of the values falling within 15 mg/dL (0.9 mmol/L) of the reference glucose <23>. The sensitivity of the Glucowatch Biographer to detect hypoglycemia when the alarm was set to 60 mg/dL (3.3 mmol/L) was only 23 percent, with a false alarm rate of 51 percent. In its present stage of development this device does not reliably detect hypoglycemia, which significantly impairs its clinical usefulness.
Iontophoresis causes mild skin irritation and can cause pain. Another potential drawback to the device is the need to replace the sensor gels every twelve hours. Each time the gels are replaced the plate must be moved to a different place on the arm (which may need to be shaved), and a blood glucose measurement is required every twelve hours to calibrate the device.
Continuous glucose monitoring system (CGMS) — Another device that can monitor interstitial-fluid glucose concentrations for up to three days has been approved by the FDA (Continuous Glucose Monitoring System, or CGMS, from Medtronic Minimed, Northridge, CA). A needle sensor containing a glucose oxidase enzyme system is placed subcutaneously and attached by a wire to a recording device (about the size of an insulin pump or pager) that records the blood glucose concentration every 15 minutes. In the current version of this system, the patient receives no information while wearing the device. Results can be determined in a physician's office and graphed, providing useful information about the extent of within-day and between-day variations in blood glucose and the frequency of unrecognized hypoglycemia <24>.
When compared with venous plasma glucose values the interstitial fluid glucose sensor yields lower values when blood glucose concentrations are rapidly rising <24>. Also, the reproducibility of results has been called into question. When 11 adults (6 type 1 diabetic patients, 3 type 2 diabetic patients, and 2 normal subjects) wore two interstitial fluid glucose sensors simultaneously differences between the two readings were over 10 percent in 70 percent of the measurements and over 50 percent in 7 percent of the measurements <25>. These studies emphasize that continuous glucose sensing devices should not be relied upon exclusively to give diabetic patients information about their blood glucose concentrations. Patients should continue to do several fingerstick tests every day to verify that the sensor readings are accurate.
The CGMS may also be be inadequate for reliably detecting hypoglycemia in children. In the a study described above <23> where 91 children and adolescents also wore one or two CGMSs, the absolute median difference between over 400 paired blood glucose values was 19 mg/dL (1.0 mmol/L), with 42 percent of values falling within 15 mg/dL (0.9 mmol/L) of the reference glucose <23>. Although modifications to the CGMS device were made during this study it was still relatively insensitive and unreliable at detecting hypoglycemia, which significantly impairs its clinical usefulness.
Future versions of this device should provide information to the patient while wearing it, similar to the GlucoWatch. In addition, it is hoped to connect the information from the glucose sensor to an insulin pump so that the changes in the rate of insulin delivery could be linked automatically to changes in blood glucose concentrations.
Implantable subcutaneous glucose sensors — A device the size of a AA battery (DexCom, San Diego, CA) can be implanted in the subcutaneous fat of the abdomen (although this device is not available or approved by the US Food and Drug Administration). The device measures glucose levels every five minutes and sends the data via wireless radio transmitter to an externally worn pager-sized receiver device. Glucose readings are displayed in real time every five minutes and as 1-, 3-, or 9-hour trend graphs. The receiver device also provides vibratory and auditory alerts/alarms when the glucose levels are high or low.
Preliminary data on 15 adults with type 1 diabetes showed that after 50 to 90 days of continuous use, 96 percent of the sensor glucose results fell within the A and B regions of the Clarke error grid (an analysis which describes the clinical accuracy of SMBG systems over the entire range of blood glucose values, taking into account 1) the absolute value of the system-generated glucose, 2) the absolute value of the reference blood glucose, 3) the relative difference between these two values, and 4) the clinical significance of this difference <26,27>. When subjects were allowed to see and use these data to guide their therapeutic decisions they were able to maintain significantly fewer episodes of hypo- and hyperglycemia. While further larger scale clinical trials are needed with this type of device, these data suggest great clinical potential for continuous implantable glucose monitoring systems.
USING THE INFORMATION — Blood glucose should be measured four to seven times daily by patients with type 1 diabetes attempting to achieve strict glycemic control: before breakfast, lunch, dinner, and bedtime, and occasionally after meals or during the night if nocturnal hypoglycemia is suspected. At a minimum, monitoring should be used to avoid and/or help treat potentially dangerous hypoglycemia. (See "Cases illustrating problems with intensive insulin therapy for diabetes mellitus", section on Morning hyperglycemia).
However, this regimen will be effective only if the patient is able to use the information to make appropriate dietary or therapeutic adjustments. As an example, patterns of glycemic control can be most easily identified if the blood glucose values are entered in columns, corresponding to times of the day, and the relation to both food intake and exercise noted. (See "Cases illustrating problems with intensive insulin therapy for diabetes mellitus", section on Variable glycemic control).
Many meters now include a memory function and can be downloaded onto computers to display the results. However, unless results are reviewed on a frequent basis to detect and address blood glucose patterns, self-monitoring will not fulfill its purpose. Relying on the automatic data storage of the meters, without regularly reviewing the results, may detract from the clinical utility of monitoring.
Optimal use of the data obtained is best done in two stages:
Pattern identification — Patterns, as opposed to intermittent problems, are best identified if there is a relatively large number of measurements. Thus, blood glucose values should be recorded four to seven times daily for several days before the therapeutic regimen is changed.
Insulin algorithms — Once a basic regimen of eating, exercise, and insulin dosing has been established, there will still be a day-to-day variability in blood glucose values due, among other factors, to the vagaries of insulin and food absorption. (See "Insulin therapy in type 1 diabetes mellitus"). This can be effectively treated by an insulin algorithm in which the before-meal dose of short-acting insulin is adjusted according to the blood glucose value. The adjustments should be small in patients who are very sensitive to insulin or who are taking low doses of insulin (as with a continuous insulin pump). (See "Cases illustrating problems with intensive insulin therapy for diabetes mellitus", sections on Insulin algorithm, Late afternoon hypoglycemia, and Late morning hyperglycemia).
In patients with type 2 diabetes, blood glucose is usually measured less often than in patients with type 1 diabetes, but the need varies, depending upon the stage of the disease, the targets being set, and the treatments being used. (See "Case illustrating blood glucose monitoring in type 2 diabetes").
With a well-educated and motivated patient, therapeutic advice can often be given over the telephone or even via fax or e-mail. It is important not to recommend many changes at the same time. Having made a change, it is usually best to wait several days until the effect of that change can be assessed from further blood glucose measurements.
Patients with special needs — Visually impaired patients may have difficulty using glucose meters; with help this problem can be overcome with "talking meters" or large-screen meters. Patients or providers may contact:
American Association of Diabetes Educators (AADE) 444 N. Michigan Ave., Suite 1240 Chicago, IL 60611-3901 Tel: 1-800-338-3633
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