Self-blood glucose monitoring

INTRODUCTION — People with diabetes have an important role in their own medical care, and self-glucose monitoring is an opportunity for people with diabetes to take control of their health.

Although diabetes is a chronic condition, it can usually be controlled with lifestyle changes and medication. The main goal of treatment is to keep blood glucose levels in the normal or near-normal range. Monitoring blood glucose levels is one of the best ways of determining how well a diabetes treatment plan is working. (See "Patient information: Lifestyle modifications in type 2 diabetes" and see "Patient information: Diabetes type 1: Insulin treatment").

A healthcare provider will periodically order laboratory blood tests to determine blood glucose levels and hemoglobin A1c (A1C). The results of these tests gives an overall sense of how blood glucose levels are controlled (show figure 1). However, fine-tuning of blood glucose levels and treatment also requires that patients monitor their own blood glucose levels on a day-to-day basis.

Self-blood glucose monitoring allows patients to know their blood glucose level at any time and helps prevent the immediate and potentially serious consequences of very high or very low blood glucose. Monitoring also enables tighter blood glucose control, which decreases the long-term risks of diabetic complications.

HOW TO TEST — The following steps include general guidelines for testing blood glucose levels; specific details for individual blood glucose monitors should be obtained from the package insert or a healthcare provider. Wash hands with soap and warm water. Dry hands. Prepare the lancing device by inserting a fresh lancet. Lancets that are used more than once are not as sharp as a new lancet, and can cause more pain and injury to the skin. Prepare the blood glucose meter and test strip (instructions for this depend upon the type of glucose meter used). Use the lancing device to obtain a small drop of blood from the fingertip or alternate site (like the skin of the forearm) (show picture 1). Alternate sites are often less painful than the fingertip. However, results from alternate sites are not as accurate as fingertip samples when the blood glucose is rising or falling rapidly (show picture 2).

Patients who have difficulty getting a good drop of blood from the fingertip can try rinsing the fingers with warm water, shaking the hand below the waist, or squeezing ("milking") the fingertip. Apply the blood drop to the test strip in the blood glucose meter. The results will be displayed on the meter after several seconds. Dispose of the used lancet in a puncture-resistant sharps container (not in household trash).

FREQUENCY OF TESTING — Studies have proven that patients with type 1 and 2 diabetes who maintain normal or near normal blood glucose levels have a lower risk of diabetes-related complications. The frequency of monitoring will depend upon the type of diabetes (1 or 2) and treatment used (insulin versus oral medications).

Type 1 diabetes — For patients with type 1 diabetes, frequent testing is the only way to safely and effectively manage blood glucose levels. (See "Patient information: Diabetes mellitus, type 1").

The recommended frequency of testing varies from patient to patient, though most patients need to test at least four times per day. Patients using intensive insulin therapy and women with type 1 diabetes who are pregnant may need to test as many as seven times per day.

Patients who test frequently, especially those using intensive insulin therapy, may consider purchasing several blood glucose monitors to keep at home, work, school, or in a purse or backpack. This allows a patient easier access to testing equipment, which can increase testing frequency and therefore improve blood glucose control. However, patients who like to track data using meters with a memory function may have difficulty if some blood glucose results are on one meter and others are on a different meter.

Type 2 diabetes — Blood glucose monitoring is also important for patients with type 2 diabetes. The recommendations for frequency of testing varies from one patient to another based upon individual factors such as type of treatment (diet versus oral medication versus insulin), level of hemoglobin A1c (A1C), and treatment goals. A healthcare provider can help a patient know how frequently they should test. (See "Patient information: Diabetes mellitus, type 2").

INTERPRETING RESULTS

Blood glucose testing — The results of blood glucose testing indicate if diabetes treatments are on target. However, blood glucose results can be affected by activity levels, foods eaten, and medications (include insulin and oral diabetes medications). To interpret results, patients must consider all of these potential factors.

Patients should discuss their blood glucose results and the best way to interpret them with a healthcare provider. Patients should understand how to record results (either with paper and pen or electronically) and how to use them to optimally control blood glucose levels. Many meters have a memory function that allows results to be stored and downloaded to a computer. Results can then be analyzed and printed for a healthcare provider to review. All patients who monitor their blood glucose levels should bring their blood glucose records to each visit with a healthcare provider.

Patients should generally record blood glucose results, the time and date, and dose of medication used; additional notes about food intake, exercise, and difficulties with illness or stress can also be helpful but are not generally required every day.

Several days of monitoring are usually needed to identify daily patterns, which can be used to make lifestyle or medication adjustments. Patients who use intensive insulin therapy should adjust their insulin dose before meals based upon the blood glucose readings (ie, give a higher dose of very-rapid or rapid acting insulin when blood glucose levels are high).

Need for urine testing — People with type 1 diabetes should perform urine testing for ketones if their blood glucose level is above 240 mg/dL (13.3 mmol/L), during periods of illness or stress, or if there are symptoms of ketoacidosis, such as nausea, vomiting, and abdominal pain. Ketones are acids that are formed when the body does not have enough insulin to break down glucose, causing the body to break down fat for energy. Ketones can also develop during illness, if an inadequate amount of glucose is available (due to skipped meals or vomiting). Ketoacidosis occurs when high levels of ketones are present, which can lead to serious complications such as diabetic coma.

Urine ketone testing is done with a dipstick, available in pharmacies without a prescription. Urine can be collected and then tested with the dipstick, or the dipstick may be held in the urine stream. A color change occurs if ketones are present, indicating a trace, small, moderate, or large concentration of ketones. If a moderate to large concentration of ketones is present, the patient should consult with a healthcare provider immediately to determine the best treatment. An additional dose of insulin may be required, or the provider may instruct the patient to go to the nearest emergency room.

ADJUSTING TREATMENT — Home blood glucose monitoring can provide useful and motivating information. However, patients should make slow and careful changes to their treatment, allowing the body time to respond to changes. Most patients will need to consult with their provider frequently as they learn to make adjustments in treatment, especially with insulin. However, with time and experience, most patients are able to learn how to make adjustments on their own.

Patients should avoid making multiple treatment changes at the same time unless instructed to do so by a healthcare provider. Changing one aspect of treatment at a time allows for more careful evaluation of the effects of that change on blood glucose levels. Furthermore, it can take several days before changes are reflected in blood glucose results.

ACCURACY OF HOME GLUCOSE MONITORING — Accuracy refers to the ability of a glucose-measuring system to report a result that reflects the actual blood glucose level. Accuracy can be affected by several factors, including the type of blood glucose strip and monitor.

Patients should check the accuracy of their blood glucose monitor occasionally by bringing it to visits with their healthcare provider when blood work is done. Patients can use their monitor to check the blood glucose at the same time that a laboratory blood glucose level is drawn.

Hospital or office laboratories report glucose levels in a part of the blood, called plasma. Most blood glucose monitors also report plasma results (this should be stated on the test strip or glucose monitor packaging). Older meters reported whole blood glucose results, which can differ from plasma results by as much as 15 percent. Patients who compare their glucose monitor results with those from their provider's laboratory should see no more than a 15 percent difference; larger differences suggest a possible problem with the monitor, blood glucose strips, or monitoring technique.

Blood glucose meters — Blood glucose meters are reasonably accurate. However, there can be some variability from one unit to the next, so it is always wise to exercise caution and common sense when using the readings from these machines. As an example, if a reading seems incompatible with physical symptoms (or lack of symptoms), take a second reading or use an alternate method for testing your blood glucose (such as a different meter). All blood glucose machines are least accurate during episodes of hypoglycemia (low blood sugar).

Blood glucose strips — Some brands of glucose strips have batch-to-batch variations. These variations may require recalibration to the meter every time a new batch of strips is opened. It is also important to be systematic about the storage of glucose strips: never mix different batches of strips together, and quickly recap the container after removing a strip. Individually wrapped strips tend to be more reliable, but are also more expensive.

Alternate site testing — Blood glucose results can be less accurate if sites other than the fingertips are used for testing (eg, arm, hand, leg). This should not be a problem if the patient uses one site exclusively. However, during times when the blood glucose is rising rapidly (such as immediately after food ingestion) or falling rapidly (in response to rapidly acting insulin or exercise), blood glucose results from alternate sites may give significantly delayed results compared with fingerstick readings. In these situations, fingertip testing is preferred.

Help for people with vision impairment — People with vision impairment may have difficulty using glucose meters. Patients with impaired vision can get assistance from the American Association of Diabetes Educators (AADE) at (800) 338-3633.

SELECTING A BLOOD GLUCOSE MONITOR — There is no single blood glucose monitor that is better than others. A number of factors should be considered when choosing a monitor: Expense — Special offers, rebates, and trade-ins are almost always available on blood glucose monitors, making them affordable for most people. However, patients should also check the cost of the supplies that go along with a monitor, including test strips; over time, these will be more costly than the monitor. Many insurance carriers cover the cost of the monitor and/or supplies. Medicare now covers all of the costs of blood glucose monitoring. Ease of use — Some monitors are easier to use than others. Some require a tiny sample of blood, meaning that a smaller and less painful finger stick is possible. Meters vary in how quickly they give the result (some in as little as five seconds). Patients should check with friends or family who have a blood glucose monitor and with their healthcare provider (especially diabetes educators) for a recommendation. Accuracy — Glucose meters take a reading from a drop of blood applied to strip. Older meters require more careful preparation of the strip and leave more room for error. Newer meters perform more of the steps automatically, so there is less room for error. Newer meters typically provide the most reliable monitoring results. Sophistication — Some meters allow entry of events (like eating or exercising) and come with software programs that allow the data to be downloaded to a computer. This is most helpful for persons who test frequently and use intensive insulin treatment.

CONTINUOUS GLUCOSE TESTING — Researchers are currently evaluating continuous, less invasive, and less intrusive methods of glucose testing.

How it works — Continuous glucose monitoring systems (CGMS) use a glucose sensor (contained in a small needle) to determine the level of glucose in the interstial fluid, found between cells under the skin. The sensor wirelessly transmits results to a small recording device (the size of a pager or cell phone), which can be worn on the clothing, carried in a purse, or placed within a short distance of the sensor (eg, a bedside table). The sensor records and displays the blood glucose level every few minutes, allowing the patient to observe the trend of their blood glucose levels. The receiver can also be set to alarm if the blood glucose level is above or below a pre-set level, which can be especially helpful for patients who cannot feel when they have low blood glucose (hypoglycemia).

The sensor must be removed and reinserted in a different area every few days. Patients must continue to measure blood glucose levels with a traditional monitor several times daily to ensure that the continuous monitor is correctly calibrated.

Currently, continuous blood glucose monitors are recommended only for patients with type 1 diabetes who use intensive insulin therapy, often with an insulin pump. A combined insulin pump and continuous glucose monitor is also available.

Drawbacks — The continuous glucose sensors currently available are not as accurate as most blood glucose monitors, especially when blood glucose concentrations are rapidly rising. In one study, over 70 percent of the blood and continuous glucose values differed by 10 percent or more, and 7 percent of the readings differed by over 50 percent [5]. The CGMS tends to be less accurate in the lower glucose range (<70 mg/dL or 3.9 mmol/L) and may be inadequate for reliably detecting hypoglycemia.

Thus, continuous glucose sensing devices should not be relied upon exclusively to give patients information about blood glucose levels. Patients must continue to do several fingersticks daily to calibrate the currently available devices and to verify that the sensor readings are accurate.

In addition, the costs associated with continuous glucose monitors are much greater than those of traditional glucose monitors.

WHERE TO GET MORE INFORMATION — Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two patients are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.patients.uptodate.com). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable. National Library of Medicine

(www.nlm.nih.gov/medlineplus/healthtopics.html)
National Institute of Diabetes and Digestive and Kidney Diseases

(www.niddk.nih.gov/)
American Diabetes Association (ADA)

(800)-DIABETES (800-342-2383)
(www.diabetes.org)
The Hormone Foundation

(www.hormone.org/public/diabetes.cfm, available in English and Spanish)


[1-4]


Use of UpToDate is subject to the Subscription and License Agreement. REFERENCES 1. Faas, A, Schellevis, FG, van Eijk, JT. The efficacy of self-monitoring of blood glucose in NIDDM subjects. Diabetes Care 1997; 20:1482.
2. Nettles, A. User error in blood glucose monitoring. Diabetes Care 1993; 16:946.
3. Self-monitoring of blood glucose. American Diabetes Association. Diabetes Care 1994; 17:81.
4. Most, RS, Gross, AM, Davidson, PC. Richardson, P. The accuracy of glucose monitoring by diabetic individuals in their home setting. Diabetes Educ 1986; 12:24.
5. Metzger, M, Leibowitz, G, Wainstein, J, et al. Reproducibility of glucose measurements using the glucose sensor. Diabetes Care 2002; 25:1185.