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Self-monitoring of Blood Glucose (SMBG): From Theory to Clinical Practice

Authors: Thomas Danne, MDFaculty and Disclosures

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Introduction

Self-monitoring of blood glucose (SMBG) has been accepted as an important instrument that empowers people with diabetes to achieve and maintain therapeutic goals. Nevertheless, it is underprescribed and underused by patients. On the other hand, determination of hemoglobin A1c (HbA1c) is accepted as the gold standard for assessing glycemic control, but its limitations are not sufficiently appreciated. Patients with normal or near-normal HbA1c levels may still display postprandial hyperglycemia, putting them at risk for long-term adverse outcomes.[1] In addition, frequent unrecognized hypoglycemia may lead to falsely low HbA1c levels, and HbA1c does not allow any estimate of glycemic variability. Determination of immediate blood glucose control is best assessed by SMBG because this provides timely information of hyperglycemia and hypoglycemia. Thus, SMBG is a prerequisite for implementing strategies to optimally treat, as well as to avoid, out-of-range glucose values. Healthcare professionals must be capable of making evidence-based clinical decisions in regard to the use of SMBG and balance issues, such as patient abilities, costs, and clinical outcomes.

Barriers to optimal use of SMBG include limited knowledge, both by clinicians and patients, as well as from perceived inconvenience or discomfort with the measurement. Motivational/behavioral issues, particularly in the adolescent subgroup, may also be a barrier. These issues, however, should never distract from the fact that failure to achieve glycemic control with SMBG is often the result of a failure to properly educate patients how to monitor blood glucose levels and the importance of accuracy in doing so. Thus, clinicians must be aware of these potential barriers and be prepared to address them with individual patients and other caregivers, such as families or guardians.

Choice of Meter and Need to Ensure Meter Accuracy

Many types of excellent monitors for SMBG are available. Of paramount importance is the accuracy of measurement. Recommendations for standardized testing of glucose meter performance have been published.[2] Healthcare professionals should choose and advise on a type that is robust, precise, accurate, and familiar to them as well as affordable to their patients.

Significant inaccuracies may arise from operator-related errors.[3] Potential human errors, such as not cleaning the sampling site (ie, fingers covered with sugar from a previous meal or using strips with the wrong coding), are frequent. In addition, methodologic issues, ie, strips that have not been stored correctly or interference of other substances with the strip methodology, need to be considered. A US Food and Drug Administration warning has been issued because patients receiving maltose-containing immune globulin products, whose blood glucose levels are monitored with glucose dehydrogenase pyrroloquinolinequinone (GDH-PQQ) systems -- which may overestimate measurements when blood maltose levels exceed 0.9 mmol/L -- are at even more risk than other patients with diabetes for life-threatening or fatal hypoglycemia. Overestimated GDH-PQQ blood glucose measurements may obscure recognition of insulin-induced hypoglycemia, delay medical intervention, and result in morbidity or mortality.[4] Other factors that may affect blood glucose measurements include diminished peripheral blood flow (eg, hypovolemia, peripheral vascular disease), hematocrit (eg, anemia vs polycythemia), blood specimen (eg, capillary vs venous, whole blood vs plasma), glucose methodology (eg, GDH-PQQ vs glucose oxidase), and interfering substances of exogenous and endogenous origin (eg, maltose, lipids, acetaminophen, uric acid).

Patient acceptance of SMBG may be enhanced by including the opportunity for testing alternative sites in addition to the fingertips (eg, the palm of the hand or the forearm). In the fasting state, glucose readings from the forearm are similar to the fingertip.[5] These alternative sites may be slower to reflect falling blood glucose levels; therefore, it is advised that fingertips be used when symptoms of hypoglycemia are present and to recheck the glucose with the fingertip if the alternative-site test is in a low range.[6]

Frequency and Timing of Monitoring

Frequent and accurate blood glucose monitoring and concomitant optimal adjustment of insulin to carbohydrate intake and exercise[7] are the basis of diabetes treatment in type 1 diabetes mellitus (T1DM) and for insulin-treated type 2 diabetes mellitus (T2DM). The frequency of SMBG is associated with improved HbA1c levels in patients with T1DM.[8] This is thought to be due to both better insulin adjustment for food consumed and an improved ability to quickly correct out-of-target glucose values. In addition, early detection of lower glucose values prior to symptomatic hypoglycemia may allow correction with a reduced risk for overcorrection and resultant hyperglycemia. The use of SMBG during exercise may also allow improved insulin management and a reduced risk for hypoglycemia during and following exercise.[9]

Blood glucose is best measured at different times in the day to show levels of blood glucose at the following phases: after the overnight fast; during the night to detect unnoticed hypoglycemia and hyperglycemia; in response to the action profiles of insulin (at anticipated peaks and troughs of insulin action); after food intake (1.5-2 hours after a meal); and in association with vigorous physical activity (during and several hours after) so that changes may be made in management to improve blood glucose profiles, to confirm hypoglycemia and to monitor recovery, and during intercurrent illness to prevent hyperglycemic crises.

The number and regularity of SMBG should be individualized depending on availability of the equipment, type of insulin regimen, and ability of the patient to identify hypoglycemia. Successful application of intensified diabetes management with multiple injection therapy or insulin pump therapy requires frequent SMBG ( 4-6 times a day) and regular review of the results to identify patterns requiring adjustment to the diabetes treatment plan. Optimal frequency and timing depend very much on the individual situation and will vary from day to day depending on activity, overall health, or recent changes in the overall therapeutic approach. Fasting, preprandial targets, and postprandial targets for SMBG have been outlined in several guidelines.[10,11] However, patients should have their targets individually determined, with the goal of achieving a value as close to normal as possible while avoiding severe hypoglycemia as well as frequent mild-to-moderate hypoglycemia.

Recordkeeping of Glycemic Control

It is common practice for a monitoring diary, logbook, or some type of electronic memory device to be used to record patterns of glycemic control and adjustments to treatment. The record book is useful at the time of consultation and should contain the time and date of SMBG levels, insulin dosage, notes of special events affecting glycemic control (eg, illness, parties, exercise, menses, etc), hypoglycemic episodes, description of severity, potential alterations in the usual routine to help explain the cause for the event, and episodes of ketonuria/ketonemia.

Monitoring records should not be used as a judgment but as a vehicle for discussing the causes of variability and strategies for improving glycemic control. Frequent home review of records to identify patterns in glycemic levels and subsequent adjustment in diabetes management are required for successful intensified diabetes management. In some instances, especially among teenagers, maintaining written monitoring records is difficult. If the family has access to a computer and can upload the SMBG monitoring data for review, this may serve as a substitute for a manual record, although details of management may be lost with this method.

Appropriate SMBG Use in T1DM and T2DM

Meta-analyses of studies on SMBG in T2DM have not shown a clear benefit. Of note, 6 of 8 randomized studies in T1DM also did not show a reduction of HbA1c. One study showed a reduction of HbA1c after 2 years, both in the groups with SMBG and in the group with urine glucose measurement.[12] Another study comparing SMBG and urine glucose showed a significant reduction after 9 months.[13] When looking at these historical references, the scope of the problem assessing the use of SMBG in T1DM or T2DM becomes apparent. SMBG can only be successful if embedded in an overall concept of care. Study patients are always intensively followed up. Therefore, this alone will reduce any potential benefit of a diagnostic intervention. Thus, SMBG should not be judged on the basis of randomized studies alone. In addition, SMBG plays an important role in risk management. Insulin and several oral agents potentially bear the risk for hypoglycemia. SMBG is therefore essential to avoid risks during activities, such as driving.[14] However, in the elderly, in whom driving or prevention of late complications may be less of an issue, hypoglycemia is associated with poor outcomes and should be prevented.[15]

New Technologies

Proper identification of patients likely to benefit from new technologies, such as continuous glucose monitoring, software to aid patient management, or telemedicine, is one of the challenges to healthcare teams. SMBG may be used as either a primary or an adjunctive tool with these technologies. Devices are available, and others are in development that measure interstitial fluid glucose every 1-20 minutes (ie, continuous measurement). Minimally invasive sensors use a catheter or a small plastic chip containing a sensor inserted into the subcutaneous space to measure the interstitial glucose.[16] They are replaced every 3-7 days and require calibration 2-3 times daily with SMBG. These sensors transmit glucose levels to a pagerlike receiver box or to an insulin infusion pump for readout by the user. The continuous glucose results are available to the wearer during the monitoring time and are stored in the receiver device or pump for downloading to a computer at a later time. The download allows the patient and/or the physician to review the results and make insulin dose adjustments. Currently, these devices are expensive and may not be available in many countries. Insurance coverage is increasing, as a recent randomized trial has shown a significant sustained beneficial effect if these devices are used continuously even in patients who already had an HbA1c level in the target range, < 7.0%.[17] If continuous glucose monitoring technologies and wearability improve further, it can be anticipated that they may replace SMBG in the not-too-distant future.

International Perspective and Summary

SMBG is an essential tool in the optimal management of diabetes and, when financially possible, should be made available for all patients with insulin-treated diabetes and for select patients with non-insulin-treated T2DM. SMBG should be prescribed at a frequency to optimize each patient's diabetes control because frequency of SMBG correlates with glycemic control,[1] and benefits of such management have been shown for T1DM,[18] T2DM,[19,20] or gestational diabetes.[21] In many countries, the cost of SMBG monitoring is very expensive relative to the cost of living. However, we should urge nations, states, and healthcare providers to ensure that insulin-treated patients with diabetes have adequate glucose monitoring equipment available to them. It must be recognized that without accurate monitoring, the risks for acute crises and long-term vascular and other damaging complications are greatly increased, leading to high levels of healthcare costs and personal disability.

Countries and healthcare systems are adapting differently to the increased cost of diabetes care.[22,23] Some countries or health insurance systems are considering or have already restricted use of SMBG and certain diabetes drugs, such as insulin analogs and newer technologies requiring those choosing these technologies to bear up to 100% of the cost. However, these international differences are not explained on economic grounds alone. They may relate more to societal factors and approaches toward preventive medicine. SMBG use and coverage may reflect society's attitude toward the individual patient's health belief model (ie, personal responsibility in maintaining health, the provision of patient empowerment and education) as well as national long-term healthcare strategies vs short-term cost reduction issues. Thus, cost-efficient use of SMBG from the onset of diabetes to prevent devastating and expensive late complications of diabetes later on depends on a responsible partnership between patients, the healthcare team, and society.

This activity is supported by an independent educational grant from Bayer HealthCare Diabetes Care.