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Blood glucose monitoring in diabetes

Self-monitoring of blood glucose (SMBG) dates back to the mid 1960s with the introduction of the Dextrostix monitor. A drop of finger-prick capillary blood was applied to a reagent strip, wiped clean and a semi-quantitative blood glucose result was obtained by comparing the colour change against a standard chart.1

Subsequently, automated strip readers were introduced in the early 1970s which generated fully quantitative readings. Since then, the use of SMBG has increased markedly with the current annual NHS expenditure on self-monitoring in excess of £130 million for England and Wales alone. There have been major advances in meter technology, which now require only minute volumes of blood (0.6-5 uL) and rapid measurement times - literally seconds. Meters may also incorporate on-board software to allow downloading of results with the possibility of remote supervision by healthcare teams, the generation of basic summary statistics on glycaemic control  (eg. average glucose concentration) and the individualised calculation of insulin doses based on blood glucose concentration and projected carbohydrate intake. Furthermore, meters are technically easy to use, although still do require some degree of manual dexterity. For visually impaired patients, talking meters are also now available. 

Much of the impetus for the expansion of SMBG has been based on the evidence for both type and 1 type 2 diabetes that good glycaemic control is associated with better clinical outcomes and a reduced rate of tissue complications of diabetes. This article will explore how SMBG might contribute to better glycaemia and in what situations or in which patient groups monitoring might be useful. It will also consider the increasing role of continuous glucose monitoring.

Theoretical framework for monitoring

How might SMBG contribute to better outcomes for people with diabetes? It hardly needs to be stated that the mere act of monitoring without any subsequent action is unlikely in itself to contribute to better patient care. For SMBG to contribute to improved outcomes for patients, someone - the healthcare professional or the patient with diabetes - must do something useful with the result. In the case of the health professional, SMBG data can be used to adjust treatment regimens and to monitor the impact of changes. In the case of the patient, applications of SMBG data might include insulin dose adjustment, an enhanced understanding of how diet and exercise affect blood glucose concentrations and therefore contribute to more effective self-management, identification of hypoglycaemia, and greater patient empowerment in the management of a complex and life-long condition. 

It is obvious that SMBG can only be effective if used in conjunction with an educational package that ensures not only that patients are able to generate accurate results, but that they are able to interpret the results and use the information in a constructive way. There must be a shared agreement between the healthcare professional, such as the nurse, and the individual patient on what is the aim and purpose of monitoring. As well as the potential benefits of SMBG, it is important to recognise that some patients find finger-pricking unpleasant and that a requirement for regular SMBG can be intrusive and give rise to anxiety. Despite the widespread adoption of monitoring, robust evidence for its impact on patient outcomes is in many situations surprisingly sparse. This reflects the difficulty in studying such a complex intervention in a condition in which self-management plays a central role.1

Insulin-requiring diabetes

SMBG is regarded as an integral component of the management of insulin-requiring diabetes as it provides real time information on glycaemia. Patients can therefore use the capillary glucose results to adjust their insulin dose. This is most commonly employed by patients on intensive insulin regimens (including insulin pump therapy - continuous subcutaneous insulin infusion) for instance where the pre-meal bolus dose of short-acting insulin can be adjusted on the basis of the glucose concentration, projected carbohydrate intake and planned exercise. A number of observational studies have indeed supported an association between self-monitoring and better glycaemic control in insulin-requiring patients.1

Pregnancy

Pregnancy may occur in patients with pre-existing type 1 or type 2 diabetes or may be complicated by the development of gestational diabetes. Diabetes in pregnancy is associated with increased perinatal mortality and congenital malformation. There is a very strong body of evidence supporting the importance of tight control of glycaemia from the time of conception onwards in minimising maternal and foetal complications.2 Women with diabetes in pregnancy are therefore asked to monitor closely (four to six times daily) and are given targets for pre- and post-prandial glucose concentrations with regular, real-time titration of insulin dose to achieve these. SMBG forms a core element of the management of diabetes in pregnancy.

Driving

Hypoglycaemia occurring during driving poses a risk to the safety of the driver and other road users. Patients with diabetes have, under law, a responsibility to prevent hypoglycaemia. In addition to insulin, some oral anti-diabetic agents - sulphonylurea agents or glinides - that work by stimulating insulin release from the pancreas may also cause hypoglycaemia. The law on driving in diabetes changed in 2011. People with diabetes who are started on medication that will increase the risk of hypoglycaemia (insulin, sulphonylureas or glinides) are required to notify the Driver and Vehicle Licencing Agency (DVLA).3 Patients with a Group 1 licence (car or motorcycle) are required to undertake appropriate monitoring before driving and every two hours during driving. People with a Group 2 licence (lorries and buses) who are treated with insulin or sulphonylurea agents are required to undertake regular monitoring - at least twice daily and at times relevant to driving - using a meter with a memory function for a three-month period prior to their annual examination, so that a proper assessment of glycaemic control can be made. 

SMBG in non-insulin-requiring type 2 diabetes

The role of SMBG in non-insulin-requiring diabetes has been the area of greatest controversy and uncertainty.1,4 There is evidence of limited education being provided to patients on how to use results and sometimes lack of interest in the results from healthcare professionals. A recent systematic review showed that SMBG, when used alongside a structured education package, resulted in reduction in HbA1c of ~4mmol/mol (0.52%).4 However in the absence of a structured education package on how to interpret and react to monitoring results, SMBG was associated with only a small reduction of HbA1c, which was of doubtful clinical significance. These findings reinforce the importance of effective patient education.

For non-insulin-requiring patients, SMBG should be available to those on sulphonylurea agents because of the associated risk of hypoglycaemia (such as when driving, as discussed earlier). Patients not treated with sulphonylurea agents should only perform SMBG if there is some agreed purpose to the monitoring, for example if the information from SMBG is used support and reinforce lifestyle change. Furthermore the requirement for SMBG may only be intermittent, eg. for short periods following alteration of anti-diabetic medication or during intensive lifestyle change. SMBG need not otherwise be routinely undertaken in this patient group.4 

Continuous glucose monitoring

The use of continuous glucose monitoring (CGM) is increasing. In CGM, a small electrochemical sensor (generally

CGM may be used as either a diagnostic or therapeutic tool. As a diagnostic tool it provides valuable retrospective data on glycaemia, for example the detection of asymptomatic hypoglycaemia, particularly nocturnal hypoglycaemia, or in the assessment of patients where HbA1c remains high despite apparent optimisation of treatment with SMBG. As a therapeutic tool, CGM may be used to provide real-time information on glycaemia to allow adjustment of insulin dose. There is evidence that the use of CGM may be associated with improved glycaemia in some patient groups.5

One of the holy grails of insulin therapy is to develop a system that links real time CGM outputs with insulin pump infusion rates - a so-called 'closed loop system' or 'artificial pancreas'. Although significant progress has been made, problems still remain to be resolved.

Conclusion

SMBG and increasingly CGM have an important role to play in the management of diabetes. Relevant patient education on how to interpret and use monitoring data is essential. 

 

References

1. O'Kane MJ, Pickup J. Self monitoring of blood glucose: is it worth it? Ann Clin Biochem 2008;46:272-82 .

2. Jovanovic L, Druzin M Peterson CM. Effect of euglycaemia on the outcome of pregnancy in insulin dependent diabetic women as compared with normal control subjects. Am J Med 71:921-7.

3. DVLA's current medical guidelines for professionals on diabetes. 

4. A report prepared by an NHS Diabetes Working Group. Self monitoring of blood glucose in non-insulin-treated Type 2 diabetes.

5. JDRF Continuous GM Study Group. Continuous glucose monitoring and intensive treatment of type 1 diabetes. N Engl J Med 2008;359:1464-76.