Frequency of blood glucose monitoring in relation to glycaemic control: observational study with diabetes database

Self monitoring of blood glucose

The number of blood glucose monitoring reagent strips dispensed to patients during the study period was determined from the MEMO dispensed prescribing database, a computerised record of all prescriptions dispensed in community pharmacies in Tayside since 1993.14 Patterns of use were investigated by sex, age, and duration of diabetes. The Carstairs social deprivation categories of the study patients, ranging from category 1 (most affluent) to category 7 (least affluent) and based on four census variables, were also determined from details of the patients' postcodes.20

Blood glucose control

Patients who had at least one glycated haemoglobin (A_1c) concentration recorded between July 1993 and December 1995 were identified. The numbers of reagent strips that were dispensed to these patients during a 6 month period before their first haemoglobin A_1c measurements were calculated. Linear regression models for patients with type 1 and type 2 diabetes were constructed separately, with haemoglobin A_1c as the outcome and age, sex, duration of diabetes, and deprivation category as covariates. The effect of body mass index (available for 70% and 75% of patients with type 1 and type 2 diabetes, respectively) was also investigated. The analyses were repeated in subgroups of patients who obtained at least one pack of strips.

Costs and analysis

The total cost of the blood glucose test strips dispensed was calculated with 1998 tariffs obtained from the British National Formulary (number 35)21 (including the pharmacist's dispensing fee). All statistical analyses were carried out with SPSS.

Self monitoring of blood glucose

Among 367 051 Tayside residents there were 807 (0.2%) patients with type 1 diabetes and 5601 (1.5%) with type 2 diabetes, 790 of whom were included in the study (table 1). In total, 13 382 prescriptions for blood glucose monitoring reagent strips were dispensed to 1307 of these patients during the 3 year period.

In those with type 1 diabetes, 128 (16%) of the 807 patients obtained no reagent strips at all. The 679 remaining patients obtained between 50 and 6220 strips (between 0.05 and 5.68 strips a day). One hundred and sixty one (20%) patients obtained more than 1095 strips (equivalent to one a day), and only eight (1%) patients obtained enough strips to measure at least four times daily. In those with type 2 diabetes, 162 (21%) of the 790 study patients obtained no strips. The 628 remaining patients obtained between 50 and 10 100 strips (that is, between 0.05 and 9.2 strips a day), with 131 patients (17%) obtaining more than 1095 strips.

Table 2 shows patterns of uptake of reagent strips by sex, age, duration of diabetes, and deprivation category. There was a particularly high uptake in children with type 1 diabetes, which then decreased sharply among young adults (15-24 years). Elderly people were another group with low uptake. Analysis by duration of diabetes showed a similar pattern, probably partly because of confounding by age. A pattern of decreasing uptake with increasing age was also evident among patients with type 2 diabetes. A trend of decreasing uptake with increasing deprivation was evident, particularly among patients with type 2 diabetes.

Blood glucose control

There were 258 patients with type 1 diabetes who had at least one valid haemoglobin A_1c concentration recorded (ranging from 4.2% to 17.4% of total haemoglobin), of whom 152 had obtained at least one pack of reagent strips in the previous 6 months. In a linear regression model the total number of reagent strips dispensed was a predictor of haemoglobin A_1c concentration (P<0.001; table 3), with a decrease in haemoglobin A_1c concentration for every extra 180 test strips dispensed (equivalent to one a day) of 0.7%. Sex was the only other independent predictor of haemoglobin A_1c concentration (P=0.002), with higher values in female patients (table 3). In the subgroup of 152 patients (those who obtained at least one pack of strips in the 6 month period) the relation between strip uptake and haemoglobin A_1c concentration was still strong (regression coefficient −0.672, P<0.001).

There were 290 insulin users with type 2 diabetes who had at least one valid haemoblobin A_1c concentration recorded (ranging from 4.2% to 14.3%). In a linear regression model none of the covariates, including the number of strips dispensed (P=0.357), were independent predictors of haemoglobin A_1c concentration (table 4). Similarly, there were no associations in the 171 patients who obtained at least one pack of strips.

Cost

The total cost of the 7002 prescriptions for glucose monitoring test strips dispensed to patients with type 1 diabetes was £155 912 (an average of £64.40 per patient per year). The 6381 prescriptions for reagent strips dispensed to patients with type 2 diabetes who used insulin cost £134 907 (£56.92 per patient per year).

Self monitoring of blood glucose

This study provides an insight into blood glucose monitoring habits in all diabetic patients in Tayside who used insulin and shows that many patients with either type of diabetes did no testing at all. Less than one fifth tested daily.

It is encouraging to note that large numbers of strips were dispensed to children who are learning to adjust to type 1 diabetes. The decline in teenagers and young adults, for whom parental control is presumably less influential, however, is worrying and backs up previous work which showed that some are not even compliant with insulin treatment.17 The Carstairs score is a crude measure of social deprivation, but a link with strip uptake was still evident, as has been documented elsewhere for income and education level.5 Diabetic patients do not pay for their prescriptions in the United Kingdom so it is not that they cannot afford them. The decline in self monitoring with age in patients with type 1 and type 2 diabetes has also been reported elsewhere.5

We believe that these results are reliable. Data from the DARTS/MEMO collaboration are well validated.14 15 The dispensing of reagent strips was used as a proxy measure for self monitoring, and, although patients may not actually have used them (resulting in overestimation of use), the misclassification effect of primary non-compliance is eliminated,22 and the method is probably more objective than direct questioning of patients.23 We had no information on strips dispensed to patients in hospitals, however, or on glucose measurements performed by a third party (for example, by district nurses).

Blood glucose control

The regression analysis suggested a direct association between haemoglobin A_1c concentration and the number of strips obtained in a previous 6 month period in patients with type 1 diabetes. This result indicates either that self monitoring is important for maintaining good diabetic control or that it is a proxy measure of good health behaviour and practice that is associated with good control. In other words, patients who self monitor are also likely to be compliant with their diabetic regimen, with confounding by factors such as diet and exercise. When the analysis was restricted to patients who obtained at least some strips and could be regarded as partially compliant (providing some control for confounding), however, the association was still evident. Even if it is non-causal, self monitoring might improve quality of life by giving patients more control over their disease.4

The analysis could be carried out only for patients who had haemoglobin A_1c concentrations recorded (predominately those residing in the geographical area served by the main teaching hospital). This was not dependent on the clinical characteristics of the patient and is therefore unlikely to distort the results of the study, and indeed the age and sex distributions of patients with and without haemoglobin A_1c concentrations recorded were similar. Only one value was analysed per patient as more regular readings might be taken in patients judged to have poor control. Haemoglobin A_1c concentration is a biological correlate of diabetic control in the previous 3 months. The 6 month period for obtaining strips was chosen to allow for patients collecting them “in bulk” at infrequent intervals.

No association was found between haemoglobin A_1c concentration and self monitoring in patients with type 2 diabetes who used insulin, as reported elsewhere.12 It may be that blood glucose monitoring is more effective in true insulin deficiency as opposed to the insulin resistant state. Or patients with type 2 diabetes might be less familiar with insulin use, more anxious about the risks of hypoglycaemia, and hence less likely to act on the results of tests. It is also important to note that patients with type 2 diabetes are a heterogeneous group, particularly in terms of pancreatic β cell reserve. An alternative explanation for the study findings is that self monitoring may be recommended particularly in those patients who are the most difficult to control.

Cost

The average cost of glucose monitoring per patient was relatively low when compared with other costs associated with diabetes care.24 Even for a patient who is self monitoring four times daily, the approximate cost per year is only £409. We therefore suggest that self monitoring of blood glucose should be further encouraged, particularly in those subgroups of patients who do not monitor their blood glucose concentrations regularly—for example, young deprived men with type 1 diabetes.