Dr Angela Murphy explains:
The incentive to test sugar in the early days of diabetes was centred on diagnosis – hence initial urine test kits would just have established if sugar was present or not. Once insulin was discovered, and eventually oral treatments as well, it became important to monitor blood glucose levels to assess how well treatment regimens were doing. Large trials have conclusively shown that good glucose control reduces the incidence of complications of diabetes. Another strong reason to test is to check for hypoglycaemia – low blood sugar levels. Blood glucose testing was greatly improved in the mid 1960’s with the invention of the Ames dextrostix and gradually home glucose testing kits became available. When continuous glucose monitoring (CGM) devices were released, even more accuracy was expected.
Blood glucose targets are partly determined from knowing what the non-diabetic blood glucose range is, but also from the data of the trials. There are 3 parameters that are used for these targets: fasting blood glucose, post-prandial (2 hours after a meal) blood glucose and HbA1c. The HbA1c is a measure of the average of glucose levels over the previous three-month period.
The Society for Endocrinology, Metabolism and Diabetes of South Africa (SEMDSA) give guidelines for target levels for these parameters. Younger, otherwise fit people living with diabetes should aim for stricter control indicated by an HbA1c <6.5%. Older people with multiple complications and/or poor general health can aim for an HbA1c <7.5%. Most patients should aim to at least keep HbA1c under 7.0%. At these levels we know that complications of diabetes are reduced, but despite that, it is estimated only about a third of people living with diabetes have good control.
Fasting and post-meal blood glucose are measured on home glucose monitors, and there are many on the market now. HbA1c levels must be done in the laboratory or by a specialized point of care machine at the doctors’ rooms. The following the recommended frequency of testing:
- Every 6 months if control stable
- Every 3 months if level not at target and treatment schedule is being adjusted
- SMGB (self-monitoring of blood glucose)
- For patients on tablets only – 3-5 tests weekly, and this can be a combination of fasting and 2 hour post-meal glucose
- For patients on insulin:
- Basal insulin – measure fasting glucose daily
- Twice daily insulin – measure before injections twice daily
- Basal and bolus insulin – measure before mealtime injections and before bedtime
- SMGB must be done with a purpose, not just to show the doctor or diabetes nurse. Blood glucose values help make decisions as follows:
- Fasting glucose
- Guide to efficacy of tablets
- Helps get basal insulin dose correct
- Before meals
- Can reflect if between meal snacks are influencing the sugar level
- Indicates if previous tablets or mix insulin is effective enough
- Guides meal-time insulin dose
- 2-hours post meals
- Shows the effect certain foods and meal portions have on the blood sugar which can help with meal planning
- Indicates the accuracy of meal-time insulin dose and timing of the injection of that dose o In times of illness, during pregnancy, when new medication has been introduced or with exercise it might be useful to increase frequency of blood glucose measuring.
We have now entered the age of Continuous Glucose Monitoring (CGM). This provides so much more information than a single glucose value, viz., the trend of the glucose values preceding that measurement, the direction the glucose level is going (higher or lower) and the speed of change of the blood glucose. There are two types of CGM:
- rtCGM: provides real-time numerical and graphical information about the current glucose level, glucose trends, and the direction/rate of change of glucose. The devices will have alarms to alert that allows action to be taken to bring glucose levels back towards normal. The systems available in South Africa are DEXCOM and GUARDIAN CONNECT.
- iCGM: provides the current glucose value with the direction and rate of change predicted as well as retrospective glucose data for a specified time period upon “scanning” of the device. The system available in South Africa is FREESTYLE LIBRE. This can be converted to a real-time device with the addition of MIAO-MIAO.
CGM is most valuable for patients on multiple daily injections who have to adjust their insulin dose according to the blood glucose reading. It is particularly helpful in managing young patients with Type 1 diabetes. Some medical aids are now providing partial funding for CGM. A new parameter for assessing diabetes control has emerged with the use of CGM – the concept of TIME IN RANGE. CGM provides a plethora of data that must be interpreted in a useful way. In 2017, several societies and panels of experts proposed the Standardized Ambulatory Profile as a universal template for CGM data presentation and visualization. This is presented as a bar graph indicating what percentage of time are glucose readings ‘in range’ (taken as levels from 4.0-10.0), and then reporting the percentage of readings too low or too high. For most people the target time in range should be >70%.
THE STANDARDIZED AMBULATORY PROFILE SHOWING TIME IN RANGE
Measuring blood glucose levels is important but it must be done in such a way that it makes a difference to overall diabetes management. If done with purpose, it will be a clear way of avoiding the complications of diabetes and having a better overall quality of life.