Checking your glucose levels

The objective of diabetes treatment

The point of all treatment for diabetes – whether it’s diet, tablets or insulin – is to keep the levels of glucose in your bloodstream as close as possible to normal. The nearer you get to achieving this, the better you will feel, especially in the long term.

Blood glucose monitoring

There are two ways in which you can monitor glucose levels for yourself and your diabetes team will advise you about which one you should use and how often to do the checks. The two methods available are:

1 blood tests

2 urine tests

and neither is particularly difficult once you get the hang of it. The development of simple fingerprick blood testing methods in the last few years has transformed life for patients taking insulin. Keeping a close check on your glucose levels is very useful when you’re on insulin because it means that you can make adjustments to your dose depending on the results.
When your diabetes is being controlled by tablets and/or diet, urine tests can give you almost as much information as blood tests and may be more convenient. Recent evidence suggests little or no benefit of blood glucose tests in patients with type 2 diabetes on diet or oral hypoglycaemic agents.
In addition, there are blood tests that measure an average blood glucose level over a period before the test – from two to eight weeks. Each of these three approaches is looked at in turn.

Blood tests

There are two systems available for self-blood glucose monitoring (or SBGM as you may hear it called). Both give accurate results and, as well as helping you improve your blood glucose control, they can be useful if you suspect that you may be about to have a hypoglycaemic reaction (see page 76).
Taking an exact reading will either reassure you that all is well or confirm that you need to take action. Blood testing strips are available on prescription but the special meters for reading them may have to be purchased separately, although many diabetes centres are able to offer them for free.

Method 1

The glucose in the drop of your blood reacts with a pad or pads on the end of a plastic strip. These pads have been impregnated with chemicals and form colours when exposed to glucose. The strip is inserted into the appropriate meter, which gives a reading.
There are several different strips available, each of which has a different reaction time, so it is vital to follow the manufacturer’s instructions carefully.

Method 2

A slightly more complicated chemical reaction takes place when a drop of your blood is put on the specially designed testing strip. There is no colour change involved and the strips can be read only using a special meter. This system needs slightly less blood than the conventional colour pad system.
It is now possible to measure both blood glucose and ketones using a fingerprick test and a special strip and meter. This technique may be particularly useful in
pregnancy and in young people who are prone to recurrent episodes of ketoacidosis.

How to do the test

The main drawback for some people is that both these systems mean that you have to obtain a fingerprick sample of your own blood (although occasionally someone else may be able to do this for you).
Pricking your finger can be especially difficult if you are a manual worker or if you have very sensitive fingers. Rather than having to nerve yourself
deliberately to stab your finger, you might find it easier to use one of the devices incorporating a spring-loaded lancet (needle). It allows you to adjust the depth of the prick to suit yourself, but the disadvantage is that, although the lancets are available on prescription, the spring-loaded devices sometimes have to be paid for.

Glucose sensors

Devices implanted in the body

These use a fine needle that is placed under the skin, usually on the abdomen or tummy. This needle is connected via a fine tube to a small device worn on a belt or on the abdomen. Special enzymes on the needle break down glucose in the fluid under the skin and this creates a small electric current, which is detected by the device and converted into a glucose level. The results are stored and downloaded into a computer after three to five days or transmitted ‘real time’ to give an immediate result.
A similar device using a different method has also been developed. A small pump pushes fluid under the skin, which absorbs glucose from the tissue. This fluid is then returned back into the device and the glucose concentration is measured. This system can measure glucose levels for up to two days.
These machines can be useful to detect patterns of glucose control and provide a basis for treatment changes. Recently, they have been connected to an insulin pump so that indications can be made for dosage as well as warnings for hypoglycaemia. These devices are not yet available on the NHS and are awaiting assessment from NICE.
Newer devices measure glucose levels every three minutes and show the readings on a small screen – so-called ‘real time’. They are currently rather expensive and are undergoing further intensive research. An exciting development has been the linking of these systems to an insulin pump and, by using a small computer chip, the device recommends insulin doses to the patient before meals.

Infrared devices

A non-invasive method uses an infrared light shone across small blood vessels. The level of glucose in the blood will affect the impedance of the light across the vessel and generate a signal, which can be converted to provide a blood glucose value. This method is, however, still highly experimental.

Urine tests

Kidney (or renal) threshold for glucose reabsorption

Glucose appears in your urine when your kidneys can no longer reabsorb the amount being filtered. The problem with urine testing is that this ‘overflow point’ isn’t the same for everyone. The correct term for this overflow point is the kidney (or renal) threshold for glucose reabsorption.
Some people who don’t have diabetes have a low threshold, and they often need the glucose tolerance test, described on page 16, to confirm the fact and explain why glucose has appeared in their urine. The normal threshold is around a blood glucose level of 10 millimoles per litre (mmol/l) so, for a person
with diabetes, a negative urine test can mean that your blood glucose level is anywhere between 0 and 10 mmol/l, depending on your personal threshold.
A positive test, on the other hand, doesn’t tell you the exact level of blood glucose or by how much it exceeds your own personal threshold. Despite this relative lack of accuracy, however, testing your urine and getting mostly negative results may be all you need to confirm that you have your diabetes well under control, especially if you’re being treated with diet and/or tablets.

How to do the test

Nearly everyone these days uses stick tests similar to those used for testing blood glucose. You dip a stick either into the stream of urine or into a specimen that you’ve just passed, wait for the chemical reaction that results in a colour change, and then read off the colour against the chart, which is usually printed on the side of the container.
As with the blood testing sticks, how long you wait varies from one type of urine stick to another, so do check the manufacturer’s instructions.
The test must be done on fresh urine if it is to reflect the level of glucose in your blood at the time that it’s done. This is especially important first thing in the morning, when urine may have been accumulating in your bladder over several hours.
What you have to do is empty your bladder about half an hour before you want to do the test, then pass another sample about half an hour later which is the one that you actually check.

What the results of blood or urine tests show

When you do either a blood or a urine test, you’re really measuring how effective your previous dose of insulin or tablet treatment has been. In other words, doing a test just before lunch will tell someone on insulin the effect of the early morning injection of quick-acting insulin. In the same way, a pre-breakfast test will reflect the effectiveness of the previous night­time dose. The same interpretation applies in principle to tablets.

Adjusting doses

When the test result shows a high level of glucose, you may have to increase the size of your next dose of medication to restore the balance. This solves the problem short term, but ideally you want to prevent the problem arising in the first place by adjusting the dose that preceded the test.
It’s a good idea to vary the time of day when you do your test, and also to wait for a series of results over a period of, say, three to five days, before making too many adjustments. That way, you will see whether there is any pattern to the changes in your blood glucose level.
Until you have more experience of handling your diabetes, it would be better to consult your GP or someone in your diabetes care team before altering your insulin or tablet dosage. Later on, once you’ve learned more about your body’s reactions, you’ll be able to make the necessary adjustments on your own because you’ll know what works for you.

Clinic monitoring

There may be situations where your medical advisers feel that it would be useful to assess the effectiveness of your treatment by means of more sophisticated blood tests. They are not a substitute for your own routine testing, but can give additional information, which will help your diabetes care team decide whether your treatment needs adjustment. Both tests usually require a blood sample to be taken from a vein, although some new machines use only a fingerprick blood sample.

Glycated haemoglobin

This is a test that measures your average blood glucose level over a period of some six to eight weeks. It is reported as a percentage, unlike blood glucose (units of millimoles per litre or mmol/l), and the normal (non­diabetic) range is usually between four and six per cent.
Good control is usually defined as a value of 7.5 per cent or less; poor control is 10 per cent or more. Roughly speaking, a glycated haemoglobin of 7.5 per cent is equivalent to an average blood glucose of 10 mmol/l, whereas a value of 10 per cent is equivalent to an average of 15 mmol/l. Like all averages, however, it could be the result of lots of small variations or much larger swings in either direction. For this reason, this test isn’t useful for making day-to-day adjustments of insulin treatment, but is a good guide as to whether your treatment is working well overall.

Fructosamine

This test works on the same principle as glycated haemoglobin, but measures treatment effectiveness over a shorter period – about two to three weeks. Again it is a useful guide as to whether your current treatment is working well or needs adjustment.

KEY POINTS

• Blood tests provide accurate information about glucose control

• Blood tests are helpful to exclude hypoglycaemia (low blood glucose)

• Urine tests are perfectly adequate for monitoring patients on diet control or low doses of oral hypoglycaemic agents (OHAs), but are not very helpful for alerting the patient to hypoglycaemia