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Diabetes: blood glucose monitoring

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VOL: 97, ISSUE: 08, PAGE NO: 36

Mary Burden, RGN, MPH, is senior diabetes specialist nurse, Leicester General Hospital

Patients and nurses have somewhat different perspectives on blood glucose monitoring (BGM). Patients want the assurance of satisfactory blood glucose control, they want to be able to cope with other illnesses or new situations, to adjust and experiment with their treatment dose, and to investigate the effects on blood glucose of different aspects of their lifestyle, such as diet, physical activity or alcohol consumption.

Nurses, on the other hand, focus on estimates of blood glucose in order to adjust therapies if patients are too ill to do it themselves - such as when requiring intravenous insulin - as part of routine observations, but also as an educational tool to encourage patient self-management.

The main benefit of self-testing of blood glucose over urine testing is that patients sometimes prefer it and urine testing cannot inform people with diabetes that their blood glucose is low - only if it is high. All the same, there are several occasions when capillary BGM is contraindicated (Table 1).

Urine or blood testing?

Many patients and nurses consider BGM to be more ‘accurate’ than urine testing but, in fact, they are measuring different things. Patient preference and utility for the individual should be the deciding factors.

How does urine testing work? On average, when blood glucose levels reach about 10mmol/L the glucose spills over into the urine and the amount can be measured. However, in some people the renal threshold is low and the glucose spills into the urine at lower levels - this happens particularly in children and pregnant women. In elderly people the renal threshold tends to be higher, so blood glucose levels can be quite high before glucose enters the urine. Negative sugars in the elderly may therefore be deceptive.

Some people with renal glycosuria - an abnormal amount of sugar in the urine - do not have diabetes but do have a low renal threshold.

A tool to empower patients

A recent meta-analysis concluded that current studies do not provide evidence for the clinical effectiveness of self-BGM (SBGM), which is expensive, and that further work is needed to evaluate self-monitoring so that resources for diabetes care can be used more efficiently (Coster et al, 2000).

Even in type 1 diabetes, Starostina et al (1994) have shown no difference in outcome between urine and blood testing, the key factor being whether people received education. Measured outcomes included HbA1c (glycosylated haemoglobin), hypoglycaemia, diabetes ketoacidosis and days off sick.

So why are people with diabetes asked to do SBGM? The answer is to empower them to take an active part in their own self-management.

All too often, however, SBGM is inflicted on people with diabetes by health professionals. Some of the reasons patients don’t like BGM include:

- BGM can seem pointless;

- Checking blood glucose reminds you that you have diabetes;

- The meter rules your life;

- It makes you feel bad about yourself;

- It is painful and inconvenient;

- It takes up too much time.

When SBGM is chosen as the monitoring method the next question is, when should it be done. The patient’s preference must be taken into account. Some patients prefer to test blood glucose once a day but at different times: for example, before breakfast one day, then before lunch the next, before the evening meal the following day and before the bedtime snack the next, then repeating the pattern. Other people prefer to do several tests all in one day - once or twice a week, for example.

The point of these patterns to is to give an idea of blood glucose levels over 24 hours, either as reassurance that all is well or to indicate that adjustment needs to be made.

The regimen described above can be used routinely, but there are times when more frequent testing is useful:

- When ill;

- When planning a pregnancy, or pregnant;

- When changing therapies or changing injection sites;

- When using an insulin pump;

- Postprandial measurement for action of insulin analogues (Humalog or NovoRapid).

When teaching SBGM it is important to emphasise that this is a useful tool to help achieve HbA1c targets. HbA1c is usually a laboratory investigation that measures how much glucose sticks to the haemoglobin in the red blood cell and glycates - the higher the blood glucose the higher the HbA1c.

Since the red blood cell survives on average for 120 days, this test measures how high the blood glucose has been over the last three months. The result is expressed as a percentage and the normal range differs between laboratories, so it is important to check on your local laboratory. There are moves afoot to standardise this test (Bartlett et al, 2000), which would facilitate interpreting the results.

Since the publication of the Diabetes Control and Complications Trial in type 1 diabetes (Diabetes Complications and Control Trial Research Group, 1993) and the UK Prospective Diabetes Study in type 2 diabetes (United Kingdom Prospective Diabetes Study Group, 1998), the general targets for HbA1c are less than 7.5% for type 1 and less than 7% in type 2. These have shown that keeping the HbA1c in these ranges dramatically reduces microvascular (eye, kidney and nerve) and macrosvascular (myocardial infarction, peripheral vascular disease and strokes) complications. Self-monitoring of either blood or urine can be a useful tool for patients to achieve these important targets. 

How accurate are BGM meters?

Even when used optimally, meters give a range of acceptable results, rather than a single figure. This is illustrated in Table 2. The quality control (QC) range is usually printed on the tin of strips or occasionally in the data sheet. This means that the result is a guide only, not the equivalent of a laboratory result.

This has particular implications at the low range because treatment decisions are often based on the results. Table 2 shows that meter A could produce a result in a patient whose laboratory glucose is 4mmol/L of anything from 2.8 to 5mmol/L.

To ensure that optimal results are obtained, the Medical Devices Agency advises that independent QC procedures on all extra-laboratory measurements should be carried out regularly in collaboration with the pathology laboratory to ensue that competence is maintained by all users and that the results are comparable with those produced by a quality controlled laboratory-based instrument (MDA, 1996). 

People with diabetes using SBGM should be encouraged to purchase the QC solutions appropriate for their meter from their pharmacist or the manufacturer, so that they can run periodic checks themselves. Some diabetes centres run QC clinics to support patients in this task. It is worth noting that meter manufacturers often have free telephone helplines and patients should be encouraged to make use of them.

BGM in the hospital or clinic

Nurses should obviously comply with good practice as described by the MDA (1996) and with the manufacturers’ instructions. They should wear gloves to protect themselves from blood-related injury. Manufacturers usually offer training and support to set up a QC programme. Some meters have a ‘lock-out’ to ensure that only people trained and competent can use the meter.

On the ward, BGM is often done every four hours as part of routine observations. This does not usually relate to food and can lead to misinterpretation of results. It is more useful to relate the tests to meals and to use a problem-oriented approach.

Patients are often denied the opportunity to carry on with their usual testing routine and it becomes very difficult for them to adjust their own diabetes treatments. If someone is on intravenous insulin, BGM is often performed every hour, night and day, with little thought about the individual patient’s therapy change.

How to minimise problems

The case studies (opposite) illustrate some typical problems. The nurse needs to think through the whole procedure to eliminate hazards. Some examples include:

- Hands not washed;

- Inappropriate use of devices to prick finger, with insufficient care to prevent cross-infection;

- Use of needles rather than lancets to prick the finger (needles are designed to slip through the skin with minimal trauma), leading to insufficient blood for the test;

- Poor disposal of lancets, cotton wool, BG strips;

- Gloves not worn.

The manufacturers of blood glucose strips recommend using soap and water and drying thoroughly when cleaning hands. But this is not always possible when the patient is bed-bound and some nurses choose to use sterets to clean the skin. Potentially this can cause problems since they contain alcohol, which can itself contaminate the skin and lead to poor results.

Dunning et al (1994), however, found using sterets had no clinical effect on meter accuracy. In practice, if using sterets or alcohol wipes the skin should be allowed to dry before pricking the finger, although in the hustle and bustle of the ward or clinic it is tempting to wipe the skin with a steret and perform the test immediately.

Although meter manufacturers insist that the meter will warn if there is insufficient blood to get an accurate reading, in nurses’ and patients’ experience this is not always the case - a finding supported by the MDA.

When performing BGM in wards and departments there needs to be policy of only using finger-prickers that are fully disposable or that require disposal of both platform and lancet. Some years ago there was an outbreak of hepatitis in a paediatric ward caused by removing only the lancet between patients. Rudoni et al (1999), however, found no problems with newer devices.

If the meter is the sort that requires the strip to be put into the meter before the blood is put on to the strip, this is a potential cause of cross-infection and, therefore, should not be used in the hospital setting. If a patient is nursed in isolation they will need their own meter and adequate decontamination procedures for re-use of equipment.

Nurses should ensure they comply with safe procedures of disposal and share this knowledge with patients and carers. Refuse collectors have been known to refuse to collect rubbish bins if they see sharps (or, even worse, experience a needlestick injury). Different local authorities have different policies about this, so it important to find out the local policy when advising patients.

Conclusion

Blood glucose monitoring by the patient is costly, painful and time-consuming. It can, however, be very valuable when it is used to adjust and change therapies or make appropriate adjustments for lifestyle changes. Nurses should be aware of the difficulties that self-BGM poses and work with patients to devise individual solutions.

Newer devices are being developed to allow non-invasive methods of measuring glucose. The role of glucose sensors has not yet been established, but may give 24-hour blood glucose patterns, for example to detect unrecognised hypoglycaemia (American Diabetes Association, 2001).

The nurse is accountable for ensuring the training has been received and that competencies have been tested, particularly if delegating this task to others. Steps must be put in place to ensure safe and effective practice.

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