On the followup visit SMBG values pre and postprandial remained unchanged HbAc was percent HPLC method and the rest of the laboratory results were within normal range

There is an obvious inconsistency between HbA1c and the SMBG values of the patient. According to SMBG values, HbA1c should be much higher (see Table 4.2). When there is a discrepancy between these two parameters, which anyway are just the basic methods of glycaemic control evaluation, the physician has to discover the cause and clarify which of the two values correctly reflects the patient's glycaemic status. There are, consequently, two alternatives:

• (B) deceptive indications of SMBG.

Possible causes of alternative (A) have been mentioned earlier in this chapter. None of them seems valid for this patient. Despite that, HbA1c measurement was repeated using another method (affinity chromatography) and the result was 7.1 percent.

Possible causes of alternative (B) are shown in Table 4.3.

A frequent and easily detected SMBG problem is the recording of measurements that do not represent the whole 24 hours of the day. In the

Table 4.3. Possible causes of misleading values of SMBG with a portable glucose meter

1. Measurements only in certain selected times

2. Erroneous handling of the meter by the patient

3. Factitious report of results by the patient

4. Exogenous factors that affect measurements

5. Malfunctioning or damaged meter current patient, after advice to measure her blood sugar level frequently, this problem was ruled out.

When there is suspicion of false measurements, the correct operation of the portable meter by the patient should be ascertained by the physician or trained medical personnel. Despite the fact that modern meters are quite simple as regards their usage, mistakes are not infrequent, especially from elderly people.

Untruthful reports constitute a relatively frequent phenomenon in children and adolescents, although 'beautification' of the results is not rare in adults either. A sincere and truthful relationship between the patient and physician is very important and decreases the chance of such phenomena. Modern glucose meters can store a large number of measurements in their memory, so that retrieval of the past results is possible. Therefore, when doubt exists about the truth of the results, the patient can be asked to bring in the glucose meter to the clinic. Our patient's meter was 10 years old and had no memory storage capabilities.

There are some factors that can affect the precision of measurements. Many of them, like the time of blood drop deposition, the need to remove excess blood and adequacy of the quantity of blood, have been eliminated with the advent of modern, new generation glucose meters. Nevertheless, altitude, temperature, humidity, haematocrit changes, hypotension, hypoxia and hypertriglyceridaemia are factors that can affect the results. It should also be noted that most meters do not have a good precision at very low or very high blood glucose concentrations. Another problem is the variability of the results among different meters, even from the same manufacturer.

All meters use a stripe that contains oxidase or hexokinase and employ a photometric or electrochemical method. The advantages of modern meters, which render them superior to the older ones, are shown in Table 4.4.

Table 4.4. Technical advantages of modern portable glucose meters

• Automatic start of measurement

• Speed of measurement

• Indication of 'wrong' when quantity of blood is insufficient

• Automatic reading of barcode

• Storage of many measurements in memory, capability of connection with a computer and analysis of results

• Smaller size and weight

Every meter should be checked regularly (for example, every six months) as regards the precision of its measurements. Evaluation is performed by simultaneous draw of venous blood and measurement of plasma glucose concentration at a reliable biochemical laboratory. For meters that report plasma values, a difference of 10 percent from the laboratory is considered acceptable.

Our patient's meter was compared to the value that the local hospital laboratory was giving and her meter was found to have a significant divergence. She was advised to change the meter and continue with her current regimen. With the new meter, capillary blood glucose values were now compatible with the HbA1c value.

If the discrepancy between HbA1c and SMBG values cannot be resolved from the above mentioned factors, an alternative solution that may help is to place a glucose sensor on the patient that continuously monitors glucose concentration for 24-72 hours. These sensors measure interstitial fluid glucose concentration through a catheter that is placed subcutaneously. The technology used differs according to the manufacturer. The sensor is calibrated using capillary glucose measurements taken with a usual glucose meter, at regular intervals (4-6 times in a day). For the time being, these glucose sensors enable only the recording of the trend of glucose values in order to evaluate the glycaemic status of Type 1 diabetic patients who have great variability in their measurements, unexplained hypo- or hyper- glycaemias or a discrepancy between HbA1c and SMBG values.

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