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Everyone is different and the blood glucose level (BGL) targets for one person may be too low or too high for another. The table below shows ideal BGL targets before and after meals, which are the same as the values that occur in people without diabetes.

The 2009 National Health & Medical Research Council (NHMRC) values were recommended after a review of the evidence of the benefits of lowering blood glucose in terms of reducing the risk of diabetes complications and the cost of achieving lower values (the extra effort and the increased risk of weight gain and hypoglycaemia).

Two ways of checking BGLs
1. Blood glucose monitoring shows you your current BGLs and testing at different times of the day indicates the pattern of your blood glucose through the day and from day to day.
2. The laboratory measurement of glycosylated haemoglobin (glucose attached to the haemoglobin in your red blood cells; HbA1c) indicates the average BGL over the last few weeks (24 hours a day, seven days a week). The laboratory reports HbA1c two ways [see ‘The measures of HbA1c’ table]. The usual target for HbA1c is less than 7%. The relationship between HbA1c and the average BGL is: Average blood glucose (mmol/L) = 2 x HbA1c (%) – 6

The various measurements tell you:
– If you feel strange and you think you might be having a ‘hypo’, a blood glucose test will indicate it
– If you and your doctor are reviewing your blood glucose management, the pattern of blood glucose tests will help you decide whether you need to modify your eating, activity or medication at particular times of the day
– When you are having a full diabetes review, the HbA1c will indicate if your average BGL is on target

Taking account of variability
Blood glucose levels vary through the day: lower before than after a meal, higher after a large meal with lots of carbohydrate, and lower during and after a long exercise session.

Your BGLs also vary from day to day, even if measured at exactly the same time and if the food, activity and medication remain the same. For example, if you measure your BGL before breakfast for 10 days and divide the sum of the 10 results by 10, the mean might be 8.4 mmol/L, although one BGL was over 13 and another lower than 5.* That’s why you, your doctor, diabetes educator and
dietitian should look at the blood glucose pattern over at least several days so you get an idea of the average and also the swings of BGLs over 24 hours and between days.

In the above example, you wouldn’t want to increase your medication to lower the average BGL before breakfast from 8.4 to 5.4 (i.e. by 3 mmol/L) because that might mean that the BGLs that were less than 5 drop to less than 2! Similarly, if the average BGL before breakfast was 5.4 but the BGL before dinner was 8.4 mmol/L, you would increase a medication that works through the day but not a medication that works over the whole 24-hour period, which might lower the BGL before breakfast and make you hypoglycaemic.

If you measure blood glucose after a meal (usually two hours after starting to eat), always measure the BGL before the meal as well because the BGL after the meal is affected by the BGL before you eat as well as by the meal itself. Also, remember that the BGL after a particular meal (e.g. breakfast) will vary daily even if the breakfast is the same each day.

The main points about blood glucose targets are:
– Targets before and after meals and for HbA1c are set individually so can vary for different people
– Blood glucose monitoring gives you the BGL reading ‘right now’ and can also give you the pattern of blood glucose over the 24-hour period and from day to day
– The HbA1c approximates the average blood glucose over the last few weeks: average blood glucose (mmol/L) = 2 x HbA1c (%) – 6
– Blood glucose at the same time of day can vary markedly between days, so the average blood glucose value is important when reviewing lifestyle and medication

Derived from Diabetes Technology and Therapeutics 2008;10:149–59.

Dr Pat Phillips, MBBS, MA, FRACP, Dip Ed Health Economics, is Editor-in-Chief of Conquest and Past Director of the Diabetes Centre and Endocrinology, Queen Elizabeth Hospital & Health Service. Angela Close is a pharmacist.

This article was originally published in Conquest magazine published by Health Publishing Australia