Your HbA1c value depends on the average blood glucose levels during the last 2–3 months. A 1% increase in HbA1c (measured with the DCCT-equivalent method) means that you have had an average increase of approximately 2 mmol/L (35 mg/dl) in blood glucose levels compared with when your last test was taken. The graph shows readings from the American DCCT study and is redrawn from American Diabetes Association’s Clinical Practice Recommendations 2003. The solid line shows the relationship between HbA1c and plasma glucose (11% higher than blood glucose in this study). Aim to have an HbA1c value of 7% or below if possible. If it is above 8%, you and your diabetes team have some work to do together, assessing and revising your diabetes care. This table (from the Rohlfing et al. reference) shows the mean glucose values that a certain HbA1c value represents. Most meters used nowadays display plasma glucose.
Unfortunately, different laboratories have different reference values for HbA1c but they are slowly becoming more standardized. The American Diabetes Association recommends that the goal of therapy in adults and adolescents should be an HbA1c below 7% and that the treatment regimen should be re-evaluated in patients with repeated HbA1c above goals. The Canadian Diabetes Association recommends that adults and adolescents should aim for an HbA1c below 7%. The International Diabetes Federation (IDF) now recommends an even lower HbA1c target of 6.5%.
Many studies have shown that with an HbA1c value of less than 8% the risk of long-termblood vessel complications will be considerably lower. If your HbA1c is above 9% we feel that this is unfair to your body since we know that in
the long run it will sustain damage from this. Studies of adults have shown that those with a lower HbA1c experience better levels of psychological wellbeing. This includes less anxiety and depression, improved self-confidence and a
better quality of life.
The risk of severe hypoglycaemia makes it difficult to achieve very low HbA1c levels. If someone with Type 2 diabetes has an HbA1c within the range for individuals without diabetes, this usually means they are at high risk of severe hypoglycaemia and/or hypoglycaemia unawareness. In the DCCT study, patients with low HbA1c had a significantly higher risk of severe hypoglycaemia. However, the risk decreased during the years of the study.
Why check your HbA1c?
Is checking your HbA1c worthwhile? For whose benefit is the HbA1c test being done? Many individuals feel as if they are visiting a ‘control station’ and being examined by health professionals to see how well they have ‘behaved themselves’. From the professional point of view, however, the HbA1c test is most valuable to you yourself. When you see the reading, you will know if your way of life over the last three months has allowed you to achieve the average blood glucose level you want for the future. When the HbA1c method was introduced, 240 adults with diabetes measured it every third month without otherwise changing their diabetes treatment. After a year, the average HbA1c. value was unchanged but it turned out that those with very low values had increased them and those with high values now had lower values, showing the benefit of knowing your HbA1c level.
DCCT method and equivalent
Normal value person without diabetes 4.1–6.1%
Adolescents and young adults < 7.5%
Adults < 7.0%
Needs improving and re-evaluation of treatment 8–9%
Not acceptable: High risk of complications >9%
May have high risk of severe hypoglycaemia < 6%
There may be individual differences in the HbA1c value it is realistic to achieve. Discuss with your diabetes team what value may be realistic for you.
For how long do blood glucose levels affect HbA1c?
Your recent blood glucose level affects HbA1c much more than that from 2–3 months ago. However, your values during the last week will not show on most methods since this fraction of HbA1c is very unstable. For a given HbA1c value, the contribution of the blood glucose is (counting backwards):
Day 1–6 : very low
Day 7–30 : 50%
Day 31–60 : 25%
Day 61–90 : 15%
Day 91–120 : 10%
Many countries (US, Australia, UK, Denmark, France, The Netherlands among others) have standardized their HbA1c monitoring methods to show DCCT-equivalent numbers. This means that if you read about studies on the Internet, you can compare like with like between one study and another. Set up your own personal goal for your HbA1c in collaboration with your diabetes team. This goal will be different for different people and perhaps also different during different times of your life. It may be more difficult to achieve the same HbA1c level for example at times when you are having problems at home or at work. By competing with yourself and setting a reason
Everyone with Type 2 diabetes should check their HbA1c regularly, every 3–6 months. A high level (8–9% with DCCT numbers or an equivalent method) is not acceptable, considering the risk of future complications. In older people, it may be up to 0.5% higher simply due to increasing patient age. After a visit to your diabetes healthcare team, you may feel more motivated to ‘get your act together’ and keep your blood glucose readings low. Use this motivation to address any problems with diet, lifestyle, your medication or insulin regimes. Some clinics send their HbA1c tests to the laboratory, so it may be some days before you get the result while others ask you to send in a blood sample a week before the clinic. Others use a desktop method that gives a result after a few minutes.
Even if your blood glucose control is improving and your tests are showing lower readings, it will still take some time for this to show in your HbA1c. Half the change will show after about amonth, and three quarters of the change after 2 months. If you start with a very high HbA1c (12–13%) and normalize your blood glucose levels completely (as often happens at diagnosis) it will go down by approximately 1% every 10th day.
If you have a very low HbA1c your average blood glucose is low and you may have a high risk of developing serious hypoglycaemia without any warning symptoms. If you have a low HbA1c (6–7%) and problems with severe hypoglycaemia or hypoglycaemia unawareness, it may be a good idea to aim for a slightly higher blood glucose level.
Monitoring fructosamine is a method of measuring the amount of glucose that is bound to proteins in the blood. The value reflects the blood glucose level during the last 2–3 weeks. Fructosamine can be good indicator during times of rapid changes in glucose control, for example when you start with a new method of treatment. However, if you take a fructosamine test every third month only, you will not get a representative measurement of your glucose control over a longer period of time. This method, therefore, is not recommended for routine monitoring of long-term glucose control.
The HbA1c result relies on the fact that red blood cells last about 120 days. Any medical problem which shortens the life of the red cell will tend to artificially reduce the HbA1c percentage. Such conditions include haemolytic anaemia, kidney failure and pregnancy. HbA1c
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