by Lesh Karan
Do you have a handle on HbA1c? It may seem complicated, but getting a grip on HbA1c doesn’t have to be hard. Whether you have a basic understanding or no idea at all, this article explains all you need to know about HbA1c.
1. What is HbA1c?
Think of it this way – sugar sticks. And inside the body, sugar sticks (binds) to red blood cells, creating a red blood cell–sugar complex. Technically, sugar sticks to haemoglobin – a specific part of red blood cells. (Haemoglobin is what gives red blood cells their colour.) This sugar–haemoglobin complex is called glycosylated (or glycated) haemoglobin, otherwise known as HbA1c (HbA = haemoglobin; 1c = glycosylated). Once red blood cells become glycosylated (stuck to sugar), they stay glycosylated until they die (about 3 months). As red blood cells die, new ones are produced. If the new blood cells are not glycosylated – due to better blood glucose control – then the overall HbA1C will decrease.
2. How is HbA1c tested?
Unlike Blood Glucose Level (BGL) tests that you do daily with a glucose meter, HbA1c tests need to be done by healthcare professionals – ideally 2 to 4 times a year. They most commonly use the traditional method of withdrawing blood from a vein in the arm (venous blood draw). The blood is sent to a laboratory to measure the HbA1c levels.
In the last few years, however, some clinics (hospitals and private practice) and pharmacies have begun to provide HbA1c ‘point of care testing’ (POCT) where a person’s HbA1c is tested on the spot using a special machine. The machine uses a finger prick of blood to perform the test and provides the result within minutes. If you would like to know more about POCT, and where you can have it done, speak to your doctor or pharmacist.
3. What do HbA1c tests measure?
The HbA1c test counts the number of red blood cells that are glycosylated (attached to sugar) and reports it as a percentage – for example, if 7 out of every 100 red blood cells are attached to sugar, the HbA1c result will be 7%. This is different from BGL tests, which measure the amount of sugar that is freely circulating in the blood – and, as such, use a different unit of measure (millimoles of sugar per litre of blood – mmol/L).
4. What can affect the accuracy of the HbA1c test?
There are some situations where the HbA1c test may not be reliable. To be accurate, the test relies on the 3-month lifespan of red blood cells. In circumstances where the lifespan of the red blood cells is affected, the test will be inaccurate. Some examples of such situations include anaemia, iron deficiency, blood transfusion (including blood donation), kidney failure and any illness that affects red blood cell survival. Vitamin C and E intake has also been reported to influence HbA1c levels.
5. Is Hba1c an average of blood glucose levels?
A misunderstanding of the difference between the two measures is probably one of the reasons why this question gets asked often. Since HbA1c and BGLs measure different things (see section 3), a HbA1c test result is not an average of BGLs. However, it is logical (and correct) to assume that consistently high BGLs will result in more glycosylated red blood cells, thereby increasing HbA1c. A fairly recent study, reported in 2008, found a strong correlation (relationship) between the two measures. This allowed the researchers to develop a mathematical equation to convert HbA1c levels into average BGLs – known as A1c-Derived Average Glucose (ADAG) or estimated Average Glucose (eAG). If you would like more information, check out the following links:
* ADAG study: http://care.diabetesjournals.org/content/31/8/1473.full.pdf+html
* Calculator to convert HbA1c levels into eAG (use the mmol/L option): http://professional.diabetes.org/GlucoseCalculator.aspx
6. What should your HbA1c levels be?
For most adults with diabetes, regardless of whether it’s type 1 or type 2, the general HbA1c target is <7.0%; that is, ideally, 7% or less of red blood cells should be glycosylated. HbA1c levels of 8.0% or above may indicate that tighter control of blood glucose levels is required. As a reference point, in adults who don’t have diabetes, the HbA1c levels are usually between 4.0 to 6.0%. The lower the HbA1c level in people with diabetes, the higher the risk of hypoglycaemia, so this and other factors, such as pregnancy, are also taken into consideration. Doctors may recommend lower (tighter) or higher HbA1c targets for some individuals.
The HbA1c target for children and adolescents is slightly higher (<7.5%) than it is for adults and is more likely to be individualised. This is because maintaining good sugar control in children is trickier, as a number of factors can put them at higher risk of developing hypoglycaemia, such as, variable food intake, hormonal changes during puberty, and varying rates of growth and development.
Large, well-conducted clinical studies – namely the Diabetes Complications and Control Trial (DCCT; type 1 diabetes study) and the United Kingdom Prospective Diabetes Study (UKPDS; type 2 diabetes study) – have shown that having an HbA1c below the recommended target reduces the risk of developing diabetes-associated complications, such as stroke, heart attack, nerve damage or damage to small blood vessels in the kidneys and eyes.
If your HbA1c is not in the ideal range, don’t panic! Speak to your diabetes educator or endocrinologist about strategies to improve your control, and jump on over to the Reality Check forum for to chat to other people about their experience.
If you are interested, you can get more information on HbA1c targets, and the studies that support them, from the following guidelines:
* National Health and Medical Research Council (NHMRC) guidelines for Type 1 diabetes in children and adolescents (Chapter 6), available at: http://www.nhmrc.gov.au/_files_nhmrc/file/publications/synopses/cp102.pdf
* Australian Diabetes Society Position Statement on individualising HbA1c targets in adults with diabetes, available at: http://www.diabetessociety.com.au/downloads/positionstatements/HbA1ctargets.pdf
7. Why is testing HbA1c levels important?
HbA1c levels give doctors and people living with diabetes invaluable information: HbA1c levels do not fluctuate quickly because once red blood cells are glycosylated, they remain that way until they die (up to 3 months). For this reason, HbA1c levels give a picture of blood glucose control over a long period of time. HbA1c is also the only proven indicator of a person’s risk of developing complications, as described in the previous section.
Blood glucose level tests, on the other hand, cannot give this type of information. As discussed in section 3, blood glucose tests measure the amount (or level) of sugar that is freely circulating in the blood. As a person’s blood glucose level can be significantly (and immediately) influenced by various factors, including activity, food intake and stress levels, it can fluctuate many times during the day. Consequently, blood glucose levels are a picture of a moment in time and cannot be relied upon to accurately give information to reflect a person’s long-term diabetes management.
8. So why still measure blood glucose levels (BGLs)?
HbA1c and BGL results give us two different sets of information – both are useful. HbA1c levels tell us about long-term glucose control to help with overall diabetes management but does not give us ‘real-time’ feedback. Blood glucose levels give us information about the present – that is, what’s happening this very minute. Thus, people with diabetes can detect any instances of hypoglycaemia or hyperglycaemia. This then allows appropriate (and immediate) changes to treatment (e.g. insulin), food intake and activity levels. In summary, both tests are important, as they give both immediate and long-term information that helps to make decisions about managing diabetes.
Hopefully this article has answered some of the questions you may have had about HbA1c. If you are still unsure about something, or have any additional questions, speak with your doctor or diabetes educator.
© Lesh Karan 2010