What is the HbA1c test?

The HbA1c test measures your average blood glucose over 2–3 months and gives an indication of your longer-term blood glucose control. It is used to diagnose diabetes and to check your blood glucose levels if you have been diagnosed with diabetes.

Key points

  1. The HbA1c test can be performed at any time of the day and does not require any special preparation such as fasting.
  2. In New Zealand, the HbA1c test is measured in millimoles per mole (mmol/mol).     
  3. If you have diabetes, learning what your target range for HbA1c and having regular HbA1c tests helps you and your healthcare team monitor how well your diabetes is controlled and whether any changes in lifestyle or medication are needed.

What is HbA1c?

Haemoglobin (Hb) is the protein in your blood that carries oxygen around your body. Glucose in your blood can stick to the haemoglobin protein. When this happens the haemoglobin is called HbA1c. The higher the glucose levels are in your blood, the more of it gets stuck to your haemoglobin and the higher your HbA1c is.

Because red blood cells only last for about 4 months (120 days), the HbA1c level only gives an indication of your blood glucose level over that time.

The amount of HbA1c used to be reported as a percentage of haemoglobin that had glucose stuck onto it, but in New Zealand HbA1c is now measured in millimoles of HbA1c per mole of haemoglobin (mmol/mol). Roughly speaking, an HbA1c of 6% is the same as 42 mmol/l and means the average blood glucose is about 7 mmol/l. To compare new units with old units you can use HbA1c unit converters like this one.

How useful is the HbA1c test?

HbA1c reflects the average plasma glucose (sugar in your bloodstream) over the previous 2–3 months and measures how much glucose has become stuck onto your red blood cells.

  • It can be performed at any time of the day and does not require any special preparation such as fasting.
  • The HbA1c test is the preferred test for diagnosis of diabetes.

There are some situations in which HbA1c can overestimate or underestimate the amount of glucose in your blood, in particular, in people who have anaemia (a lack of haemoglobin) or other red blood cell problems, or people with kidney disease.

If you do have diabetes, learning what your target range for HbA1c is very important. Having regular HbA1c tests helps both you and your healthcare team monitor how well your diabetes is controlled and whether any changes in lifestyle or medication are needed. 

How is HbA1c used for diagnosing prediabetes and diabetes?

As a general guide, HbA1c levels of: 

  • less than or equal to 40 mmol/mol are normal
  • 41 to 49 mmol/mol are prediabetes or ‘impaired fasting glucose’
  • 50 mmol/mol and above suggest diabetes if you have symptoms. (If you have no symptoms of diabetes, 2 tests on separate occasions are needed).

Read more about prediabetes, type 1 diabetes and type 2 diabetes.

HbA1c in pregnancy

HbA1c is also used in early pregnancy, up to about 12 weeks, to diagnose diabetes. It is also used to monitor a pregnant woman’s blood glucose levels if she has been diagnosed with diabetes. However, because of the changes in a woman’s blood and haemoglobin levels in pregnancy, HbA1c is not used to diagnose diabetes later in pregnancy. A glucose tolerance test, which measures blood glucose levels before and after a special sugary drink, is used to diagnose diabetes in pregnancy.

What are healthy HbA1c levels for people with diabetes?

An ideal range or target HbA1c level varies from person to person and depends on age, type of diabetes and other health conditions or stages (such as pregnancy). Ask your doctor or nurse what your target HbA1c is. 

The following ranges provide a general guide: 

  • Less than or equal to 53 mmol/mol is a very healthy HbA1c level.
  • Between 54 mmol/mol and 63 mmol/mol is a fair HbA1c level and needs work to improve it.
  • Between 64 mmol/mol and 86 mmol/mol indicates your blood glucose levels are much too high.
  • Above 86 mmol/mol indicates your blood glucose levels are extremely high and urgent action is required. 

If you are taking insulin and your HbA1c level is too low, you are at higher risk of having low blood glucose levels (‘hypos’ or hypoglycaemia). In this case, talk with your doctor/nurse about whether you need to reduce your insulin dose or frequency. 

Why is tight diabetes control so important?

A number of studies have shown many benefits of reducing a high HbA1c level. In these studies, the equivalent of an 11 unit decrease in HbA1c, eg, from 86 to 75 mmol/mol or 75 to 64 mmol/mol leads to: 

  • a 16% decrease in risk of heart failure
  • a 14% decrease in risk of fatal or non-fatal myocardial infarction (heart attack)
  • a 12% decrease in risk of fatal or non-fatal stroke
  • a 21% decrease in risk of diabetes-related death
  • a 14% decrease in risk of death from all causes
  • a 43% decrease in risk of amputation
  • a 37% decrease in risk of small blood vessel disease (eg, retinal blood vessel disease causing vision loss).

The sooner you take control of your health and do all you can to get your blood glucose and HbA1c levels down into the optimal range, the better. Learn more about type 2 diabetes, including balancing food, activity levels and medications.

If you are overweight (which most people with type 2 diabetes are) then losing some weight is often the best thing you can do. In some cases, people have managed to reverse their diabetes by getting back to a healthy body weight and adopting healthier eating and activity patterns. 

Work with your doctor/nurse and diabetes team to learn what approach is best for you to regain good diabetes control.

If you are on insulin or have high HbA1c levels, then also ask about self-monitoring of your blood glucose levels. 

Do I need to check both my blood glucose and HbA1c levels?

Yes you do! HbA1c and blood glucose levels give different information. A blood glucose level tells you what your level is right now. Your blood glucose level goes up and down all the time as you eat and exercise and is usually measured by a finger prick test. HbA1c is a way of averaging out all these results to see if your diabetes treatment needs to be adjusted. See type 2 diabetes management.

Note: If you visit American websites, watch out for different units. In the USA, blood glucose is measured as mg/dL. As a rule of thumb, a blood glucose of 5.0 mmol/l, which is a common target for blood glucose before meals, is the same as 90 mg/dL.

Visit Diabetes UK to convert blood sugar/glucose from mmol/L (UK standard) to mg/dL (US standard) and vice versa. 

References

  1. Yozgatli K et al. Accumulation of advanced glycation end products is associated with macrovascular events and glycaemic control with microvascular complications in Type 2 diabetes mellitus Diabet Med. 2018 Apr 23;35(9):1242–1248.
  2. The new role of HbA1c in diagnosing type 2 diabetes BPAC, NZ, 2012
  3. Braatvedt, G D. et al. Understanding the new HbA1c units for the diagnosis of type 2 diabetes NZ Med J. 2012 Sept 23;125(1362): 70–80.
  4. Management of type 2 diabetes NZ Primary Care Handbook, 2012
  5. Stratton IM, Adler AI, Neil HA, et al. Association of glycaemia with macrovascular and microvascular complications of type 2 diabetes: prospective observational study British Medical Journal 2000 Aug 12;321(7258): 405-12.

Credits: Health Navigator Editorial Team .