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What is the HbA1C?

The HbA1c

Enlarge  Haemoglobin, in your blood, joins up with glucose to form the chemical called HbA1c

In the blood stream are the red blood cells, which are made of a molecule, haemoglobin. Glucose sticks to the haemoglobin to make a 'glycosylated haemoglobin' molecule, called haemoglobin A1C or HbA1C. The more glucose in the blood, the more haemoglobin A1C or HbA1C will be present in the blood.

Red cells live for 8 -12 weeks before they are replaced. By measuring the HbA1C it can tell you how high your blood glucose has been on average over the last 8-12 weeks. A normal non-diabetic HbA1C is 3.5-5.5%. In diabetes about 6.5% is good.

The HbA1C test is currently one of the best ways to check diabetes is under control; it is the blood test that gets sent to the laboratory, and it is done on the spot in some hospital clinics. Remember, the HbA1C is not the same as the glucose level.

Coincidentally the glucose/HbA1C numbers for good control are rather similar though in the UK and Europe: glucose levels averaging 6.5 mmols/l before meals is equivalent to 7% HbA1C (glucose levels are higher after meals)   (see below).

 

Two examples

Below are two examples of people who have their HbA1c measured. One is poorly controlled, one well controlled.

 

A graph of glucose changes over 9 weeks. The glucose (green line) changes between 7-12. This results in an HbA1c level of 10% at the end of the 9 weeks (red line). Poorly controlled.
Here the glucose changes between 5-9. This results in an HbA1c level of 7% at the end of the 9 weeks. Well controlled.

 

 

When should the HbA1C be measured?

Measure HbA1c
  • every 3 months if trying to improve
  • every 6 months if very stable

If your diabetes is controlled (basically an HbA1C lower than 7%), every 3-6 months.

But if the last reading is above 7% and you are in reasonable health, you will need to achieve a lower level if possible, and the next reading should be sooner. This assumes you will make changes to improve your control. There is no point in having your HbA1c measured if you are not trying to achieve good control of your diabetes, although the level does predict the likelihood of complications from your diabetes.

Your ideal HbA1c depends on your general health, and whether or not you use insulin, etc, and is discussed here.

 

 

Illustrated diagrammatically

Haemoglobin in the blood (red, rectangle) combines with glucose in the blood (green, circle) to form glycosylated haemoglobin.
This reaction occurs over a 10 week period.

 


Controlled diabetes, not much glucose, not much glycosylated haemoglobin
Uncontrolled diabetes, more glucose, much more glycosylated haemoglobin

 

Glucose levels fluctuate from minute to minute, hour to hour, and day to day. Thus for hour to hour control, or day to day, a glucose level is the best guide.
The HbA1C level changes slowly, over 10 weeks, so it can be used as a 'quality control' test.
In diabetes glucose tend to rise more than usual, dropping with exercise, rising after food, rising a lot more after sweet food, and can make it hard to control.

For how to lower the HbA1C to achieve better control (and better health in the long term), see the pages type 1 diabetes/insulin (type2) or type 2 diabetes (no insulin).   Good control produces benefits that lasts 10 years at least (NEJM 2008).

 

How does your glucose level compare with your HbA1c

HbA1c %
Average blood glucose level mmol/l
13 18
12 17
11 15
10 13
9 12
8 10
7 8
6 7
5 5

HbA1c levels by coincidence nearly equate to glucose levels. So an HbA1c level of 10% means the average glucose level for the previous 10 weeks was 13mmol/l.

But at lower levels there is even less difference, so an  HbA1c of 7% means the average glucose level was 8mmols/l.

See HbA1c to blood glucose convertor (link).

 

 

 

 

Diagnosing diabetes

Diabetes may be defined as having an HbA1c>6.5% (Pulse 2010). So,

 

Duration of diabetes

Type 2 diabetes may be present for many years before it is diagnosed. Blood pressure levels and proteinuria may be proxy measurement for the length of time diabetes has been present...the higher the blood pressure and the greater the proteinuria, then the longer time diabetes has been present.

This combination of blood pressure levels and proteinuria may also be used to determine how long a type 1 diabetic has had poor control.

Converting to mmol/l

DCCT- HbA1c(%)
IFCC-HbA1c(mmol/l)
6.0 42
6.5 48
7.0 53
7.5 58
8.0 64
9.0 75

 

see

>6.6%/48mmol/l = chronic diabetes .

HbA1c may be normal in acute stages, such as in pregnancy or sudden onset type 1, or young children, or HIV, so in such circumstances blood glucose testing should be used.

 

 

 

References

 

 

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