The HbA1c Test

What is HbA1c?

HbA1c is a modified form of haemoglobin. Haemoglobin is the molecule in our red blood cells which carries oxygen around the body; most of the haemoglobin is of a type called HbA. A small fraction of HbA becomes modified during its lifetime (HbA1). In the late 1950s it was found that HbA1 could be separated further into HbA1a, HbA1b and HbA1c. A decade later, the clinical significance of HbA1c became apparent when it was revealed that many people with diabetes had uncharacteristically high levels of this haemoglobin variant.

HbA1c is formed when glucose molecules attach to HbA molecules in a process known as glycosylation. Glycosylation reactions are common and occur naturally in the body. Normally though, they take place in a controlled environment, regulated by enzymes. The process by which HbA1c is formed, however, occurs in the absence of enzymes and is therefore referred to as non-enzymatic glycosylation, or simply ‘glycation’.

What controls the process then? Essentially, the reaction between glucose and protein is time and concentration dependent. This means that the amount of haemoglobin which forms HbA1c will depend on:

The concentration of glucose that the haemoglobin is exposed to

Not all cell types in the body require insulin for the uptake of glucose. Red blood cells have ‘insulin-independent’ glucose transporters on their surface. So if glucose levels in the plasma (the watery part of the blood) are high, then glucose levels inside the red blood cells will also be high. The higher the blood glucose level, therefore, the more glycosylation of haemoglobin will occur.

The length of time that the haemoglobin is exposed to a given concentration of glucose

The longer the blood glucose level is high, the more glycosylation will occur.

Note, also, that this factor is influenced by the age of the red blood cell - many cells in our body are continually being broken down and replaced; the average life span of a red blood cell is about 120 days. The haemoglobin is continuously being glycated at a rate which is proportional to the prevailing blood glucose level. At any given time, there will be a mixture of old and new red blood cells circulating in the bloodstream. Old cells will have been exposed to recent, and not so recent, blood glucose levels. New cells will only have been exposed to recent blood glucose levels. So the more recent glycaemia will have the largest influence on the overall HbA1c reading. Indeed, it has been suggested that half of an HbA1c value is attributable to the previous month, a further quarter to the month before that, and the remaining quarter to the two months before that.

Red blood cells in patients with haemolytic anaemia have a short lifetime and they are therefore subject to less glycation; HbA1c levels in these people are therefore lower.

Chemical reaction

The actual chemistry of the reaction between glucose and protein is complex. In the short term, an intermediate is formed which may dissociate into glucose and protein again. However, over a period of many hours, the intermediate undergoes molecular ‘rearrangement’ and becomes a stable entity. So the initial linking of glucose to protein (haemoglobin in this case) is ‘reversible’ but the overall process is ‘irreversible’.

Measuring glycated haemoglobin or HbA1c levels

Since HbA1c levels depend primarily upon time-averaged blood glucose levels, it makes sense that HbA1c levels provide a reflection of glycaemic control in people with diabetes. The use of glycated haemoglobin in clinical practice was well received - the healthcare team had a simple tool which purportedly gave an accurate assessment of blood glucose control. Treatment decisions could be based on something much more solid than a patients description of symptoms, or home urine/blood test records. Now glycated haemoglobin is used in setting treatment goals and its measurement is routine for people with diabetes.

The test

There are a number of ways in which glycated haemoglobin can be measured. Methods of separating haemoglobin molecules are based on differences in their electronic charge, differences in their molecular structure or differences in their immunological reactivity. Unfortunately, the different methods measure slightly different things and there is no standard method, and therefore, no standard reference range.

Many laboratories now provide DCCT-standardised HbA1c testing. This means that the result is adjusted such that it can be directly compared to results obtained using the methodology that was used in the Diabetes Control and Complications Trial.

So far we have concentrated on HbA1c, i.e. the haemoglobin-glucose adduct. Some methods, however, actually measure the whole HbA1 fraction, which includes other sugar-modified haemoglobins. Some methods also measure other haemoglobin variants. In such instances, the reference range is slightly higher, and the test may be referred to as HbA1 or GHb (‘glycohaemoglobin’ or total glycated haemoglobin).

Advances in technology have recently enabled ‘on-the-spot’ testing, which is now employed by some diabetes clinics.

The result

The test result is given as a percentage value and relates to the percentage of haemoglobin that is glycated. It is NOT a direct measurement of average blood glucose, but it IS a reliable index of blood glucose control over the preceding 6 to 8 weeks.

The lack of standardisation means that glycated haemoglobin measurements made by different laboratories cannot be easily compared and for your result to make sense you will need to know the reference range provided by your laboratory. In addition you should be careful not to confuse a target range with a normal range. A target range may indicate acceptable values for a person with diabetes and this may differ slightly from the normal range seen in the non-diabetic population.

Normal ranges for HbA1c usually lie between about 4 and 6 per cent. People with diabetes should ideally be aiming for values below 7 or 7.5 per cent - but remember that the exact figures are different for each method used so check with your diabetes clinic.