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HbA1c reporting: getting on top of the changes

This summer, HbA1c results will start being reported as two figures as a new unit is phased in. Chemical pathologist Dr Eric Kilpatrick guides GPs through the confusion as part of his update

This summer, HbA1c results will start being reported as two figures as a new unit is phased in. Chemical pathologist Dr Eric Kilpatrick guides GPs through the confusion as part of his update

The case

A 59-year-old man with a six-year history of type 2 diabetes attends for review. He takes metformin 500mg twice a day, and glipizide 10mg daily. Disappointingly, his glycated haemoglobin (HbA1c) is unsatisfactory at 8.6%. He insists he is adherent with his medication and follows the usual dietary advice – he also points out that the readings on his home monitor seem fine, averaging around the 6mmol/l mark. You increase his glipizide to 15mg a day and arrange a further appointment for two months' time.

At this subsequent review, he clutches a further batch of satisfactory home readings. Yet his repeat HbA1c has, at 8.4%, barely shifted.

What does glycated haemoglobin actually measure?

HbA1c refers to the non-enzymatic binding of glucose to haemoglobin throughout the life of the red cell. A result of 8.5% means that 8.5% of the haemoglobin has glucose bound to it. The higher the mean glucose, the higher the HbA1c. Although the glucose attaches throughout the life of the red cell, glycaemia over the month prior to sampling contributes to about half of the HbA1c value, with months one to two contributing another 25% and months three to four before testing responsible for the remaining 25%.

How reliable is it? What factors, other than diabetic control, might affect the result?

A recent study found HbA1c provides an estimate of mean blood glucose within about 20% either side of the true average glucose for a patient. Although that confirms the usefulness of the test, it also means it is possible at the extremes for a patient with an HbA1c of 6.5% to have the same average glucose as one who has an HbA1c of 9%.

Any condition that can reduce red cell survival, such as haemolytic anaemia, can lead to a falsely low HbA1c and iron deficiency anaemia can lead to spurious rises of up to 1.5% that will subsequently reverse following iron treatment. Patients with abnormal haemoglobins (other than HbA) can also have HbA1c values that are discrepant to their mean glucose, even if the haemoglobinopathy itself does not lead to a change in red cell life. It would certainly be worth checking iron status (blood count and ferritin) in this patient to exclude iron deficiency.

How can the discrepancy between the glycated haemoglobin and home readings be explained?

It could be that this patient's HbA1c is not as concordant with his mean glucose as it is in most other patients – as described above – or that he has a haematological abnormality.

It is also possible that he is falsifying his meter readings. This was originally identified with the availability of glucose meters that recorded the result in their memory. Studies performed without informing patients of the memory function found about a third could have different readings in their monitoring diary than those recorded in the meter memory. Occasionally some patients still falsify readings, either by testing their non-diabetic partner as often as they do themselves, or by testing the control solutions that come with the meter.

Can this test replace home monitoring in some patients? If so, who?

Home blood glucose monitoring is necessary for patients with type 1 diabetes as well as those with type 2 diabetes who are treated with insulin. As such, HbA1c will be an adjunct, not a substitute, for these patients. For type 2 patients who are not treated with insulin there is a stronger argument that HbA1c could be solely used instead, with glucose monitoring reserved for those having problems such as hypoglycaemia on tablets .

Are there likely to be future refinements to this test? What will they involve?

There has been a niggling doubt about the HbA1c results we have become accustomed to. Put simply, they are not the true concentration of HbA1c, but rather the best estimation that HbA1c instruments in the 1980s could deliver.

More recently a definitive way of measuring HbA1c has been developed that has shown the ‘true' HbA1c values are between 1.5 and 2% lower than the ones we are used to.

There has been concern that simply moving to these numbers could cause confusion with existing ones, so a complete change of units has been proposed. Rather than a new result of, say, 6.5%, the new unit equivalent will be 48mmol/mol.

As of June 2009, it is intended that the numbers used at present and the new ones will both be reported by UK labs, then as of June 2011 only the new figure will be used.

Obviously, the change to these new figures is going to be a challenge for everyone who treats patients with diabetes, and so professional organisations such as Diabetes UK and the Association for Clinical Biochemistry are creating educational material to ease the transition.

Dr Eric Kilpatrick is a consultant in chemical pathology at Hull Royal Infirmary and an honorary professor in clinical biochemistry at Hull York Medical School

Competing interests None declared

changes units Changes in HBA1C reporting - what you need to know

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