By Dr Tim Welborn, Endocrinologist
Glycated haemoglobin (HbA1c) measures long-term glycaemia in most diabetic patients and has recently been approved as a screening test for undiagnosed diabetes and prediabetes. Glucose is chemically an aldehyde with affinity for protein, so it attaches itself to haemoglobin in the red cell. HbA1c is commonly expressed as a percentage of the total haemoglobin and it estimates the average blood glucose levels for the preceding 2 to 3 months (red cells survive ~120 days).
Glycated haemoglobin can be misleading when there is abnormal red cell turnover. Sickle-cell anaemia and other haemolytic states, and the anaemia of renal failure, can cause falsely low levels. Cases of thalassaemia trait can have prolonged red cell survival, with higher readings than expected. Therefore, be guided by the patient’s home blood glucose monitoring where possible.
Fructosamine is an alternative measure of short term glycaemia in the preceding 2-3 weeks. It reflects glycosylation of plasma proteins, predominantly albumin. Order this when misleading HbA1c readings are suspected, or when ascertaining recent blood sugar control after short-term improvement. The formula for equivalent conversion of Fructosamine to HbA1c is: HbA1c = Fructosamine x 0.171 +1.61
HbA1c for diagnosing diabetes and prediabetes has recently been approved as an annual test in those at risk (Medicare rebate available); more patient friendly than a 2-hour glucose tolerance test, and there is no need to fast. Results are much less influenced by recent illness or stress, although there is caution with interpretation in abnormal red cell states.
In general, an HbA1c of 6.0–6.4% is consistent with pre-diabetes, and values > 6.5% indicate probable diabetes. It is recommended that a fasting random glucose is also measured to support the diagnosis.
Glycated haemoglobin enables the doctor and patient to set agreed targets. In general “satisfactory” control is described as HbA1c <7.0%, “fair” control 7.0-8.0 % , and suboptimal control > 8.0%. Targets must be individualised and realistically adjusted for the elderly and the cognitively impaired. Females with gestational diabetes should be given a target of 6.0–6.5%.
The pivotal ACCORD study (2008) aimed to achieve “normal” glycated haemoglobin levels in type 2 diabetes (target <6.0%), and the intensively treated group did achieve steady levels of 6.5% (on multiple medications plus insulin). But the study had to be terminated because of increased mortality in this group as compared to conventional treatment.
Most laboratory reports interpret HbA1c by including a statement of “estimated average blood glucose”, derived from a complex formula (but in day-to-day practice, a simple and quite accurate way of doing this is: “Double the number and subtract 6” (see Table).
Expressing HbA1c as a “percentage” is a time-honoured format, and should be retained because we are all familiar with it. Some authorities say the measure should properly be expressed as mmol/mol., yielding numbers that will be difficult for most of us to interpret. Let’s stay with the old way!
Table: Easy approximation of blood glucose
|HbA1c(%)||Lab estimated av.||Bl. glucose approximation|
|7.0%||8.6 mmol/L||8.0 mmol/L|
Author competing interests: nil relevant.
Disclaimer: Please note, this website is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional’s advice. Opinions expressed at this website do not necessarily reflect those of Medical Forum magazine. Medical Forum makes no warranties about any of the content of this website, nor any representations or undertakings about any content of any other website referred to, or accessible, through this website.