What is the HbA1c blood test?

The HbA1c blood test or glycosylated haemoglobin is used as a general indicator of average blood sugar levels present in plasma over about 3 months.  It does not indicate whether a person is insulin resistant or not, whether they have high blood sugar or low blood sugar at times, only what the long term average blood sugar level is.

It makes no differentiation between someone who has a steady blood sugar level of 5-6 mmol/L and someone who cycles constantly between hypo and hyperglycaemia going to 15 mmol/L after a meal and then crashing down to 3 mmol/L or less soon afterwards, the latter being a typical expression of insulin resistance.  It is frequently used to monitor diabetics in addition to regular blood glucose monitoring at home, or to diagnose diabetes.

There are many factors which can affect the results of an HbA1c test, such as any process affecting the red blood cell volume or turnover rate including blood loss, anaemia, surgery, blood transfusions, erythropoietin treatment as well as chronic kidney or liver disease and even high doses of vitamin C.  Different assay techniques also provide different HbA1c results.  It is possible for 2 people with the same average blood glucose level to have an HbA1c result which differs by as much as 3 percentage points, which can be quite significant!

How does the HbA1c work?

Haemoglobin is the oxygen carrying component of a red blood cell and along with water, makes up the bulk of a red blood cell.  When haemoglobin is exposed to glucose in the blood stream a percentage of the haemoglobin binds to glucose forming glycosylated haemoglobin.  The higher the blood sugar rises, the greater percentage of haemoglobin in red blood cells is glycosylated and this occurs in a predictable way.  As red blood cells have a lifespan of roughly 3-4 months, by measuring the amount of glycosylated haemoglobin in a red blood cell, we can calculate the average blood sugar level for the last 2 – 16 weeks, although there is some disagreement amongst experts as to how long exactly that time period is.

In summary:

• HbA1c cannot be used to diagnose insulin resistance
• Regular home monitoring of blood glucose levels is the best way to monitor for blood glucose control and to isolate instances of hypoglycaemia and hyperglycaemia
• An HbA1c test can be used to provide an average value for blood sugar levels over the preceding few weeks.  If it is above 6.5%, this can justify a diagnosis of diabetes.

More Information:

Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ (2008). “Translating the A1C assay into estimated average glucose values.”. Diabetes Care 31 (8): 1473–8.

http://www.ncbi.nlm.nih.gov/pubmed/18540046