Basically, this test measures what your average blood glucose level (bg) has been for the last few months. There's some debate as to how far back in time it goes, but it's about 2 or 3 months, with a bit more emphasis on the most recent month.

This test has various other names: hemoglobin test, hemoglobin A1c, glycosylated hemoglobin A1c, glycohemoglobin, A1c and so on. It's all the same test. Some spell it haemoglobin. It's also more correctly written HbA_1c, with the 1 and the c going below the line.

In your blood is something called a hemoglobin molecule. It contains iron and carries oxygen around the body, delivering it to cells. The molecule is made in your bones, in the bone marrow, and lives for about 3 months. When it dies it is cleared from the blood stream by your spleen (a funny little organ which lives in the lower left rib cage). During it's life, glucose molecules one by one "stick" to the hemoglobin molecules. The more glucose that's in the bloodstream, the more sticks to the hemoglobin. If you remove the hemoglobin from the blood, and measure how much glucose is stuck to it, you can find out roughly how much glucose has been in the blood during the life of the molecule. In other words, an average for the last 2 or 3 months, but the most recent month has a stronger effect on the result than the other 2 months.

There are several different ways of measuring the amount of glucose stuck to the hemoglobin. Each way gives different results. They are all "right", it is just that the figures are slightly different. With your HbA1c result, the lab will also quote a normal range. This is also called a reference range, and tells you what a non-diabetic person could expect their HbA1c to be. This reference range may vary between different labs.

Be very careful when comparing your results with anybody elses: find out if their lab's reference range is the same as yours. Your aim is usually to get your figure near the upper end of that range. If your HbA1c is too low, near the bottom end of the reference range, it may be that your control is superb, but it's more likely that you're having far too many hypos.

A typical normal range might be 4.3%-5.9%
At this reference range, if your HbA1c is above 10%, there's plenty of room for improvement.
If it's 6.5% that's pretty good.
5.9% would be classed as excellent.

The result is always given as a percentage: how much glucose is stuck to the hemoglobin as a percentage of the maximum that could be stuck to the molecule.

Increasingly the test is being standardised to the one used by the Diabetes Control and Complication Trials (DCCT), but not all labs use this yet.

But be warned: a low HbA1c is not necessarily good. It may disguise the fact that you are having a lot of highs and a lot of hypos. If these are during the night, you may not know that they are happening. What you want is a steady bg, so the HbA1c is not a substitute for regular bg testing.

There is also a small minority of people for whom the HbA1c test doesn't work properly. This is because their hemoglobin cells have a shorter life than normal. This does not cause any health problems, but it means that the standard test results will a much higher figure than they should, since more glucose sticks to a "new" molecule than an "old" one.

There is another long-term average test called a fructosamine test. This gives a shorter, more recent average than the HbA1c, and is not used so often.

For further reading, see Karen Reed's discussion of HbA1c's at http://www.reed-biomedical.com/diabetes/management/hba1c.asp