One number, drawn at any time of day, that describes roughly three months of your blood sugar. It is the closest thing metabolic medicine has to a memory.
Glucose in your blood attaches itself to haemoglobin, the protein inside red blood cells that carries oxygen. The attachment is slow, permanent for the life of the cell, and proportional to how much glucose was around. HbA1c is the percentage of your haemoglobin carrying that attachment.
Red cells live about 120 days, so the figure covers roughly three months — weighted towards the most recent four to six weeks, because the cells circulating now are on average younger than the ones that have already gone.
This is why it is the most useful single number in metabolic medicine. A fasting glucose tells you about this morning. HbA1c tells you about the season.
| HbA1c | Interpretation |
|---|---|
| Below 5.7% | Normal |
| 5.7% – 6.4% | Pre-diabetes |
| 6.5% or above | Diabetes range, on two occasions |
Some laboratories report in mmol/mol instead of a percentage. They are the same measurement on a different scale: 5.7% is 39 mmol/mol, 6.5% is 48. If your report shows a number in the thirties or forties, you are reading mmol/mol.
A single result in the diabetes range is not a diagnosis. It is repeated before anybody acts on it, which is standard practice and not a sign that something has gone wrong.
It is not a mild illness. It is a statement about trajectory: at 5.7% to 6.4%, the process that produces type 2 diabetes is already underway, and the body is compensating. The compensation is the point. It is also the opportunity, because this is the band in which the change is most reversible.
The range is wide and a decade can be spent inside it. "Come back in a year" is reasonable advice at 5.8% and considerably less reasonable at 6.4%, and the difference is invisible unless somebody is tracking it.
HbA1c assumes your red cells live a normal length of time. When they do not, the result shifts without your glucose shifting at all.
Falsely low — anything that shortens red cell life or replaces cells quickly: haemolytic anaemia, recent significant blood loss, a recent transfusion, pregnancy in the later stages, some liver disease.
Falsely high — anything that lengthens it: iron deficiency anaemia, vitamin B12 deficiency, and some chronic kidney disease.
Unreliable either way — haemoglobin variants, which are more common in some ancestries than others and which some assay methods handle better than others.
This is the practical reason a full blood count and an iron study usually sit alongside HbA1c on a baseline panel. Without them, an abnormal HbA1c has two possible explanations and no way to choose between them.
Note it, note the date, and note which laboratory ran it. The value of HbA1c is almost entirely in the series. One reading is a snapshot of a process; three readings over a year tell you which direction the process is going, and that is the thing worth knowing.
What it means for you specifically, and whether anything should be done about it, is a question for a physician who can see the rest of your results.
Every patient starts the same way: thirty minutes with a physician, then a full biomarker panel drawn at a laboratory near you. Whether any medicine follows is your doctor’s decision, made after reading your results with you.
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