HbA1c explained: what your number means

What HbA1c actually measures

Glucose in your blood attaches to haemoglobin, the oxygen-carrying protein inside your red blood cells. The more glucose has been circulating, the more of your haemoglobin ends up “glycated”. HbA1c measures the proportion of haemoglobin that has glucose stuck to it, which is why it works as a running average rather than a snapshot.

Because red blood cells live for about three to four months, HbA1c reflects your average glucose over roughly the previous eight to twelve weeks, weighted towards the most recent few. This makes it far more stable than a single glucose test, which can swing depending on when you last ate.

UK units: mmol/mol and the older percentage

Here is the part that confuses people most. The UK standard unit is mmol/mol (the IFCC unit), but you will still see the older percentage (the DCCT unit) used internationally and in some apps and older reports. They describe the same thing on different scales, so it helps to recognise both.

You do not need to do the maths yourself - your report will usually state the unit - but a rough sense of how they line up makes results easier to read:

• 42 mmol/mol is about 6.0%
• 48 mmol/mol is about 6.5%
• 53 mmol/mol is about 7.0%
• As a rule of thumb, the mmol/mol number is the larger, two- or three-figure value; the percentage is the smaller one. If your result looks like “5.9” it is almost certainly a percentage, and if it looks like “41” it is mmol/mol.

The general UK bandings

Clinicians group HbA1c results into broad bands to help interpret them. These are widely used reference points rather than hard lines, and the diagnosis of any condition is always a clinical judgement that takes the whole picture into account - not something a single number settles. The commonly cited UK bandings are:

It is worth stressing that a result near a threshold does not place you in a category by itself. Doctors often repeat the test, look at your symptoms and other risk factors, and consider anything that might have skewed the reading before drawing any conclusion.

• Normal (non-diabetic): below 42 mmol/mol (below 6.0%)
• Non-diabetic hyperglycaemia, sometimes called “prediabetes”: 42–47 mmol/mol (6.0–6.4%)
• Diabetes threshold: 48 mmol/mol (6.5%) or above, usually confirmed on more than one test or alongside symptoms

What can skew an HbA1c result

Because HbA1c depends on red blood cells and the haemoglobin inside them, anything that changes how long those cells live or how they behave can shift the number - independently of your actual glucose. This is why a result occasionally needs interpreting with care rather than at face value.

If any of these apply to you, it is worth mentioning to your doctor, who can decide whether HbA1c is the right test or whether another approach gives a clearer picture.

• Anaemia, including iron-deficiency anaemia, can move the result
• Recent significant blood loss or a blood transfusion
• Certain haemoglobin variants (haemoglobinopathies) such as some forms common in particular ancestries
• Pregnancy, and conditions affecting red-cell turnover such as kidney or liver disease

Lifestyle factors generally linked to glucose regulation

Several everyday factors are broadly associated with how the body regulates glucose over time. These are general wellbeing observations rather than treatment advice, and any plan to change a result should be guided by a clinician - particularly if you are already managing a condition or taking medication.

Tracking HbA1c across several tests, rather than reacting to one reading, gives a far more honest sense of the trend. A single number is a snapshot of an average; the direction of travel over time is usually the more meaningful signal.

• Dietary patterns, particularly the overall balance and quantity of carbohydrate and added sugars
• Regular physical activity, which is generally linked to how the body handles glucose
• Body weight and waist measurement
• Sleep and stress, both of which can influence glucose regulation