HbA1c measures the percentage of haemoglobin in your red blood cells that has glucose permanently attached to it. Because red blood cells live for approximately 120 days, HbA1c reflects your average blood sugar level over the past 2 to 3 months rather than a single point in time. This makes it a more reliable indicator of overall blood sugar control than a one-off glucose measurement.
The test is used in two main contexts. First, it is a standard screening tool for prediabetes and type 2 diabetes. An HbA1c in the prediabetes range means your blood sugar has been consistently above normal but not yet at the threshold for a diabetes classification. This window matters because lifestyle changes at this stage, including dietary improvements and regular physical activity, have been shown to reduce or delay progression to type 2 diabetes. Second, for people already managing diabetes, HbA1c is the primary measure of how well blood sugar is being controlled over time. GPs and endocrinologists use it to guide treatment decisions and set individual targets.
One of the practical advantages of HbA1c is that it does not require fasting. You can eat and drink normally before your blood draw, making it convenient to include alongside other tests. Unlike fasting glucose, which can be influenced by what you ate the night before, stress, or illness on the day of collection, HbA1c is not affected by short-term fluctuations.
There are some situations where HbA1c may not accurately reflect your true average blood sugar. Conditions that alter red blood cell lifespan, such as certain anaemias, thalassaemia, or significant recent blood loss, can cause HbA1c to read artificially low or high. If any of these apply, your practitioner will factor it into their interpretation and may use alternative measures.
HbA1c is used to screen for prediabetes and type 2 diabetes, to assess diabetes when symptoms are present, and to monitor how effectively blood sugar is being managed in people already diagnosed. Because it reflects a rolling 2 to 3 month average, a single result is highly meaningful.
