A single glucose reading is a snapshot. It depends on what you ate, how you slept and even how stressed you were on the way to the test. The HbA1c test works differently: it looks back over a longer period, which is why it has become one of the most used blood tests for following blood sugar.
This article explains what HbA1c measures, how it compares with glucose and fructosamine, who tends to check it and how often.
What the HbA1c test measures
Glucose in the blood sticks to haemoglobin, the protein inside red blood cells. The more glucose there is over time, the more of this "glycated" haemoglobin, called HbA1c, builds up. Because red blood cells live for a few months, HbA1c reflects average blood sugar over the previous 2 to 3 months (Chen et al., 2022).
A review of laboratory guidance from the American Diabetes Association and the American Association for Clinical Chemistry states that diabetes can be identified either by raised glucose in a venous sample or by raised HbA1c, and that HbA1c is also a central measure for following glucose control (Sacks et al., 2023).
The HbA1c test in the BPR Clinic catalogue is processed by the LML. The LML describes it as the reference test for checking for diabetes and for monitoring it in people who already know they have it.
Why it matters even without symptoms
According to the LML, more than a million people in the UK have type 2 diabetes without knowing it, and type 2 accounts for about 90% of diabetes cases. The NHS notes that not everyone with type 2 diabetes has symptoms, and that when symptoms such as tiredness, thirst or passing more urine do appear, they may develop gradually.
The NHS also describes prediabetes: blood sugar higher than normal but not high enough to be called diabetes. In the UK, NICE guidance focuses on finding adults at high risk and offering them an intensive lifestyle-change programme.
HbA1c, glucose and fructosamine: how they differ
Glucose
The Glucose test measures the sugar in your blood at the moment of collection. The LML lists many uses: checking for high or low blood sugar, routine check-ups, assessments before surgery and following treatment. It is quick and useful, but it varies from hour to hour.
HbA1c
HbA1c averages that variation over 2 to 3 months. It does not need fasting for most purposes, which makes it practical. Its limitation is that it depends on red blood cells. A review in Experimental and Therapeutic Medicine explains that conditions affecting the lifespan of red cells, or inherited differences in haemoglobin, can change HbA1c independently of blood sugar (Chen et al., 2022).
Fructosamine
The Fructosamine test measures glucose attached to proteins in the blood rather than to haemoglobin. According to the LML, it reflects average glucose over the previous 2 to 3 weeks. A review of alternative markers describes fructosamine as useful for following shorter-term control and in situations where HbA1c is hard to interpret, such as some blood disorders (Lee, 2015).
In practice, many people only need HbA1c. Glucose and fructosamine are added when a professional wants a different angle.
When the HbA1c test tends to make sense
The NHS lists groups with a higher chance of type 2 diabetes, including:
- being white and over 40, or over 25 and of South Asian, Chinese, Black African or Black Caribbean background;
- having a parent, brother or sister with type 2 diabetes;
- being overweight and not very physically active;
- having a condition that raises the risk, such as high blood pressure.
People already living with diabetes check HbA1c regularly to see how their plan is working. The LML notes that these checks are usually repeated every few months.
How often
As a general orientation, people over 40 or with risk factors often check HbA1c once a year, and people being followed for raised blood sugar every 3 to 6 months. Your history changes that rhythm, so talk to a healthcare professional about what fits you.
