HbA1c Test Explained: What the Numbers Mean and When to Repeat Test

If you have recently had a blood test through your GP and been given an HbA1c result, you may have been told it is normal, borderline, or raised, and then sent on your way with little further explanation. What the test actually measures, what the different thresholds mean in practice, and when a private HbA1c test adds something that a single NHS result cannot – these are the questions worth answering.

What Does The HbA1c Test Measure?

The HbA1c test measures the proportion of haemoglobin in your red blood cells that has become bonded to glucose over time.¹ Because red blood cells live for approximately 90 to 120 days, the result reflects your average blood glucose level over roughly the preceding two to three months, rather than a single snapshot at one point in time.²

This is what makes it a more clinically useful indicator of longer-term glucose control than a standard fasting glucose reading, which can be significantly affected by what you have eaten or how active you have been in the preceding hours. In the UK, HbA1c results are expressed in millimoles per mole (mmol/mol), following the International Federation of Clinical Chemistry standardisation used across NHS laboratories.

What Do The Numbers Mean?

The key thresholds used in UK clinical practice, as set out in NICE guidance, are as follows:

A result below 42 mmol/mol is considered normal, indicating that average blood glucose is within a healthy range. A result of 42 to 47 mmol/mol indicates prediabetes, also referred to clinically as non-diabetic hyperglycaemia. This means blood glucose is consistently higher than it should be, but has not yet reached the threshold for a type 2 diabetes diagnosis. A result of 48 mmol/mol or above meets the diagnostic threshold for type 2 diabetes. In people without symptoms, a confirmatory second test is required before a diagnosis is made.^2,3

A prediabetes result is a meaningful clinical signal, not a result to be filed and forgotten. Research confirms that individuals in the 42 to 47 mmol/mol range carry a significantly elevated risk of progressing to type 2 diabetes without intervention. A meta-analysis of 50 trials published in the BMJ found that structured lifestyle interventions reduced the relative risk of progression by 31% in individuals with prediabetes, on the condition that the intervention lasted between six months and two years.⁴

The NHS Diabetes Prevention Programme, which is offered to people with a prediabetes result, has been shown to reduce the risk of developing type 2 diabetes by 20% compared to those not referred to the programme.⁵ Knowing where you sit within the prediabetes range, and tracking whether that figure is moving in the right direction, is therefore clinically useful information rather than simply a number to monitor passively.

When Can Private Retesting Add Value?

Most people who search for information about HbA1c are looking at a result they have already received from their GP. A private HbA1c test is not always necessary, but there are specific circumstances where it provides something an NHS result alone does not.

If your result was in the prediabetes range and you have made meaningful lifestyle changes in the months since, a repeat test tells you whether those changes have had the effect you intended, without waiting for your next annual review. If your result was borderline, either close to 42 or just below 48 mmol/mol, retesting after a defined interval gives a clearer picture of whether your glucose control is improving, stable or worsening.

If your initial test was taken during a period of illness or significant stress, results can be temporarily elevated, and a repeat under stable conditions may give a more representative reading. ² A private HbA1c test at The Health Suite also comes with clinician interpretation, meaning your result is contextualised within your broader health picture rather than delivered as a number without explanation.

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