HbA1c Explained: Understanding Your Average Blood Sugar Results

HbA1c, also known as glycated haemoglobin, is a laboratory blood test that estimates your average blood glucose level over the previous two to three months.

Red blood cells contain a protein called haemoglobin, which carries oxygen from your lungs to the rest of your body. As glucose circulates in your bloodstream, a small proportion naturally attaches to haemoglobin. This process is known as glycation.

The higher your blood glucose remains over time, the greater the amount of glucose that becomes attached to haemoglobin. Because red blood cells typically live for around 120 days, measuring the percentage of haemoglobin that has glucose attached provides an estimate of your average blood glucose during that period.

Unlike a blood glucose reading taken before breakfast or after a meal, HbA1c is not influenced significantly by what you ate earlier that day. Instead, it reflects your longer-term glucose exposure. For this reason, HbA1c has become one of the most valuable tools for assessing long-term glucose control.

Many people assume HbA1c and blood glucose tests measure the same thing, but they provide different information.

A finger-prick blood glucose test measures your glucose level at the exact moment the sample is taken. Your result can change considerably throughout the day depending on factors such as meals, physical activity, stress, illness, sleep, medication and time of day.

HbA1c, by contrast, smooths out these day-to-day fluctuations by estimating your average blood glucose over several weeks. This makes it particularly useful for identifying long-term patterns rather than short-term changes.

Blood glucose naturally rises and falls throughout the day. After eating, glucose levels usually increase before gradually returning towards their usual range. Exercise, stress, illness, sleep and medications can all influence blood glucose throughout the day.

A single glucose reading therefore tells only part of the story. HbA1c helps answer a different question: "What has my average blood glucose been over the past two to three months?"

Understanding this longer-term picture is important because persistently elevated blood glucose may gradually damage blood vessels and nerves throughout the body. Over many years, poorly controlled blood glucose has been associated with an increased risk of complications affecting multiple organs, including the eyes, kidneys, heart, brain, blood vessels, peripheral nerves and feet.

Monitoring HbA1c over time allows healthcare professionals to identify trends, assess whether glucose control is improving or worsening, and support conversations about long-term health. Importantly, HbA1c is only one part of the overall assessment. It should always be considered alongside your symptoms, medical history, physical examination and any additional investigations that may be appropriate.

Your healthcare professional may recommend an HbA1c test for many different reasons. It is commonly used when someone has symptoms or risk factors associated with diabetes, but it may also form part of a routine health assessment.

People who may be offered HbA1c testing include those who:

• Have a family history of diabetes
• Are living with overweight or obesity
• Have high blood pressure
• Have raised cholesterol levels
• Have previously experienced gestational diabetes
• Have polycystic ovary syndrome (PCOS)
• Belong to an ethnic group known to have a higher risk of Type 2 diabetes
• Have symptoms that could suggest raised blood glucose

Symptoms that may prompt further assessment include increased thirst, passing urine more frequently than usual, feeling unusually tired, blurred vision, unexplained weight loss, slow wound healing and recurrent infections. These symptoms are not specific to diabetes and can occur in many different medical conditions. An HbA1c test is therefore interpreted alongside your clinical assessment rather than on its own.

HbA1c testing is straightforward and widely available. Most commonly, a healthcare professional takes a small blood sample from a vein in your arm. In some clinics, surgeries or pharmacies, HbA1c may also be measured using a finger-prick blood sample analysed with a point-of-care device.

The laboratory measures the proportion of haemoglobin that has glucose attached and reports the result either as a percentage (%) or millimoles per mole (mmol/mol). Many laboratories report both units to make interpretation easier. The blood sample itself usually takes only a few minutes to collect, and results are commonly available within a few days, depending on the laboratory.

One of the major advantages of the HbA1c test is its convenience. Unlike a fasting blood glucose test, HbA1c does not usually require fasting. In most cases, you can eat and drink normally before your appointment, unless your healthcare professional has asked you to have other blood tests at the same time that do require fasting.

This means you can usually eat your normal meals, drink water as usual, take your prescribed medicines unless advised otherwise, and attend your appointment at any time of the day. Because HbA1c reflects average blood glucose over the previous two to three months rather than your glucose level at a single moment, what you eat on the day of the test has very little influence on the result.

The appropriate testing interval depends on why the test is being performed. Healthcare professionals recommend repeat testing based on your individual circumstances, medical history and previous results.

People without diabetes: HbA1c may only be measured as part of a routine health assessment or when clinically indicated. There is no single testing interval that applies to everyone.

People at increased risk of diabetes: Periodic monitoring may be beneficial. The frequency varies according to age, family history, body weight, previous HbA1c results, other medical conditions and clinical judgement.

People living with diabetes: HbA1c is commonly measured at regular intervals to monitor long-term glucose control. Testing may occur more frequently when treatment has recently changed, blood glucose levels are unstable, there has been a recent illness, additional support is required, or your healthcare professional wishes to assess progress. The exact schedule is individual and should always follow the advice of your healthcare team.

Receiving your HbA1c result can sometimes be confusing, particularly if it is reported using unfamiliar units. Most laboratories report HbA1c as either Percentage (%) or Millimoles per mole (mmol/mol). These represent the same measurement but use different reporting systems.

It is important to remember that HbA1c should never be interpreted in isolation. Healthcare professionals consider your result alongside your symptoms, medical history, previous HbA1c results, current medications, other blood tests and overall health. For this reason, two people with the same HbA1c result may receive different advice depending on their individual circumstances.

The table below summarises commonly used reference ranges for adults. Exact interpretation may vary depending on the laboratory, local guidance and individual circumstances.

UK diagnostic note: In a person without symptoms, a diabetes diagnosis is not made on a single HbA1c result; a second confirming test at 48 mmol/mol or above is needed. In someone with symptoms, a single result is sufficient.

US criteria

For international readers: US guidance from the American Diabetes Association uses a different prediabetes band of 5.7–6.4% or 39–46 mmol/mol. These are US criteria, not the UK figures shown above.

A lower HbA1c generally indicates that average blood glucose has remained within a lower range over the previous two to three months. For many people, this reflects healthy glucose regulation.

However, HbA1c should always be interpreted in context. Certain medical conditions affecting red blood cells may make HbA1c appear lower than expected without accurately reflecting average blood glucose. This is one reason healthcare professionals sometimes recommend additional investigations.

A higher HbA1c suggests that average blood glucose has been elevated over several weeks or months. This does not mean that one meal or one week of unhealthy eating caused the increase. Instead, HbA1c reflects longer-term patterns.

A raised HbA1c may prompt your healthcare professional to consider your symptoms, previous HbA1c results, lifestyle factors, other laboratory investigations and whether additional assessment is appropriate. The result itself does not explain why glucose has been elevated, which is why interpretation always considers the wider clinical picture.

Although HbA1c is one of the most useful laboratory tests available, it is not perfect. Certain medical conditions can influence the result independently of your true blood glucose.

Examples include:

• Iron deficiency anaemia
• Recent significant blood loss
• Blood transfusion
• Certain inherited haemoglobin conditions
• Advanced kidney disease
• Pregnancy
• Conditions affecting red blood cell lifespan

When HbA1c may not provide an accurate reflection of long-term glucose exposure, healthcare professionals may recommend other investigations.

Many people find HbA1c easier to understand when it is converted into an estimated average glucose (eAG). While HbA1c is reported as a percentage (%) or millimoles per mole (mmol/mol), eAG expresses the result using the same units as everyday blood glucose measurements.

It is important to remember that eAG is an estimate rather than an exact value. Individual differences, glucose variability and certain medical conditions can influence the relationship between HbA1c and average blood glucose. For many people, however, eAG provides a useful way of translating an unfamiliar laboratory result into numbers that are easier to understand.

These values should be viewed as approximate averages rather than exact equivalents.

Many people wonder why several different blood tests exist for glucose. The answer is simple: each test answers a different clinical question.

HbA1c is one of the most reliable laboratory tests available, but it is not suitable for every situation. Because the test depends on the normal lifespan of red blood cells, conditions affecting those cells can influence the result.

Conditions that may affect HbA1c accuracy include:

• Iron deficiency anaemia
• Recent significant blood loss
• Blood transfusion
• Certain inherited haemoglobin conditions (such as sickle cell trait or thalassaemia)
• Advanced kidney disease
• Pregnancy
• Medications that affect red blood cell lifespan (such as some HIV treatments)
• Recent splenectomy

When HbA1c may not provide an accurate reflection of long-term glucose exposure, healthcare professionals may recommend alternative investigations such as fructosamine testing, oral glucose tolerance tests or continuous glucose monitoring.

Symptoms are your body's way of signalling that something may need attention. While many symptoms have common, benign causes, some patterns are important red flags.

If symptoms become severe or you are concerned about your health, seek appropriate medical advice promptly.

Although HbA1c provides valuable information, it cannot answer every question about glucose control. For example, HbA1c cannot show whether blood glucose rises after meals, overnight glucose fluctuations, episodes of low blood glucose, or day-to-day variability.

This is why healthcare professionals sometimes use HbA1c alongside other investigations rather than relying on a single test. HbA1c should be viewed as one important piece of your overall health picture. Healthcare professionals may also consider blood pressure, cholesterol, kidney function, body weight, waist circumference, physical activity, family history, lifestyle factors and previous laboratory results. Taken together, these measurements provide a much broader understanding of long-term health than any single blood test alone.

One HbA1c result provides useful information. A series of HbA1c results collected over months or years provides something much more valuable—a picture of how your health is changing over time.

For example, a downward trend from 8.2% → 7.5% → 6.9% suggests meaningful improvement, even if the July result may still be above an individual's target. Similarly, an upward trend from 5.8% → 6.2% → 6.7% may prompt further assessment because it suggests that blood glucose has been increasing over time.

• HbA1c estimates your average blood glucose over approximately the previous two to three months.
• It is one of the most widely used blood tests for assessing long-term glucose control.
• Fasting is usually not required.
• HbA1c provides different information from finger-prick glucose testing.
• Looking at trends over time is often more valuable than focusing on a single result.
• Certain medical conditions can affect HbA1c interpretation.
• Results should always be discussed with a qualified healthcare professional.