HbA1c Explained: What the Test Measures

The HbA1c test — also called the A1C test in the United States — measures a form of haemoglobin that reflects average blood glucose levels over the past two to three months. It is one of the most important tests in the diagnosis and monitoring of diabetes, but it is often misunderstood.

What HbA1c measures

Haemoglobin is the protein in red blood cells that carries oxygen. Glucose (sugar) in the bloodstream naturally attaches to haemoglobin in a process called glycation. The resulting product is glycated haemoglobin, or HbA1c (haemoglobin A1c). The more glucose there is in the blood over time, the more of it attaches to haemoglobin, and the higher the HbA1c percentage.

HbA1c is reported either as a percentage (e.g. 6.5%) or in millimoles per mole (mmol/mol) in the UK (e.g. 48 mmol/mol). Both refer to the same measurement; the mmol/mol unit is the international standard recommended since 2009, though many clinicians and patients still use the percentage.

Why it reflects the last two to three months

Red blood cells live for approximately 90 to 120 days before being broken down and replaced. Because haemoglobin persists for the entire life of the red cell, the glucose attached to it also persists. The HbA1c result therefore reflects the average blood glucose across the lifespan of circulating red cells — roughly the past two to three months — rather than a single moment in time.

A helpful way to think about it: a fasting glucose test is like a photograph of one moment; HbA1c is more like a time-lapse over several months. The most recent few weeks of glucose exposure contribute more to the result than earlier weeks, because newer red cells are more numerous in the circulation.

How HbA1c differs from a finger-prick glucose reading

A finger-prick blood glucose test (or a fasting plasma glucose test in the laboratory) measures the concentration of glucose in the blood at the moment of testing. The result reflects what is happening right now — what you ate recently, whether you exercised, the time of day, and your stress levels, among other factors.

HbA1c, by contrast, averages out those fluctuations. A person can have a perfectly normal fasting glucose on the day of a blood draw but still have an elevated HbA1c if their glucose has been running high for several months. Conversely, restricting diet for a few days before a test will lower a fasting glucose but will have little effect on HbA1c.

This is why HbA1c is useful for assessing long-term glucose control, while daily glucose monitoring (finger-prick or continuous glucose monitor) is used to manage day-to-day fluctuations.

When HbA1c is used

Diagnosis of type 2 diabetes and prediabetes. HbA1c is used as a diagnostic test for type 2 diabetes in many countries. In the UK, an HbA1c of 48 mmol/mol (6.5%) or above on two separate occasions (or once if accompanied by symptoms) is consistent with a diabetes diagnosis. An HbA1c of 42–47 mmol/mol (6.0–6.4%) is classified as high risk (sometimes called prediabetes in other classification systems).

Monitoring in existing diabetes. In people already diagnosed with diabetes, HbA1c is used to assess whether blood glucose has been adequately controlled over the past few months. Monitoring frequency depends on type of diabetes, stability of control, and local guidelines, but is often every three to six months.

HbA1c is not suitable as a diagnostic test for type 1 diabetes, where presentation is typically acute and symptomatic, and blood glucose tests are used instead.

How the sample is taken and processed

HbA1c can be measured from a venous blood sample (collected into an EDTA or fluoride oxalate tube, depending on the laboratory’s method) or from a fingerprick capillary sample in some point-of-care settings. Unlike fasting glucose, HbA1c does not require fasting. The sample can be taken at any time of day.

In the laboratory, HbA1c is most commonly measured using high-performance liquid chromatography (HPLC) or immunoassay methods. Both methods have been standardised internationally to allow comparability between laboratories. For more on how blood samples are processed, see our separate guide.

What can affect or distort HbA1c

Because HbA1c depends on the lifespan and haemoglobin content of red blood cells, any condition that alters red cell turnover, haemoglobin structure, or blood volume can produce falsely high or falsely low results. This is an important limitation to be aware of.

Conditions that may cause a falsely low HbA1c include haemolytic anaemia (where red cells are destroyed faster than normal, so there is less time for glucose to attach), recent blood transfusion (which dilutes the patient’s own glycated cells with donor cells), iron deficiency anaemia (which can affect certain measurement methods), pregnancy (due to increased red cell turnover), and chronic kidney disease.

Conditions that may cause a falsely high HbA1c include iron deficiency anaemia measured by certain methods (where the reduced turnover of red cells means more time for glycation to accumulate), and haemoglobin variants such as HbS (sickle cell), HbC, or HbE, which can interfere with some analytical methods.

In populations with a higher prevalence of haemoglobin variants (for example, people of African, South Asian, or Mediterranean descent), alternative measurement methods or alternative tests may be preferred to ensure accuracy.

Why context still matters

HbA1c is a useful average, but averages hide variation. A person who swings between very high and very low glucose levels throughout the day may produce an HbA1c that looks similar to a person whose glucose remains relatively stable, even though the patterns of exposure are very different. This is one reason that continuous glucose monitoring (CGM) has become an increasingly important tool alongside HbA1c.

Target HbA1c levels are also individualised. A target suitable for a healthy adult with newly diagnosed type 2 diabetes may not be appropriate for a frail older person, a pregnant woman, or someone with frequent hypoglycaemia. Understanding how reference ranges work and how clinical targets are set is important context for interpreting any HbA1c result.

Common misunderstandings

“HbA1c measures today’s blood sugar.” No. It reflects the average over approximately two to three months. Eating well the week before a test does not significantly change the result.

“HbA1c is always reliable.” Not in all circumstances. As described above, certain conditions and haemoglobin variants can make HbA1c unreliable. Clinicians need to know about relevant medical history to interpret the result accurately.

“A normal HbA1c means my glucose control is perfect.” Not necessarily. A normal average can mask significant glucose excursions (highs and lows). Glucose variability, particularly hypoglycaemia, carries its own risks even when the HbA1c is within target.

Related tests and follow-up questions

HbA1c is rarely used in isolation. Related tests that may be ordered alongside or in follow-up include fasting plasma glucose, random plasma glucose, oral glucose tolerance test (OGTT), renal function tests (since kidney disease affects glucose metabolism and can complicate HbA1c interpretation — see our kidney function test guide), and lipid profiles. The full blood count may also be relevant if anaemia is suspected as a confounder.

References

1. MedlinePlus. Hemoglobin A1C (HbA1c) Test. US National Library of Medicine. medlineplus.gov
2. Centers for Disease Control and Prevention (CDC). Prediabetes – Your Chance to Prevent Type 2 Diabetes. cdc.gov
3. Mayo Clinic. A1C Test. mayoclinic.org
4. NICE. Type 2 Diabetes in Adults: Management. NG28. nice.org.uk
5. World Health Organization (WHO). Use of Glycated Haemoglobin (HbA1c) in the Diagnosis of Diabetes Mellitus. 2011. who.int