What is the Hb A1c test and why shouldn’t it be used as the only diagnostic tool for diabetes?

To diagnose diabetes, there are a number of tests that can be used. But not all tests are created equal.

Using only one testing method for diagnosing type 2 diabetes may result in missing people, leaving them undiagnosed. If someone is not diagnosed with diabetes, they will continue going about their normal daily lives, not realising that they could be making the condition worse.

The HbA1c test should not be used alone to diagnose diabetes. It should be used in combination with other blood glucose tests. Recently, a study was presented at the US Endocrine Society’s annual meeting, discussing the consequences of using a single HbA1c test to diagnose diabetes.

The prevalence of type 2 diabetes

It’s no secret how devastating the effect of diabetes is worldwide. Globally, diabetes is a major contributor to early illness and death. The World Health Organisation estimated that diabetes was the seventh leading cause of death in 2016, with diabetes being a direct cause of 1.6 million deaths worldwide (Diabetes).

Global statistics

The 8th edition of the International Diabetes Federation’s (IDF) Diabetes Atlas released in 2017, estimated that:

  • One in 11 people have diabetes (425 million)
  • One in two adults with diabetes are undiagnosed (212 million)
  • 12% of global health expenditure is spent on diabetes (USD$727 billion)
  • Every 6 seconds a person dies from diabetes

IDF also estimated that by 2040:

  • One in 10 adults will have diabetes
  • Diabetes-related health expenditure will exceed USD$802billion

What is the HbA1c test?

The Hb A1c test, or the glycated haemoglobin A1c test, is a blood test that can be taken at any time and is not affected by food. You don’t need to fast before taking the test. It gives an indication as to what your average blood glucose levels have been over the previous three months, by measuring the percentage of a certain type of haemoglobin, called HbA1 that has been altered by a glucose molecule has attached to it. After glucose has attached to haemoglobin A1 (HbA1) it is given a new name, Haemoglobin A1c or Hb A1c.

Try using our HbA1c tool to see where you are at.

What is haemoglobin?

Haemoglobin is the iron-containing protein that is responsible for transporting oxygen in red blood cells. It helps to carry oxygen from our lungs to the rest of the body.

What do the different HbA1c readings mean?

It is normal for all people to have some glucose attached to haemoglobin. In people with diabetes, the higher the blood glucose levels are, the more glucose you have attached to haemoglobin.

A healthy person, without diabetes, has a HbA1c level below 5.7%

Someone with prediabetes (sometimes called borderline diabetes), has a HbA1c level between 5.7 and 6.4%

In people with diabetes, a HbA1c level of 6.5% or higher, from two separate tests, can indicate diabetes.

Is HbA1c a useful test?

HbA1c can be a useful measure of blood glucose control, but it’s important to remember that it is only an indirect measure of blood glucose control. For some people with diabetes, it is possible to have a low HbA1c reading but have high blood glucose readings because they can’t handle glucose well. Further on in the article, we will also discuss what other factors could contribute to falsely high or low HbA1c readings.

Hb A1c and corresponding estimated average glucose level table

Hb A1c and corresponding estimated average glucose values
Hb A1c % Hb A1c mmol/mol eAG mg/dl eAG mmol/l

Good blood glucose control is essential in preventing diabetic complications. HbA1c readings can indicated glycaemic control from the previous 3 months in people with diabetes.

People who have diabetes have higher levels of HbA1c, which could be two to three times higher than normal. There is a strong relationship between high Hb A1c readings and the risk of developing diabetes-related complications, including cardiovascular disease, retinopathy, and neuropathy.

Why is Hb A1c tested every 3 months?

Your Hb A1c should be tested every 3 months because our bodies make new red blood all the time and our blood is completely turned over every 3 months. Approximately 1% of our red blood cells are destroyed every day, while a similar amount of new red blood cells is formed. So Hb A1c levels are highly variable and can only indicate the average blood glucose concentration over the lifespan of the complete pool of red blood cells.

Newly formed red blood cells have little glucose attached to haemoglobin when it enters the blood stream, compared to red blood cells that have been circulating our bodies for two months. However, red blood cells are easily able to bind with glucose. As a result, glucose stays stuck to the haemoglobin of red blood cells and the amount that stays bound to haemoglobin is related to how much glucose is present in the blood.

Why shouldn’t HbA1c be used to diagnose type 2 diabetes?

Diagnosing people with diabetes solely based on HbA1c results can be problematic. When analysing the results of the 2005-14 National Health and Nutrition Examination Survey (NHANES), it was found that the sensitivity of HbA1c for detecting diabetes was only 27%. What sensitivity means, is the percentage of sick people who are correctly identified as having the condition. This means that up to 73% of people with diabetes could be missed when using a HbA1c test as the only form of diagnosis.

Age is another factor that may impact on HbA1c results. When the original studies which provided the basis of recommending HbA1c as a tool to diagnose diabetes, were only conducted in adult populations. It is possible that current HbA1c cut-offs are not suitable for children and adolescents (Prevalence of Diabetes and High Risk for Diabetes Using A1C Criteria in the U.S. Population in 1988–2006).

What can affect Hb A1c readings?

There are a number of factors that can contribute to misleading HbA1c levels. Any condition that affects the life of a red blood cell, whether it prolongs red cell life, causes decreased red cell turnover or excessive blood loss can result in false HbA1c readings.

Falsely elevated Hb A1c

When red blood cell turnover is low, it can provide falsely high HbA1c values. This is because there is a disproportionate number of older red blood cells. This can occur in patients who have iron, vitamin B12, or folate deficiency anaemia (Pitfalls in Hemoglobin A1c Measurement: When Results may be Misleading). Anaemia is a condition that results in a lack of red blood cells or haemoglobin.

If someone with diabetes uses metformin, there are at risk of vitamin B12 deficiency. If they have a vitamin B12 deficiency, then their Hb A1c readings may be falsely elevated. You can read more about diabetes and vitamin B12 deficiency.

In people with renal failure (kidney failure) there is also a reduction in the amount of a chemical called erythropoeitin produced. Erythropoeitin stimulates the bone marrow to make new blood so as levels of erythropoeitin fall so does red cell production leading to higher Hb A1c levels.

Falsely lowered Hb A1c

On the other hand, when red blood cell turnover is high, leading to a greater number of young red blood cells, it can provide falsely low HbA1c readings. Conditions that may cause falsely low A1c include:

  • Patients treated for iron, vitamin B12, or folate deficiency anaemia
  • Haemolytic anaemia, which causes the abnormal breakdown of red blood cells
  • Patients treated with erythropoietin, which is a naturally occurring hormone in the kidneys that promotes an increased rate of red blood cell production
  • Acute or chronic blood loss, for example injury, operations, menstruation and blood loss from the gut.
  • Supplements and medication, including Vitamin E, Ribavirin (an antiviral medication), and interferon alpha (medication to treat cancers and viral infections)


During pregnancy, HbA1c is not a true reflection of glycaemic control, as A1c levels can be variable across the entire pregnancy (Red blood cell survival and kinetics during pregnancy). This is due to both the decreased life span of red blood cells and increased erythropoietin production. Since A1c levels are generally falsely low in pregnant women, an oral glucose tolerance test is the only screening and diagnostic tool for gestational diabetes that should be used.

Chronic kidney disease

In chronic kidney disease, a complication of type 2 diabetes, A1c levels generally present falsely low. False decreases in HbA1c may be due to haemodialysis and erythropoietin treatment, and can underestimate a patient’s average blood glucose control.

Racial variations

When examining blood glucose levels, various studies have shown that A1c readings are high in some ethnic groups, including African American, Hispanic, and Asian, compared to Caucasian groups.

Blood disorders affecting HbA1c

Specific blood conditions called haemoglobinopathies can affect readings of HbA1c, causing the values to appear falsely elevated or lowered. This can result in HbA1c readings that do not accurately represent a blood glucose control in a person with diabetes.

Sickle cell trait

Sick cell disease is a group on conditions that affects haemoglobin. People with this condition have abnormally shaped haemoglobin, which causes red blood cells to form and ‘s’ or crescent shape. When someone has sickle cell trait, it means that they are a carrier of the condition, usually with none of the symptoms of sickle cell anaemia or sickle cell disease. Sickle cell trait is common in people with African backgrounds.

In people with sickle cell trait, there is a decreased amount of haemoglobin compared to red blood cells of individuals with normal haemoglobin. The 2017 study, Association of Sickle Cell Trait With Hemoglobin A1c in African Americans found that Hb A1c levels are significantly lower in participants who have the sickle cell trait than those without the trait (5.72% vs 6.01%). The study suggested that using HbA1c may underestimate past blood glucose control and should be used with caution when screening, diagnosing, and monitoring prediabetes and diabetes.

African Americans typically have a higher rate of type 2 diabetes and diabetes-related complications compared to Caucasian Americans. Using HbA1c as the primary tool for diagnosis could result in inaccurately assessing diabetes risk, as well as present a missed opportunity for early intervention.


The thalassemias are a group hereditary blood disorders where the body either makes an inadequate amount of haemoglobin or an abnormal form of haemoglobin. This can cause variable degrees of anaemia.

As with sickle cell trait, using HbA1c in people with thalassemia is limited and will likely provide inaccurate results.

Why use a HbA1c test if it isn’t accurate?

Some doctors may choose to use only the HbA1c test due to convenience. The HbA1c test requires a blood sample. However, the oral glucose tolerance test requires the patient to fast overnight and then drink a sugary drink the next day, with blood glucose levels monitored before the drink and two hours after consumption.

How else is diabetes diagnosed?

Other testing methods to determine if someone has type 2 diabetes includes:

  • Random blood glucose test
  • Oral glucose tolerance test
  • Fasting blood glucose test

To learn more about the tools used to diagnose diabetes, including prediabetes, type 1 diabetes, and type 2 diabetes, read the article How is Diabetes Diagnosed?.

What’s the difference between HbA1c and the reading on my glucose meter?

A HbA1c reading is an estimated average of blood glucose levels from the previous three months. The reading on the glucose meter, is from a single point in time and can tell you what your blood glucose readings are at the time that you test. These glucose readings can be variable and depend on what you’ve eaten or what medication you use, as well as a number of other factors.

How can I lower by Hb A1c?

If you have diabetes, you can improve and lower your Hb A1c readings and reduce your risk of developing diabetic complications. You can lower your Hb A1c through:

  • Improving your diet
  • Exercising and moving your body more
  • Taking your medications as prescribed
  • Checking your blood glucose levels regularly

For more information about exercise and diabetes, read these articles:

To the point:

  • Hb A1c is a test that can that can provide insight into your blood glucose control from the previous 3 months
  • The Hb A1c test shouldn’t be used to diagnose type 2 diabetes as there are many factors that can influence blood glucose levels, including blood disorders, chronic kidney disease, and racial variations
  • Hb A1c is still a useful test as it can tell you if your current diabetes management plan is working for you
  • You can lower your Hb A1c through making healthy lifestyle changes, including eating a healthy diet, exercising more, and taking your medication as prescribed by your doctor

Do you already have diabetes?

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