Hemoglobin A1c

What is it?

According to the NIDDK, the A1C test is a blood test that measures "average" blood glucose (blood sugar) levels over the past three months. It is also called haemoglobin A1C, HbA1c or glycated haemoglobin.

How it measures: glucose attaches to haemoglobin inside red blood cells, and the higher the glucose in the blood, the more of it attaches to haemoglobin. MedlinePlus describes the result as the percentage of red blood cells coated with glucose, and notes that red blood cells live about three months, which is why the test reflects this period (MedlinePlus).

Why is it done?

According to the NIDDK, providers use it to detect prediabetes and give lifestyle-change guidance, to detect type 2 diabetes, and to monitor the disease and help with treatment decisions to prevent complications.

MedlinePlus notes that the CDC recommends the test for people over 45, and for younger people with risk factors such as prediabetes, overweight or obesity, a family history of type 2, high blood pressure, high cholesterol, heart disease, previous gestational diabetes or polycystic ovary syndrome. It is also requested when symptoms appear such as intense thirst, frequent urination, unexplained weight loss, blurred vision and tiredness (MedlinePlus).

Preparation

According to the NIDDK, there is no need to fast before the blood draw for an A1C test, so the sample can be taken at any time of day. The NIDDK also says you can eat and drink before it. MedlinePlus advises telling the provider about all the medicines you take, as some (including opioids and some HIV drugs) may affect the result, not to stop any medicine without medical guidance, and to say if you are pregnant.

How is it done?

According to MedlinePlus, a professional takes a blood sample from a vein in the arm with a small needle into a tube; the process takes under five minutes, and a slight prick may be felt.

Understanding the result

According to the NIDDK, MedlinePlus and the CDC, the percentages given for diagnosing diabetes in non-pregnant people are: below 5.7% normal, 5.7% to 6.4% prediabetes, and 6.5% or higher diabetes. Unit: percentage (%). Laboratories and guidelines differ, so what counts is your laboratory's report and your doctor's interpretation.

The NIDDK notes that the test result may vary slightly: a result of 6.8% might be recorded in a repeat test within a range of 6.4% to 7.2%.

According to the NIDDK, if a result suggesting diabetes or prediabetes appears without symptoms, a repeat test on a different day should be done to confirm. MedlinePlus says professionals often use more than one test to confirm a diagnosis.

For people with diabetes, the NIDDK notes that targets differ from person to person, that studies have shown some people may reduce the risk of complications by keeping A1C below 7%, and that this may not suit everyone (for example, those with concerns about low blood sugar). The personal target is set by the doctor.

Risks and side effects

MedlinePlus notes that the risks of a blood draw are very small, and mild pain or a bruise at the needle site may occur and usually go away quickly.

Limits of the test

According to the NIDDK, the A1C test does not diagnose type 1 diabetes, gestational diabetes or cystic fibrosis–related diabetes (and MedlinePlus confirms that it does not diagnose gestational diabetes).

The result may be misleading with: sickle cell disease, iron-deficiency anaemia, kidney failure or liver disease, recent bleeding, blood transfusion or dialysis, and haemoglobin variants in people of African, Mediterranean or Southeast Asian descent. The NIDDK says the test may not be reliable for people with such ancestry or relatives with sickle cell anaemia or thalassaemia.

After the test

According to the NIDDK, experts advise that people with diabetes have an A1C test at least twice a year, which MedlinePlus also mentions. You and your doctor decide when to repeat it.

According to the NIDDK, people with prediabetes are retested every year, and those with normal results every 3 years (as given on the NIDDK page on diabetes diagnosis tests).

Common questions