HbA1c test: what it shows and why it matters

What does an HbA1c test show?

The HbA1c blood test reflects the average level of glucose in the blood over the past two to three months. That is why it differs from a finger-prick reading, which captures one moment. Glucose attaches to a part of the red blood cell, and the more glucose there is in the blood, the more HbA1c is present. Red blood cells live for about two to three months before new ones replace them, which sets the window the test covers. Because it covers a longer window, the test does not show short-term rises or dips in glucose. Hospital guidelines say HbA1c may be unsuitable for diagnosis when diabetes symptoms have been present for less than about two to three months, and local protocols differ.

What is the test used for?

Doctors use it to diagnose diabetes. They also use it to keep an eye on levels in people at risk of developing diabetes, a state called prediabetes. In people who already have diabetes, it shows how well blood sugar has been controlled. MedlinePlus says the test is not used to diagnose type 1 diabetes. Type 2 diabetes is diagnosed with blood tests, and HbA1c is the usual first test. HbA1c is not used to diagnose diabetes during pregnancy.

How are the results given and what are the targets?

UK laboratories give HbA1c in millimoles per mole, while some countries use a percentage. NICE-based guidance says an individual HbA1c target should be agreed with each person. A higher target may suit someone who has had many problems with low blood glucose. As people age, the diabetes team may raise the target, because the risks of tight control can outweigh the benefits.

How often is it done?

NICE patient information advises that HbA1c is usually tested every three to six months. It may be done more often if blood glucose levels are changing quickly. Once blood glucose is stable, NICE patient information says the test is usually every six months.

What can make the result less reliable?

Conditions that affect red blood cells can make the result less accurate, such as anaemia or an uncommon form of haemoglobin. Where HbA1c is unreliable, glucose-based blood tests are used for diagnosis instead. Pregnancy, heavy bleeding and recent blood transfusion can also make HbA1c unreliable. An NHS Scotland resource says the HbA1c may change if diet, physical activity, weight or medication have changed. An estimated average glucose converts HbA1c into meter units, but it is an average and not an exact match for readings. Heavy bleeding or haemolytic anaemia can make HbA1c falsely low, while iron deficiency may raise it. Lab Tests Online UK says the accuracy of an HbA1c result depends on the method used to measure it, and some haemoglobin variants can interfere with the measurement.

What happens after a result in the prediabetes range?

The usual response to an intermediate result is lifestyle advice on eating, activity and weight. A GP can refer eligible people to the free NHS Diabetes Prevention Programme, and some areas allow self-referral. People with an intermediate result are usually monitored at least once a year. A GP appointment is the route to discuss any result. A raised result in someone without symptoms is usually repeated, and a second result in the diabetes range confirms the diagnosis.

Why does a prediabetes result matter?

Prediabetes means blood sugar is higher than normal but not high enough for a diagnosis of type 2 diabetes. It also means a high risk of developing type 2 diabetes. Prediabetes usually causes no symptoms, so it is found through blood tests. Feeling very thirsty, passing urine more often, feeling more tired than usual and losing weight without trying are symptoms of diabetes itself. The NHS notes that not everyone with diabetes has symptoms, which is why getting checked matters. The programme is free, lasts nine months and gives support with healthy eating, physical activity and weight.