HbA1c / Glucose Tolerance Test

The HbA1c / Glucose Tolerance Test is a blood test — or a pair of blood tests — used by endocrinologists (hormone and metabolism specialists) to measure how well your body manages sugar, helping to diagnose or rule out diabetes and prediabetes.

The two tests work in different ways. The HbA1c (glycated haemoglobin) test measures the average level of sugar that has stuck to your red blood cells over the past two to three months, giving a picture of long-term sugar control. The Glucose Tolerance Test (GTT) measures how quickly your body clears a measured sugar drink from your blood over a set period — usually two hours — showing whether your body responds to sugar normally. Your doctor may order one or both depending on what they need to find out.

These tests are among the most important tools in diagnosing and monitoring conditions related to blood sugar. They are used when a doctor suspects that the body is not processing glucose (sugar) properly, or when a patient already has a known condition that needs to be tracked over time.

• Suspected type 2 diabetes — the most common form, where the body becomes resistant to insulin (the hormone that moves sugar into cells).
• Suspected type 1 diabetes — an autoimmune condition where the body produces little or no insulin.
• Prediabetes — blood sugar levels that are higher than normal but not yet in the diabetes range.
• Gestational diabetes (diabetes during pregnancy) — the GTT is routinely offered to pregnant women, usually between 24 and 28 weeks.
• Monitoring of known diabetes — the HbA1c is used regularly to check whether a patient's diabetes management plan is working.
• Polycystic ovary syndrome (PCOS) — a hormonal condition in women that raises the risk of insulin resistance.
• Metabolic syndrome — a cluster of conditions including high blood pressure, excess weight around the waist, and abnormal cholesterol that together increase diabetes risk.
• Routine screening in people with risk factors such as obesity, a family history of diabetes, or a history of gestational diabetes.

There are some situations where a standard GTT or HbA1c result may be less reliable and the doctor will need to interpret results more carefully or choose a different approach.

• Certain blood disorders such as anaemia (low red blood cells) or haemoglobin variants (unusual forms of haemoglobin) can affect HbA1c accuracy.
• Recent blood transfusion or significant blood loss can alter HbA1c readings.
• Severe illness, surgery, or prolonged bed rest at the time of testing can make GTT results harder to interpret.
• Patients who cannot tolerate the sugar drink — for example due to gastric surgery — may not be suitable for a standard GTT.

As a blood test, this procedure carries very little risk. The discomforts that may occur are minor and almost always short-lived.

• Brief pain or stinging at the needle site during the blood draw.
• Small bruise (haematoma) at the puncture site, which usually fades within a few days.
• Lightheadedness or faintness in some people, particularly those who are anxious about needles or who have not eaten before the test.
• Nausea during the GTT — some people find the concentrated sugar drink unpleasant and may feel queasy after drinking it.
• In rare cases, the GTT sugar drink can trigger a hypoglycaemic episode (a sudden drop in blood sugar) in people whose bodies overreact to sugar. Medical staff are present throughout the GTT and can respond immediately if this happens.
• Infection at the needle site is possible but extremely uncommon when standard hygiene practices are followed.

Preparation for this test is straightforward, but following the instructions carefully is important because eating, drinking, or certain activities can directly change your blood sugar levels and affect the result.

For the HbA1c test alone, fasting is often not required because the test reflects a three-month average. However, many laboratories draw it at the same time as other fasting blood tests, so your doctor will give specific guidance. For the GTT, you will normally be asked to fast (have nothing to eat or drink except plain water) for at least eight to twelve hours before the test. This usually means not eating or drinking after midnight if your appointment is in the morning.

In the days before a GTT, your doctor may advise eating a normal diet that includes a reasonable amount of carbohydrates (foods like rice, bread, or noodles). Avoiding carbohydrates in the days before the test can produce a misleadingly abnormal result. Your doctor will also advise on whether any regular medications should be paused, particularly blood thinners or diabetes medications. Do not stop any medication without that guidance.

Smoking and strenuous exercise are usually avoided on the morning of the GTT, as both can affect how the body handles sugar. Alcohol is generally avoided for at least twenty-four hours before the test.

Common tests that may be run alongside or before this procedure include a fasting blood glucose test, a full blood count (to check for anaemia that might affect HbA1c accuracy), kidney function tests, and a lipid panel (cholesterol levels). These are often done from the same blood sample.

Here is what typically happens on the day of a combined HbA1c and GTT appointment:

• You arrive at the clinic or laboratory after fasting. You may be asked to rest quietly for a few minutes before the test begins.
• A nurse or phlebotomist (blood-draw specialist) takes a blood sample from a vein in your arm. This is your fasting baseline blood sugar reading.
• For the GTT: you are given a sweet drink containing a measured amount of glucose (sugar). You drink it within a set time, usually within five minutes.
• You wait in the clinic. You will be asked not to eat, drink (except water in some protocols), smoke, or exercise during the waiting period.
• Further blood samples are taken at timed intervals — commonly at one hour and two hours after the drink. Your doctor or laboratory may request additional draws at other time points.
• The HbA1c test is usually run on one of the blood samples taken during this process — no extra draw is needed.
• Once the final sample is taken, the procedure is complete. You can usually eat and resume normal activities straight away.

Because this is a blood test, there is no physical recovery period in the way there is after surgery or an invasive procedure. Most people leave the clinic immediately after their final blood sample is taken and return to their normal day without any restrictions. However, there are a few practical points to be aware of.

• Eat and drink as normal straight after the test — especially important after a long fasting period to avoid lightheadedness.
• If you feel faint or unwell after the GTT sugar drink, tell the clinic staff before you leave. It is safer to sit and recover than to drive or travel immediately.
• Keep gentle pressure on the needle site for a minute or two if there is any bleeding, and a small plaster (adhesive bandage) is usually enough.
• Any bruising at the needle site is normal and will fade on its own within a few days.
• Results for HbA1c are usually available within a day or two; GTT results may be available the same day or within twenty-four hours, depending on the laboratory.
• A follow-up appointment with your doctor or endocrinologist (hormone and metabolism specialist) will be arranged to discuss what the results mean for you and whether any further tests or treatment steps are needed.
• If diabetes or prediabetes is confirmed, your doctor will discuss next steps, which may include lifestyle changes such as diet and physical activity adjustments, regular monitoring of blood sugar at home, and in some cases medication.
• If you are being monitored for existing diabetes, your doctor will use the HbA1c result to decide whether your current management plan needs to be adjusted.