Glucose Tolerance Test

Overview

A glucose tolerance test (also called an oral glucose tolerance test, or OGTT) is a simple test that shows how well your body handles sugar. Sugar in your blood is called glucose, and it is your body's main fuel. You fast overnight, give a blood sample, drink a sweet drink, and then give more blood samples over the next one to two hours. The results help your doctor see whether your body is using glucose the way it should. It is mostly used to check for diabetes, and in pregnancy to check for gestational diabetes (diabetes that starts during pregnancy).

Yes. It is one of the most common tests in general practice and in pregnancy care. Millions of people have one every year, and clinics run them every day.

Anyone can be offered the test. It is most often done in pregnancy, usually between 24 and 28 weeks, and in adults who have risk factors for type 2 diabetes or symptoms of high or low blood sugar. Children and teenagers sometimes have it too, for the same reasons.

Symptoms

Common symptoms

• A glucose tolerance test does not cause symptoms — it is a test, not an illness. The symptoms below are the ones that often lead a doctor to suggest the test. Many people have no symptoms at all and are tested because of risk factors.
• Feeling very thirsty, more than usual
• Peeing more often, especially waking at night to go
• Feeling unusually tired or lacking energy
• Blurred vision that comes and goes
• Cuts, sores or infections that are slow to heal
• Repeated infections, such as thrush or urinary infections
• Tingling or numbness in the feet
• Feeling shaky, sweaty or very hungry a few hours after a meal

Symptoms in children

• Often no symptoms at all — children are usually tested because of family history or other risk factors
• Wetting the bed again after being dry for a while
• Being very thirsty and drinking much more than usual
• Peeing a lot, including at night
• Tiredness, irritability or poor growth
• Losing weight without trying
• Repeated skin or nappy-area infections

Symptoms in older adults

• High blood sugar often causes no clear symptoms in older adults
• Tiredness, low energy or general weakness may be the only clue
• Confusion or poor concentration can sometimes be a sign
• Frequent urine infections or slow-healing skin problems
• Thirst may be blunted, so it is easy to miss
• This is why screening tests are offered even when someone feels well

Causes

Main causes

• A glucose tolerance test is not caused by anything — it is a test. These are the usual reasons a doctor orders one:
• To check for type 2 diabetes or prediabetes (blood sugar that is higher than normal but not yet diabetes)
• To check for gestational diabetes during pregnancy, usually at 24 to 28 weeks
• To look into symptoms of low blood sugar that come on a few hours after eating
• To follow up a result that was unclear, such as a borderline fasting glucose or HbA1c (a test that shows your average blood sugar over about three months)
• Rarely, to check how well the bowel absorbs sugar

Risk factors

• A parent, brother or sister with diabetes
• Being overweight, especially around the waist
• Being aged over 40, or over 25 for some South Asian, African, Caribbean and Middle Eastern backgrounds
• Having had gestational diabetes in a past pregnancy
• Having given birth to a baby weighing more than about 4 kg (9 lb)
• Polycystic ovary syndrome (a hormone condition that can affect periods and blood sugar)
• High blood pressure, heart disease or a previous stroke
• Taking steroid tablets for a long time
• Smoking, low physical activity and poor sleep
• Some mental health conditions and the medicines used to treat them — ask your doctor if this applies to you

When to see a doctor

See a doctor urgently if:

• You have the urgent symptoms listed above, such as feeling faint, shaking badly, vomiting, or a very high home glucose reading while unwell
• You are pregnant and notice unusual thirst, passing lots of urine, or feel generally unwell — contact your midwife or doctor the same day

Book a routine appointment if:

• Your doctor or midwife has suggested a glucose tolerance test and you want to talk it through first
• You have two or more risk factors for type 2 diabetes
• You have symptoms such as thirst, passing urine a lot, or tiredness that will not go away
• You are planning a pregnancy and have risk factors for diabetes
• You had gestational diabetes in a past pregnancy — you will usually be offered a test again

Diagnosis

The glucose tolerance test is the diagnosis test itself. You fast overnight, a blood sample is taken to measure your starting glucose, you drink a sweet glucose drink, and then further blood samples are taken — usually after one hour and after two hours. Your doctor compares these numbers with standard ranges. Sometimes a single fasting blood test or an HbA1c test is done first, and the glucose tolerance test is used when the answer is still unclear.

Tests that may be done

• Fasting blood glucose — a sample taken before you drink anything sweet
• Two-hour glucose tolerance test — the main test, with blood samples at set times after the drink
• HbA1c — a blood test that shows your average blood sugar over about three months
• Random blood glucose — a sample taken at any time of day
• Urine test for glucose and ketones (natural chemicals made when the body burns fat instead of sugar)
• A home glucose meter reading, if you already monitor your sugar

What to expect at your appointment

You will be asked to eat and drink normally for a few days before the test, then fast for 8 to 12 hours — water is fine. On the day, a small needle takes a blood sample, and you drink a very sweet, flat, sugary drink within about five minutes. You then wait, usually sitting quietly, and give one or two more blood samples. You should not smoke, eat, or do vigorous exercise during the wait. Bring a book or your phone, and something to eat for afterwards. Ask your doctor whether to take your usual medicines on the day, and arrange a lift home if you have felt faint with needles before. The needle feels like a brief pinch, and the drink can taste very sweet and may make you feel a little sick.

Treatment

The test itself needs no treatment. What comes next depends on the result. A normal result usually means no further action, apart from routine checks. A result showing prediabetes or diabetes means your doctor will talk with you about monitoring, healthy eating, activity and regular review. If you are pregnant and the result shows gestational diabetes, you will be offered extra support from your maternity team, with closer monitoring and advice on eating and activity.

Self-care at home

• Eat your normal meals in the days before the test — do not diet or cut out carbohydrates
• Fast for the time your clinic tells you, usually 8 to 12 hours, and drink water only
• Ask whether to take your usual medicines on the morning of the test
• Avoid smoking and strenuous exercise during the test
• Bring water, something to eat for afterwards, and something to pass the time
• Tell the staff straight away if you feel dizzy, shaky, sweaty or unwell during the test

Medical treatments

There is no medicine involved in having the test. If your result shows diabetes or prediabetes, treatment is planned with your doctor. It usually starts with support to eat well, move more and lose a little weight if needed, plus regular blood tests to see how things are going. Some people later need medicines to help control blood sugar, chosen and checked by their doctor. In pregnancy, gestational diabetes is managed with closer monitoring, diet and activity advice, and sometimes medicines — your maternity team will explain what is right for you. Always ask your own doctor or pharmacist before starting, stopping or changing any medicine.

When is surgery considered?

This test does not involve surgery and is not related to any operation. If you are due to have surgery and also need a glucose test, your care team will plan the timing so the two do not clash.

Living with this condition

For most people, the test is a single morning out of their life and nothing changes afterwards. If the result leads to a diagnosis, day-to-day life centres on a few simple routines: taking any medicines your doctor prescribes, checking your blood sugar if asked to, keeping regular appointments, and noticing how food and activity affect how you feel. None of this needs to take over your life — most people fit it into their normal routine within a few weeks.

Lifestyle tips

• Build activity into your day — walking, gardening, dancing or swimming all count
• Aim for a balanced plate: plenty of vegetables, some whole grains, some protein, and fruit rather than sweet snacks
• Cut back on sugary drinks and very sweet foods rather than cutting them out completely
• Keep a regular sleep routine — poor sleep affects blood sugar
• If you smoke, ask your doctor or pharmacist for help to stop
• Keep alcohol within recommended limits
• Go for your regular check-ups, eye checks and foot checks if you are offered them

Diet and exercise

You do not need a special diet. Aim for regular meals, more vegetables and fibre, and fewer very sweet or heavily processed foods. Small swaps — water instead of fizzy drinks, wholegrain instead of white bread — add up over time. For activity, most adults do well with about 150 minutes a week of something that makes them a little warm and out of breath, plus some strength work such as carrying shopping or chair exercises. In pregnancy, gentle activity like walking and swimming is usually encouraged — check with your midwife first.

Mental health and emotional wellbeing

It is normal to feel worried while waiting for results, or to feel shocked if you are told you have diabetes. Some people feel guilty or anxious, especially in pregnancy. These feelings usually settle, and talking about them helps. Tell your doctor or midwife how you are feeling — support is available. If you ever feel overwhelmed or unsafe, contact your local crisis line or emergency services straight away; you do not have to cope alone.

Prevention

The test itself is not something you prevent — it is a check. What can often be prevented or delayed is type 2 diabetes. Eating well, staying active, keeping to a healthy weight, not smoking and getting enough sleep all lower your risk. If you already have prediabetes, these changes can sometimes bring your blood sugar back to normal.

Screening programmes

Screening is offered to people at higher risk of type 2 diabetes, usually with a simple blood test first. In the UK, the NHS Health Check offers this to adults aged 40 to 74. In pregnancy, a glucose tolerance test is offered at around 24 to 28 weeks if you have risk factors. If you have had gestational diabetes, you will usually be offered a follow-up blood test some weeks after the birth. Ask your doctor or nurse whether screening is right for you.

Complications

If left untreated

• If the test is not done, diabetes or prediabetes can go unnoticed for years
• Over time, high blood sugar can damage blood vessels and nerves
• This can lead to heart and circulation problems
• Kidney damage, which may need monitoring and treatment
• Eye problems that can affect sight if not checked
• Foot problems, including slow-healing sores
• Untreated gestational diabetes can make a baby grow larger than usual and cause problems at birth
• It can also raise the chance of the mother developing type 2 diabetes later in life

Long-term outlook

The outlook is good. Finding high blood sugar early gives you the best chance to act before any damage happens, and many people keep their blood sugar in a healthy range for years with simple changes. If you are pregnant, gestational diabetes is usually managed well and most babies are born healthy. Whatever your result, your care team will guide you step by step, and there is a lot you can do to protect your health.

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