Oral glucose tolerance test (OGTT): what happens and how results are read

After you eat, glucose rises and insulin is released to move it into cells. An OGTT challenges this system with a standard dose and sees how quickly glucose returns toward normal. Lab Tests Online UK describes it as a fasting glucose measurement, then a drink containing a specified amount of glucose, then another glucose test two hours later.

It can reveal problems that a fasting sample alone misses, because some people have a normal fasting value but handle a glucose load poorly. That state, impaired glucose tolerance, is a recognised stage on the way to type 2 diabetes.

A 75 g, two-hour OGTT may be used to diagnose diabetes, and is the usual test where HbA1c cannot be used. Lab Tests Online UK lists those situations as symptomatic children and young people, current or recent pregnancy, suspected type 1 diabetes, symptoms for less than two months, acute illness, medicines that raise glucose quickly such as corticosteroids, haemoglobin variants, anaemia and recent blood transfusion.

In pregnancy, risk is assessed at the booking appointment. Risk factors listed are a BMI above 30, a previous baby weighing 4.5 kg or more, previous gestational diabetes, a family history of diabetes, and a minority ethnic family origin with a high prevalence of diabetes. Women with a risk factor can be offered a 75 g OGTT at 24 to 28 weeks, and those with previous gestational diabetes may be offered early monitoring or an OGTT soon after booking and again at 24 to 28 weeks if the first is normal.

You fast, generally for 8 to 10 hours with only water allowed, so tests are usually booked in the morning. Follow the instructions you are given by the team. After the first blood sample you drink the glucose solution and then wait, usually resting and not eating, until the second sample is taken two hours later.

The NHS says the pregnancy OGTT takes about 2 hours. Some people feel sick or light-headed after the drink; tell the staff if so. Bring something to read, as you must stay for the full two hours.

Lab Tests Online UK gives interpretation thresholds for fasting glucose and the 2-hour OGTT value. A fasting glucose between 3.6 and 6.0 mmol/L is normal, 6.1 to 6.9 is impaired fasting glucose and 7.0 or above is probable diabetes. After the 75 g drink, a 2-hour value below 7.8 mmol/L is normal glucose tolerance, 7.8 to 11.0 is impaired glucose tolerance and 11.1 or above is probable diabetes.

For gestational diabetes, either a fasting plasma glucose of 5.6 mmol/L or above or a 2-hour value of 7.8 mmol/L or above is used for diagnosis, although other thresholds are in use, so your own service will explain which applies. To be certain of a diagnosis in someone without symptoms, the test should be repeated at a different time, for example within two weeks.

The NHS describes gestational diabetes as a type of diabetes that can happen during pregnancy and usually goes away after the baby is born, with risk reduced by lifestyle changes. If it is diagnosed, glucose testing continues through pregnancy and after delivery, and Lab Tests Online UK warns untreated high glucose in pregnancy can lead to large babies who may have low glucose after birth. Impaired tolerance outside pregnancy is a signal to reduce risk: the NHS says people with pre-diabetes may be eligible for the NHS Diabetes Prevention Programme.

Seek urgent advice for symptoms such as intense thirst, frequent urination, tiredness and weight loss; the NHS recommends an urgent GP appointment or NHS 111.