Glucose tolerance test: normal range and what the result means (GTM)

The glucose tolerance test (GTT), also called a sugar load test or OGTT (oral glucose tolerance test), shows how the body responds to an amount of glucose (sugar) over a certain period of time. Before the test, blood is taken while fasting; then a 75 g glucose solution is drunk over a few minutes, and blood is taken again 1 hour and 2 hours later. In a healthy person, after the glucose load the pancreas should release insulin quickly enough for the blood sugar level to return close to the starting level within 2 hours. If this mechanism is impaired, the glucose level stays raised for longer — this indicates impaired glucose tolerance or diabetes mellitus. The GTT is more sensitive than a simple fasting glucose test because, according to MedlinePlus, impaired glucose tolerance often appears earlier than a changed fasting glucose level. That is why this test is used when borderline cases need clarifying, or to confirm a diagnosis of diabetes or prediabetes.

• When the fasting blood glucose level is raised but does not reach the diagnostic threshold for diabetes — to clarify whether this is prediabetes or diabetes
• When there are symptoms of diabetes mellitus: increased thirst, frequent urination, blurred vision, tiredness, slow-healing wounds
• As a preventive check when risk factors are present — excess weight, diabetes in the family, a sedentary lifestyle, age over 35
• When a diagnosis of prediabetes needs confirming or ruling out, and other tests (fasting glucose, HbA1c) give borderline results

According to MedlinePlus, a glucose level of 7.8–11.1 mmol/l 2 hours after the glucose solution is considered impaired glucose tolerance (prediabetes), and 11.1 mmol/l or more is the diagnostic threshold for diabetes mellitus. The value can also be raised temporarily by severe stress on the body.

• Impaired glucose tolerance (prediabetes) — the pancreas cannot lower the glucose level quickly enough after the load
• Diabetes mellitus
• Severe stress on the body — trauma, a stroke, a heart attack or surgery, which temporarily raise the glucose level

Symptoms listed by MedlinePlus:

• Increased thirst
• Frequent urination
• Blurred vision
• Tiredness
• Slow-healing wounds

What to discuss with a doctor after a raised result

• Whether there was no acute stress during the test (trauma, surgery, an acute illness) that could have temporarily raised the result
• A repeat test or additional tests (fasting glucose, HbA1c) to confirm a diagnosis
• Risk factors — weight, physical activity, family history
• How often further monitoring should take place, together with a doctor

MedlinePlus states that vigorous physical activity during the test can lower the blood glucose level. A low glucose level (hypoglycaemia) can also occur on its own, unrelated to the test itself, and it has characteristic symptoms.

• Vigorous physical activity during the test or before it
• Too long a fast, or too little food before the test, if this did not follow the instructions

Symptoms listed by MedlinePlus:

• Shaking or feeling agitated
• Hunger
• Tiredness
• Dizziness, confusion or irritability
• Headache

What to discuss with a doctor after a low result

• Whether rest was maintained during the test (no vigorous physical activity)
• Whether the preparation was correct — a long enough time without food, but not too long
• A repeat test, if the result raises doubts
• Discussing symptoms related to a low glucose level with a doctor

Glucose tolerance test bands: what each result means

According to the American Diabetes Association (ADA), a glucose level of 7.8–11.1 mmol/l (140–199 mg/dl) 2 hours after a 75 g glucose solution corresponds to the prediabetes category — impaired glucose tolerance. This means the body handles glucose after the load more slowly than usual, but does not yet reach the diagnostic threshold for diabetes mellitus.

The ADA defines the prediabetes category as an intermediate state between normal and diabetes mellitus: the body still produces insulin and responds to the glucose load, but the response is delayed or insufficient, so after 2 hours the glucose level remains raised. This category is also often called impaired glucose tolerance — a term used to distinguish it from impaired fasting glycaemia, which is established by a different test.

A single result in this band does not confirm a diagnosis by itself — what matters is whether there was no acute stress during the test (an infection, trauma, strong emotional strain) that could have temporarily raised the reading, and whether the preparation requirements were properly followed (fasting duration, diet before the test). The ADA recommends assessing prediabetes together with other markers — fasting glucose and HbA1c — because each of them reflects a different period of time and can give different results in the same person.

Because prediabetes is defined as a stage that may be followed by further monitoring, a doctor usually assesses the result together with risk factors — body weight, physical activity, family history — rather than as an isolated number. The exact frequency and extent of the next steps depend on the overall clinical picture, which a doctor assesses.

• Whether there was no acute stress, illness or infection during the test or before it that could have temporarily affected the result
• Whether the preparation requirements for the test were followed exactly (fasting duration, diet before the test)
• The results of another marker — fasting glucose or HbA1c — in order to compare different ways of assessing this
• Whether a repeat test is needed and how often, if the result is close to the threshold
• Risk factors that a doctor assesses together with this result — body weight, physical activity, family history

Common questions about this band. Is a glucose tolerance test result of 8 mmol/l at 2 hours high? 8 mmol/l falls within the 7.8–11.1 mmol/l interval, which the ADA assigns to the prediabetes (impaired glucose tolerance) category — above the limit of normal (7.8 mmol/l), but below the diagnostic threshold for diabetes mellitus (11.1 mmol/l). Does impaired glucose tolerance mean diabetes is already present? No — according to the ADA, this is a separate, intermediate category between normal and diabetes mellitus. Diagnosing diabetes requires a higher value or additional tests, which a doctor assesses. Why can a glucose tolerance test result fall into this band when fasting glucose was normal? According to MedlinePlus, impaired glucose tolerance often appears earlier than a disturbance of the fasting glucose level, so the GTT can show a change when the other test still shows a normal result. Does the test need repeating if the result is in this band? The need for a repeat test and its timing are determined by a doctor, taking into account the circumstances of the preparation, possible acute stress during the test and other markers.

Above 11.1 mmol/l — the diabetes diagnostic threshold (ADA)

According to the ADA, a glucose level of 11.1 mmol/l (200 mg/dl) or more 2 hours after a 75 g glucose solution corresponds to the diagnostic threshold for diabetes mellitus. Such a value shows that after the glucose load the body does not bring the glucose level back close to the starting level within 2 hours — this is the highest category in the GTT classification.

The ADA states that a 2-hour glucose value of 11.1 mmol/l (200 mg/dl) or more is one of several diagnostic thresholds used to establish diabetes mellitus — alongside fasting glucose and HbA1c. Because this is a diagnostic threshold and not an isolated observation, the ADA recommends confirming the diagnosis with a repeat test on another day or with another of the tests mentioned, unless clear symptoms of diabetes are present together with a very high value.

A single result in this band does not replace a final diagnosis by itself. It is important to take into account possible acute stress during the test (trauma, an infection, surgery), which can temporarily raise the glucose level even in people without diabetes, and also whether the preparation for the test met the requirements.

Because this category is the highest on the GTT scale, a doctor usually assesses it together with symptoms (increased thirst, frequent urination, tiredness) and other laboratory markers, in order to decide on further confirmation and a monitoring plan.

• Confirming the diagnosis with a repeat test on another day or with another test (fasting glucose, HbA1c), as the ADA recommends
• Whether there was no acute stress, illness, infection or surgery during the test or before it that could have temporarily raised the result
• Discussing symptoms of diabetes — increased thirst, frequent urination, blurred vision, tiredness, slow-healing wounds
• Whether the preparation requirements for the test were followed exactly
• A plan for further monitoring and additional tests, together with a doctor

Common questions about this band. Does a glucose tolerance test result of 12 mmol/l at 2 hours mean diabetes? 12 mmol/l exceeds the 11.1 mmol/l threshold set by the ADA, which is assigned to the diabetes mellitus diagnostic category. The ADA recommends confirming such a result with a repeat test or another marker, and the final diagnosis is made by a doctor. Is a single high GTT result already a diagnosis? According to the ADA, a diagnosis usually requires confirmation with a repeat test or with another of the diagnostic tests (fasting glucose, HbA1c), unless clear symptoms are present together with a very high value. What can temporarily raise a glucose tolerance test result above 11.1 mmol/l? According to MedlinePlus, severe stress on the body — trauma, an infection, a stroke, a heart attack or surgery — can temporarily raise the glucose level, so such circumstances are worth discussing with a doctor. Which tests are done after such a GTT result? The ADA states that a repeat GTT, a fasting glucose test or HbA1c are most often used to confirm the diagnosis — the specific plan is chosen by a doctor.

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