What is the oral glucose tolerance test used for?

The oral glucose tolerance test assesses the glycaemic response after ingestion of a standardized glucose load and is used in the diagnosis of diabetes mellitus, prediabetes and gestational diabetes.

How should the oral glucose tolerance test be prepared?

The test should be performed after adequate fasting, while maintaining usual carbohydrate intake in the preceding days and avoiding acute conditions that may alter glucose levels.

Which values allow diagnosis of diabetes mellitus?

Diabetes mellitus may be diagnosed when fasting plasma glucose or 2-hour plasma glucose after oral glucose load reaches values defined by accepted diagnostic criteria.

What is impaired glucose tolerance?

Impaired glucose tolerance corresponds to a 2-hour plasma glucose value above normal but below the diagnostic threshold for diabetes, and is associated with increased risk of type 2 diabetes and cardiovascular disease.

What is impaired fasting glucose?

Impaired fasting glucose corresponds to fasting plasma glucose values above the normal range but below the diagnostic threshold for diabetes mellitus.

Is the oral glucose tolerance test used in pregnancy?

Yes. The oral glucose tolerance test is used for screening and diagnosis of gestational diabetes, with pregnancy-specific criteria.

Is confirmation of an abnormal result required?

In the absence of unequivocal symptoms of hyperglycaemia, results compatible with diabetes should be confirmed by repeating the test or using another validated diagnostic criterion.

What should be done after an abnormal test?

After an abnormal test, management depends on the result: surveillance and lifestyle intervention in prediabetes, treatment and structured follow-up in diabetes mellitus, or specific obstetric surveillance in gestational diabetes.

Interpretation of plasma glucose and OGTT — diagnostic criteria and confirmation

The diagnosis of diabetes mellitus and prediabetes is based on measurement of plasma glucose, either fasting or after an oral glucose load (OGTT), reflecting progressive dysfunction of glucose homeostasis. Fasting values ≥126 mg/dL or 2-hour values ≥200 mg/dL are diagnostic of diabetes and should be confirmed on another day, except in the presence of unequivocal signs of hyperglycaemia. Intermediate values define states of increased risk, namely impaired fasting glucose and impaired glucose tolerance, both associated with a higher probability of progression to type 2 diabetes and cardiovascular disease.
This table summarizes diagnostic thresholds, the rationale for laboratory confirmation and the clinical implications of the results, including the need for reclassification and recommendation of lifestyle measures in cases of prediabetes.

Clinical cases

Clinical cases — Interpretation of plasma glucose and OGTT

Case 1 — Initial assessment with still normal values

A 38-year-old woman, BMI 24 kg/m², with no history of diabetes, attends a routine check-up.
Fasting plasma glucose: 92 mg/dL.
OGTT (75 g): 2-hour plasma glucose: 126 mg/dL.
She has no complaints of polyuria, polydipsia or weight loss.

What is the most appropriate interpretation?

Diabetes mellitus already confirmed
Normoglycaemia (normal fasting and 2-hour values)
Prediabetes — impaired glucose tolerance
Prediabetes — impaired fasting glucose

View answer

b) Normoglycaemia.
Rationale: According to usual criteria, values <100 mg/dL fasting and <140 mg/dL at 2 hours on OGTT correspond to normal glucose tolerance.
In these patients, periodic screening should continue according to overall risk (age, family history, lifestyle), but there is no indication to repeat the test immediately.

Case 2 — Prediabetes: impaired fasting glucose vs impaired tolerance

A 52-year-old man, BMI 30 kg/m², controlled hypertension, with no classic symptoms of hyperglycaemia.
Fasting plasma glucose: 112 mg/dL.
OGTT (75 g): 2-hour plasma glucose: 162 mg/dL.

What is the best classification of the results?

Normal — no increased risk
Diabetes mellitus
Prediabetes — impaired fasting glucose and impaired glucose tolerance
Prediabetes — impaired glucose tolerance only

View answer

c) Prediabetes — impaired fasting glucose and impaired glucose tolerance.
Rationale: Fasting plasma glucose between 100–125 mg/dL defines impaired fasting glucose, while 2-hour values between 140–199 mg/dL define impaired glucose tolerance.
This patient meets both criteria and should therefore be classified as having prediabetes, with particularly high risk of progression.
It is essential to reinforce intensive lifestyle changes and consider more frequent screening for diabetes and cardiovascular complications.

Case 3 — Clearly diabetic values: confirm or not?

A 61-year-old woman, BMI 29 kg/m², hypertensive and dyslipidaemic, is referred by nursing staff after an abnormal result.
Fasting plasma glucose: 134 mg/dL.
She repeated an OGTT on another date: fasting plasma glucose 128 mg/dL, 2-hour plasma glucose 214 mg/dL.
She reports some xerostomia, but denies marked polyuria, intense polydipsia or evident weight loss.

What should be the next step?

Assume confirmed diabetes mellitus and start initial assessment
Assume diabetes mellitus, but confirm at least one value in a new sample
Classify only as prediabetes (impaired glucose tolerance)
Repeat OGTT only if classic symptoms develop

View answer

a) Assume confirmed diabetes mellitus and start initial assessment.
Rationale: Fasting values ≥126 mg/dL and/or 2-hour values ≥200 mg/dL on OGTT are compatible with diabetes mellitus.
In asymptomatic individuals, diagnostic results should be confirmed on a different day using the same test or an alternative test. In this case, abnormal diagnostic values were documented on two different dates, so the diagnosis is confirmed.
Initial assessment should include therapeutic education, cardiovascular risk assessment and planning of pharmacological management.

FAQ

Frequently asked questions — Interpretation of plasma glucose and OGTT

1) When should prediabetes be suspected?

Prediabetes should be suspected when fasting plasma glucose is between 100–125 mg/dL (impaired fasting glucose) and/or when 2-hour plasma glucose after OGTT is between 140–199 mg/dL (impaired glucose tolerance). These values do not confirm diabetes, but identify an increased risk of progression and indicate the need for lifestyle intervention.

2) When is it mandatory to repeat the test to confirm diabetes?

Whenever the value is in the diagnostic range for diabetes (fasting ≥126 mg/dL or 2-hour ≥200 mg/dL), the test must be repeated on another day for confirmation — except when classic symptoms of hyperglycaemia are present, such as polyuria, polydipsia and unexplained weight loss.

3) Which symptoms make repeat testing unnecessary?

In the presence of clear signs and symptoms of hyperglycaemia (polyuria, polydipsia, weight loss), a single value in the diabetic range is sufficient to confirm the diagnosis, without the need for repeat testing.

4) Which test should be repeated for confirmation?

Confirmation should be performed using the same method (e.g. OGTT → OGTT; fasting glucose → fasting glucose), under similar conditions (adequate fasting, same analytical method). HbA1c may be used as an alternative test if the laboratory method is adequately standardized.

5) Do values in the prediabetes range require repeat testing?

No. Values in the prediabetes range (fasting 100–125 or 2-hour 140–199) do not require repeat testing for confirmation. They are sufficient to classify risk and recommend lifestyle changes, screening for cardiovascular risk factors and clinical follow-up.

6) What is the role of the OGTT?

The OGTT is more sensitive for detecting impaired glucose tolerance and should be used when fasting glucose is borderline, in high-risk patients (obesity, PCOS, family history, cardiovascular disease) and when postprandial dysglycaemia is suspected.

7) Can HbA1c replace fasting glucose and OGTT?

HbA1c may be used for diagnosis when the method is standardized (NGSP/DCCT) and there are no haematological interfering factors. Prediabetes is defined between 5.7–6.4% and diabetes at ≥6.5%. It may be less sensitive in early stages or in specific populations (PCOS, >65 years, obesity).

8) Which patients should be screened even without symptoms?

Screening is recommended in adults aged ≥35 years, or earlier in the presence of risk factors: increased BMI, family history, dyslipidaemia, hypertension, PCOS, sedentary lifestyle, history of prediabetes or gestational diabetes.

References

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Interpretation of plasma glucose and OGTT — diagnostic criteria and confirmation

Clinical cases — Interpretation of plasma glucose and OGTT

Case 1 — Initial assessment with still normal values

Case 2 — Prediabetes: impaired fasting glucose vs impaired tolerance

Case 3 — Clearly diabetic values: confirm or not?

Frequently asked questions — Interpretation of plasma glucose and OGTT

1) When should prediabetes be suspected?

2) When is it mandatory to repeat the test to confirm diabetes?

3) Which symptoms make repeat testing unnecessary?

4) Which test should be repeated for confirmation?

5) Do values in the prediabetes range require repeat testing?

6) What is the role of the OGTT?

7) Can HbA1c replace fasting glucose and OGTT?

8) Which patients should be screened even without symptoms?

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Interpretation of plasma glucose and OGTT — diagnostic criteria and confirmation

Clinical cases — Interpretation of plasma glucose and OGTT

Case 1 — Initial assessment with still normal values

Case 2 — Prediabetes: impaired fasting glucose vs impaired tolerance

Case 3 — Clearly diabetic values: confirm or not?

Frequently asked questions — Interpretation of plasma glucose and OGTT

1) When should prediabetes be suspected?

2) When is it mandatory to repeat the test to confirm diabetes?

3) Which symptoms make repeat testing unnecessary?

4) Which test should be repeated for confirmation?

5) Do values in the prediabetes range require repeat testing?

6) What is the role of the OGTT?

7) Can HbA1c replace fasting glucose and OGTT?

8) Which patients should be screened even without symptoms?

Lupus: Diagnostic Approach

Oral cancer: screening, diagnosis and referral algorithm

Spirometry: interpretation

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