Glucose Tolerance Testing 

Updated: Nov 13, 2016
  • Author: Jiun-Lih Jerry Lin, MBBS, MS(Orth); Chief Editor: Eric B Staros, MD  more...
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Reference Range

Table 1. Fasting Blood Glucose [1, 2] (Open Table in a new window)

  Australia/United Kingdom North America
Nondiabetic < 5.5 mmol/L < 110 mg/dL
Impaired glucose tolerance 5.5-6.9 mmol/L 110-126 mg/dL
Diabetic >7 mmol/L >126 mg/dL

Table 2. Random Blood Glucose [1, 2] (Open Table in a new window)

  Australia/United Kingdom North America
Nondiabetic < 5.5 mmol/L < 110 mg/dL
Impaired glucose tolerance 5.5-11 mmol/L 110-180 mg/dL
Diabetic >11 mmol/L >180 mg/dL

Glucose tolerance testing (GTT) is used to evaluate the ability to regulate glucose metabolism and is indicated when random/fasting blood glucose testing alone is insufficient in establishing or ruling out the diagnosis of diabetes mellitus.

The reference range of serum or plasma glucose is less than 140 mg/dL at 2 hours after a 75-g glucose load.

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Interpretation

Table 3. Reference Range for Oral Glucose Tolerance Test [1, 2] (Open Table in a new window)

  Fasting 2-Hour
Normal < 5.5 mmol/L < 7.8 mmol/L
< 110 mg/dL < 140 mg/dL
Impaired glucose tolerance 5.5-6.9 mmol/L 7.8-11 mmol/L
110-126 mg/dL 140-200 mg/dL
Diabetes >7 mmol/L >11.1 mmol/L
>126 mg/dL >200 mg/dL

 

Hemoglobin A1c (HbA1c) testing

In conjunction with glucose tolerance testing (GTT), another useful indicator for the diagnosis and monitoring of diabetes mellitus includes hemoglobin A1c (HbA1c) testing. HbA1c is glycated hemoglobin. Its value, measured in percentage of all Hemoglobin, is used as an indicator of the serum glucose value over the last 3-4 months. The American Diabetes Association considers an HbA1c value of greater than 6.5% to be diagnostic of diabetes mellitus, and a value of 5.7–6.4% to be associated with a high risk for developing diabetes. [7]

Glucose tolerance testing in preventative medicine

A glucose tolerance test can be used to identify patients at high risk of developing type-2 diabetes based on clinical risk factors, with a view to beginning preventive therapy. [7] Approximately 25% of patients with either impaired glucose tolerance or impaired fasting glucose will develop type-2 diabetes over the following 3–5 years. [8] However, the results of a glucose tolerance test cannot distinguish between type-1 and type-2 diabetes mellitus, and cannot be used to predict response to therapy, or likelihood of future complications. [1] Although glucose tolerance testing is more specific than other options, fasting blood glucose and haemoglobin A1c testing are usually more convenient and thus are used more frequently for screening. [7]

Higher plasma glucose levels are linked to increased cardiovascular risk, with this risk increase extending into the non-diabetic range. [9] Impaired glucose tolerance (as demonstrated on a GTT) has also been demonstrated to be a better predictor of increased cardiovascular risk and mortality than fasting glucose. [10, 11]  

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Collection and Panels

Specifics for collection and panels are as follows:

  • Specimen type: Blood serum or plasma
  • Container: Vacutainer, red top (serum) gray top (plasma)
  • Collection method: Venipuncture
  • Specimen volume: 1 mL
  • Other instructions: Perform study with patient in fasting state

Related tests include serum or plasma glucose, hemoglobin A1c, urinalysis, C-peptide, and insulin.

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Background

Description

Glucose tolerance testing (GTT) evaluates the ability to regulate glucose metabolism. Plasma glucose reflects a combination of carbohydrate uptake from the gastrointestinal tract, hepatic glucose output, and peripheral glucose uptake. GTT is used when the serum glucose level alone is equivocal in diagnosing diabetes (ie, when the glucose level falls in the equivocal range). [1, 2]

After baseline fasting plasma glucose testing, a glucose load is administered—either intravenously or, more commonly, orally—and plasma glucose is measured at specified intervals thereafter. In standard oral GTT, plasma glucose concentration is measured 2 hours after a 75-g oral glucose load; for gestational diabetes mellitus, an additional measurement may be made at 1 hour.

Indications/Applications

Oral GTT is currently the criterion standard for the diagnosis of diabetes in Australia. It is indicated when the results of fasting or random blood glucose are equivocal towards the diagnosis of type 2 diabetes mellitus, such as the following:

  • Fasting blood glucose: 5.5-6.9 mmol/L
  • Random blood glucose: 5.5-11 mmol/L

Oral GTT is unnecessary if fasting or random blood glucose levels are clearly in the nondiabetic or diabetic range (refer to Reference Range section).

Indications for oral GTT include the following:

  • Equivocal fasting plasma/random plasma glucose results
  • To screen for gestational diabetes mellitus at 24-28 weeks of gestation in all pregnant women not known to have diabetes
  • To screen for diabetes mellitus at 6-12 weeks postpartum in women with a history of gestational diabetes mellitus, using nonpregnant oral GTT criteria
  • To screen for impaired glucose tolerance, which is associated with an increased risk of developing diabetes mellitus. [1, 2]

Oral GTT in pregnancy

There is a strong correlation between increasing maternal glucose levels at 24-32 weeks’ gestation and a range of adverse maternal and fetal outcomes. [3] Several different strategies are used worldwide in the screening and diagnosis of gestational diabetes. The American Diabetes Association offers two approaches to gestational diabetes screening; a "one-step" approach with a single 2 hour 75g OGTT, or a "two-step" approach with a 1 hour 50g screening OGTT followed by a confirmatory 3 hour 100g OGTT in patients who screen positive. [7]

The Australian Diabetes in Pregnancy Society (ADIPS) Consensus Guidelines for the testing and Diagnosis of Gestational Diabetes Mellitus in Australia [4] recommends early testing for gestational diabetes mellitus in women with risk factors, in a tiered approach to early glucose testing.

Moderate risk factors include the following:

  • Ethnicity: Asian, Indian subcontinent, Aboriginal, Torres Strait Islander, Pacific Islander, Maori, Middle Eastern, non‐white African
  • Body mass index (BMI) of 25-35 kg/m 2

High risk factors include the following:

  • Previous gestational diabetes mellitus
  • Previous elevated blood glucose value
  • Maternal age of 40 years or older
  • Family history diabetes mellitus (first-degree relative with diabetes mellitus or a sister with gestational diabetes mellitus)
  • BMI higher than 35 kg/m 2
  • Previous macrosomia (baby with birth weight >4500 g or >90th percentile)
  • Polycystic ovarian syndrome
  • Medications (corticosteroids, antipsychotics)

Women with one moderate risk factor should be considered moderate risk and should initially be screened with a random or a fasting glucose test in early pregnancy, followed by pregnancy oral GTT if clinically indicated. High-risk (one or more high risk factor or 2 moderate risk factors) women should undergo 75-g pregnancy oral GTT, with venous plasma samples taken fasting, 1 hour and 2 hours, and at the first opportunity after conception.

Moderate- or high-risk women with normal early pregnancy glucose testing should have repeat pregnancy oral GTT at 24–28 weeks’ gestation [5] or earlier if indicated. As risk stratification by risk factors is unreliable, all women should be tested, and all women not known to have gestational diabetes mellitus should have 75g pregnancy oral GTT at 24–28 weeks.

In a 75g oral GTT, a diagnosis of gestational diabetes mellitus is made if one or more of the following glucose levels is elevated:

  • Fasting: ≥92 mg/dL (5.1 mmol/L),
  • One hour: ≥180 mg/dL (10 mmol/L),
  • Two hour: ≥153 mg/dL (8.5 mmol/L)

The diagnostic criteria for 0-, 1-, and 2-hour values were chosen to identify the same risk of an adverse fetal outcome at each time point. [6]

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