What is the purpose and procedure of an Oral Glucose Tolerance Test (OGTT)?

Oral Glucose Tolerance Test (OGTT): Purpose and Procedure

The Oral Glucose Tolerance Test (OGTT) is the gold standard for diagnosing diabetes mellitus and impaired glucose tolerance, providing crucial information about glucose metabolism that can identify patients who would be missed by fasting glucose tests alone. 1

Purpose of OGTT

• OGTT is used to diagnose diabetes mellitus, impaired glucose tolerance (IGT), and impaired fasting glucose (IFG), collectively known as disorders of glucose metabolism 2
• It can detect post-challenge hyperglycemia in patients with normal fasting glucose levels, identifying approximately 25.8% of diabetic patients who would be missed by fasting plasma glucose testing alone 3
• OGTT is particularly valuable for detecting impaired glucose tolerance (IGT), which is a significant risk factor for future diabetes and cardiovascular disease 2
• It is recommended when fasting plasma glucose (FPG) and HbA1c results are inconclusive but diabetes is still suspected 2

Who Should Undergo OGTT

• Testing should be considered in adults who are overweight or obese (BMI ≥25 kg/m²) and have one or more additional risk factors for diabetes 2
• Risk factors include physical inactivity, first-degree relative with diabetes, high-risk ethnic populations, history of gestational diabetes, hypertension, dyslipidemia, polycystic ovarian syndrome, and history of cardiovascular disease 2
• In the absence of these risk factors, testing should begin at age 45 years 2
• OGTT is specifically recommended for patients with impaired fasting glucose (IFG) to better define their risk of diabetes 2
• For cardiovascular patients, OGTT should be added if HbA1c and fasting plasma glucose are inconclusive 2

OGTT Procedure

• Patient should fast for at least 8 hours prior to the test 2
• A baseline fasting plasma glucose sample is collected 2
• Patient consumes a glucose load containing 75g of anhydrous glucose dissolved in water 2
• Blood samples are collected at specific intervals, with the 2-hour post-load sample being most critical for diagnosis 2, 1
• In acute coronary syndromes, the OGTT should not be performed earlier than 4-5 days after the event to minimize false-positive results 2

Diagnostic Criteria

• Diabetes is diagnosed if 2-hour plasma glucose is ≥200 mg/dL (11.1 mmol/L) 2
• Impaired glucose tolerance (IGT) is diagnosed if 2-hour plasma glucose is between 140-199 mg/dL (7.8-11.0 mmol/L) 2
• Impaired fasting glucose (IFG) is diagnosed if fasting plasma glucose is between 100-125 mg/dL (5.6-6.9 mmol/L) according to ADA criteria 2
• Both IGT and IFG are officially termed "pre-diabetes" and are risk factors for future diabetes and cardiovascular disease 2

Advantages and Limitations

Advantages:

• Detects isolated post-challenge hyperglycemia that would be missed by fasting tests 3
• Provides more comprehensive assessment of glucose metabolism 4
• Remains the gold standard for diabetes diagnosis 1

Limitations:

• Less convenient than fasting plasma glucose testing 2
• Requires more time to perform (at least 2 hours) 2
• May cause nausea or discomfort in some patients 5
• Less acceptable to patients than fasting tests 2

Clinical Considerations

• If tests are normal, repeat testing should be carried out at least at 3-year intervals 2
• In those identified with pre-diabetes, other cardiovascular risk factors should be identified and treated 2
• Laboratory measurement of plasma glucose should be performed on venous samples with enzymatic assay techniques 2
• The test should be performed in a healthcare setting, not as community screening 2
• Certain medications, including glucocorticoids and nicotinic acid, may affect test results by producing hyperglycemia 2

Recent Developments

• Some studies suggest using 30-minute or 1-hour post-glucose load glycemia measurements for earlier detection of glucose metabolism disorders 6
• Mixed meal tolerance tests (MMTT) are being explored as alternatives that better replicate normal dietary patterns 5
• However, the 2-hour OGTT remains the recommended standard in current guidelines 2