Oral Glucose Tolerance Test (OGTT)

• An oral glucose tolerance test evaluates how effectively cells metabolize glucose after ingestion.

Introduction to OGTT

• Fasting blood glucose and hemoglobin A1c levels aid in diagnosing type 1 and type 2 diabetes, as well as prediabetes.
• Primarily, glucose tolerance tests diagnose gestational diabetes.
• Type 1 diabetes develops rapidly, marked by high blood sugar and symptoms, potentially not needing an OGTT.
• Type 2 diabetes is more common and develops over years, typically in adulthood.

Purpose of OGTT

• The oral glucose tolerance test (OGTT) is the gold standard for diagnosing diabetes mellitus.
• OGTT can diagnose impaired fasting glycemia (IFG), impaired glucose tolerance (IGT), and gestational diabetes.
• Increased cardiovascular risk is indicated by IFG and IGT.
• A variation of OGTT, with half-hourly blood collections for glucose and growth hormone, serves as the gold standard for diagnosing acromegaly.
• Reactive hypoglycemia may be demonstrated through a prolonged OGTT.
• Glucose tolerance tests measure the body's clearance of a standardized glucose load.

OGTT Procedure

• For 3 days before the test, the patient needs to maintain a normal diet with over 150g of carbohydrates daily and consistent physical activity.
• Obtain glucose from the hospital Pharmacy, using 75g anhydrous glucose dissolved in 200 ml water.
• The patient should fast for at least 12 hours before the test but can drink water as needed.
• The glucose solution can be refrigerated overnight at 4°C, this is optional.
• Counseling and consent are needed for the patient.
• Required materials include a glucose sachet, drinkable water, PPE, and blood collection supplies. Test duration is 1-3 hours.
• Confirm the patient has fasted for 12 hours and understands to remain for 2 hours in the waiting area without eating, drinking, or smoking.
• A venous blood sample is collected for glucose, labeled with the sample date and time according to protocol.
• The patient should drink the glucose solution in 15 minutes or less, ideally within 5 minutes, and the time recorded.
• The patient remain seated in the waiting area for 2 hours, without eating, drinking or smoking.
• A second venous blood sample is collected 2 hours post-glucose, labeled with the sample date and time.
• Both blood samples are sent with a completed Clinical Chemistry request form for OGTT or glucose tolerance test to the Pathology Laboratory.
• A urine sample for urinalysis is optional.

OGTT in Children

• Type 1 diabetes is the most common form of diabetes in childhood, so a formal glucose tolerance test is rarely needed.
• If a glucose tolerance test is required, the glucose dose is 1.75g/kg body weight, up to 75g maximum.

OGTT Complications

• Complications are rare, some patients may experience faintness, nausea, and vomiting after drinking the glucose solution.
• This is more significant during pregnancy.
• Lucozade may be tolerated better and the person can lie down.
• If the patient vomits, the test cannot be completed, unless it is towards the end of the test (after 1 hour).

OGTT Values for Gestational Diabetes

• The values are based on a 76-gram OGTT, elevation of only one value being needed for diagnosis.

Gestational diabetes: Two-step approach

• It involves a 1-hour screening, followed by a 3-hour glucose tolerance test if levels are elevated.
• A venous blood sample is collected to test for fasting glucose, and the patient drinks a solution with 50 grams of glucose.
• An hour later, another sample is collected to measure blood glucose for 1 hour level.
• If the blood glucose is more than 135 mg/dl i.e 7.5 mmol/l proceed to next stage.
• The second step is generally only conducted if the first has a positive result (i.e >7.5mmol/l).
• The patient will drink a glucose solution containing 100 grams of glucose for the three-hour version test.
• Testing involves collecting a blood sample at baseline (fasting), and at one-, two-, and three-hour intervals following glucose ingestion.
• Gestational diabetes can be diagnosed if two of the four blood glucose levels exceed the specified thresholds.

Screening for Gestational Diabetes (GDM)

• Conduct screening in patients with poor obstetric history.
• Conduct screening in patients with history of PCOS.
• Conduct screening if the patient has glycosuria or obesity.
• Conduct screening in patients with a previous and family history.

Interpretation of Gestational Diabetes

• Diagnostic levels for the 2 stage test:
• Fasting requires greater than 95 mg/dL or 5.3 mmol/l
• After 1 hour requires greater than 180 mg/dL or 10mmol/l
• After 2 hours requires greater than 155 mg/dL or 8.6mmol/l
• After 3 hours requires greater than 140 mg/dL or 7.7mmol/l

OGTT Curve

• Become familiar with normal, diabetic, and flat curves.