What is OGTT and what is it used for?

OGTT is the abbreviation for Oral Glucose Tolerance Test.
It is used to diagnose disturbances in sugar metabolism. It is performed by serially measuring glucose in the serum of patients, both before and after glucose is given orally.

OGTT is performed in cases of:

• Suspected gestational diabetes mellitus (GDM)
• People at risk for diabetes mellitus, whose fasting glucose levels are within the reference range
• Testing patients with borderline elevated glucose levels after meals
• Diagnosing diabetes mellitus
• Testing patients with unexplained neuropathy, nephropathy, or retinopathy
• Women diagnosed with polycystic ovary syndrome (PCOS)

How is OGTT performed?

It should be noted that there are several factors that can influence glucose tolerance. These should be controlled or eliminated before the test is performed. The following conditions need to be met for the lab results to be valid:

• The test should be performed exclusively in the morning.
• The patient should consume more carbohydrate-rich foods (more than 150 g per day) for the previous 3 days.
• Discontinue medication that affects glucose metabolism – always in consultation with a doctor!
• The patient should not eat, smoke, or engage in physical activity for 12 hours prior to the test.
• 30 minutes before the test, the patient should remain at rest.

When all conditions are met, blood is drawn and fasting glucose is measured.

How is the test performed?

During OGTT, it is recommended to use venous blood for analysis, not capillary blood (from the finger), as glucose concentrations in capillary and venous blood can vary by up to 2 units.

The patient then drinks a prepared glucose solution, with a standard dose of 75 g of glucose for adults (for children, it is 1.75 g/kg of the patient’s weight, but not more than 75 g).

For pregnant women, this dose is usually 100 g of glucose. The test lasts 2 or 3 hours.

Venipuncture is performed after 30 minutes and then chronologically, as ordered by the doctor. Usually:

• For pregnant women: at 0 min, 30 min, 60 min, 120 min, 180 min (urine sample at the beginning and end of the test).
• For others: at 0 min and 120 min (urine sample at the beginning and end of the test).

The initial glucose concentration (“fasting”) should be lower than 6.1 mmol/L.

OGTT

If the glucose concentration at 120 minutes is higher than 11.1 mmol/L, the result needs to be repeated to confirm it. If the glucose concentration is between 7.8 and 11.1 mmol/L, it indicates impaired glucose tolerance, while if the glucose concentration at 120 minutes is lower than 7.8 mmol/L, glucose metabolism is satisfactory.

Lower OGTT values may be found in:

• Polycystic ovary syndrome
• Rheumatoid arthritis
• Acute and chronic kidney damage
• Viral hepatitis
• Pheochromocytoma
• Hyperthyroidism
• Acromegaly
• Hyperfunction of the adrenal cortex
• Hyperlipoproteinemia
• Cystic fibrosis
• Thalassemia
• Cirrhosis
• Chronic pancreatitis

Doctors may occasionally recommend performing OGTT with insulin levels.

This means that glucose and insulin concentrations are measured from each blood sample. The test results indicate the presence or absence of insulin resistance, or metabolic syndrome.

For more information, consult your doctor.