ORAL GLUCOSE TOLERANCE TEST

The glucose tolerance test (GTT) is also known as the oral glucose tolerance test which measures the body’s ability to metabolize glucose. 

Indication

It is performed for the diagnosis of diabetes in patients with borderline fasting blood glucose level (i.e. between 100 to 140 mg/dL).

Preparation of Individual

Patient is advised to take carbohydrate-rich diet for at least 3 days prior to the GTT. The patient should be in overnight fasting of at least 10 hours. Drinking water is allowed during the fasting period.

Procedure

•• A fasting blood sample and urine sample is collected for estimation of the fasting blood glucose value. It provides a baseline for comparing other glucose values.

•• Patient is given 75 g glucose (amount varies with weight/age) orally, dissolved in about 300 mL of water.

•• Blood and urine samples are collected at timed intervals of 30 minutes for at least 2 hours after drinking glucose.

•• Blood samples are analyzed for glucose level and urine samples are qualitatively tested for glucose.

Amount of Glucose to be Given in OGTT

•• Standard OGTT uses 75 g of glucose.

•• In children glucose should be given based on the weight (1.5 to 1.75 g/kg).

•• In pregnant women, 100 g oral glucose is recommended.

Interpretation (Fig. 57.1)

Fig. 57.1: Oral glucose tolerance test showing variation of blood glucose level in normal, prediabetes and diabetes mellitus

•• Normal/euglycemic: On taking oral glucose load, the concentration of blood glucose increases and peak value (140 mg/dL) is reached in less than an hour which returns to normal by 2 hours. None of the urine sample shows glucose.

•• Prediabetics/impaired glucose tolerance: In patients with impaired glucose tolerance, both the fasting (110 to 126 mg/dL) and 2 hour (140 to 200 mg/dL) blood glucose levels are increased. These patients develop frank diabetes later.

•• Diabetes mellitus: The individual is diagnosed as suffering from diabetes mellitus if the fasting blood glucose levels are more than 126 mg/dL and at 2 hours more than 200 mg/dL. Diagnostic criteria for oral glucose tolerance test are shown in Table 57.1.

•• Gestational diabetes: It is defined as any degree of glucose intolerance developing during pregnancy. About 2–5% of females with gestational diabetes mellitus carry mutation in the glucokinase gene.

Other Types of GTT

•• In the mini GTT, fasting and 2 hours samples (instead of ½ hour interval) of blood and urine are collected.

•• In individuals with malabsorption, intravenous GTT is carried out.

•• Glucosteroid stressed GTT is employed to detect latent diabetes. The GTT is unphysiological way of evaluating the glucose handling by the body. Physiological way is by collecting the fasting blood sample and the individual is allowed to take heavy breakfast. The blood and urine samples are collected at 1 hour and 2 hours (postprandial—meaning after meal). This is commonly employed in established diabetic patients for monitoring the control.

GLYCOSURIA

•• Normally glucose does not appear in urine until the plasma glucose concentration exceeds renal threshold (180 mg/dL). The presence of glucose in urine is known as glycosuria. The commonest cause of glycosuria is diabetes mellitus and therefore, urine examination for glucose is the first line screening test for diabetes.

Points to note:

•• The glucose tolerance test (GTT) measures the body’s ability to metabolize glucose and helps in the diagnosis of diabetes.

•• The fasting blood sugar and urine are examined before giving the oral glucose, the dose of which depends on the weight of the individual and is usually about 75 g in an adult.

•• The blood and urine samples are collected at every half an hour at least for 2 hours.

•• In diabetes mellitus, the fasting blood glucose levels are more than 126 mg/dL and at 2 hours more than 200 mg/dL.