Oral Glucose Tolerance Test (OGTT)

Oral Glucose Tolerance Test (OGTT) - Diagnostic test for Diabetes Mellitus

This is performed to establish a diagnosis in:

1. Patients with transient or sustained glycosuria who have no clinical symptoms of diabetes (polyuria) and with normal fasting and post-prandial blood glucose levels.
2. Patients with symptoms of diabetes but with no glycosuria and normal fasting level.
3. Persons with a strong family history of diabetes but with no overt diabetes.
4. Patients whose glycosuria is associated with pregnancy, thyrotoxicosis, liver disease, and infections.
5. Women who have characteristically large babies or individuals who were large babies.
6. Patients with neuropathies and retinopathies of undertermined origin.

The patient should ingest a daily diet of at least 300gm of carbohydrate for 3 days prior to the test. Therefore, on an acutely ill-hospitalised patient, this test should not be conducted. As far as possible it should be performed on an ambulatory patient. Preferably, the test should be performed in the morning. Various malignancies, fever, cachexia, liver dysfuction and renal failure may be associated with mild to moderate degrees of abnomal GTT. There is an age-related factor that decreases glucose tolerance and hence makes the interpretation of OGTT in elderly subjects difficult. Timing of glucose administration and blood sampling must be accurate.

Patient preparation

1. Instruct the patient about the purpose and procedure of the test; Stress a normal diet with high carbohydrate (150-300gm) for 3 days preceding the test. Fasting is required for at lest 10 hours before the test and not more than 16 hours.Water is permitted and encouraged.
2. Determine the patients weight and record it.
3. Collect urine and blood samples and test for glucose, recording exact time of collection. Have the patient empty his or her bladder for each specimen;

• No food & water is to be taken during test period.
• Encourage the patient to rest.
• No smoking.

Preferably, the samples of urine and whole blood be taken at fasting, 30 minutes, 1 hour. 1 & half hours, 2 hours, 3 hours and 4 hours after ingestion of the carbohydrate meal, if nausea and vomitting occur during the test, the interpretation becomes difficult.

Interpretation

Wilkerson point system

Fasting = 130mg%plasma or more

1 hour = 195mg%plasma or more

2 hours = 140mg%plasma or more

3 hours = 130mg%plasma or more