Glucose Tolerance Test: Types, Utility and Procedure

The below mentioned article provides a short note on Glucose Tolerance Test (GTT):- 1. Types of Glucose Tolerance Test 2. Utility of Glucose Tolerance Test 3. Procedure 4. Different Types of Curves.

Types of Glucose Tolerance Test:

1. Oral glucose tolerance.

2. Intravenous glucose tolerance.

3. Cortisone stress glucose tolerance test.

4. Multiple dose glucose tolerance tests.

Utility of Glucose Tolerance Test:

1. To detect the cause of abnormality of car­bohydrate metabolism.

2. To detect cause of asymptomatic glyco­suria.

Procedure of Glucose Tolerance Test:

1. Patient should take normal meal or carbo­hydrate diet 3 or 4 days before the test.

2. Test is usually done after a overnight fast­ing. (It may be done 5 hours after fasting)

3. Fasting blood and urine samples are col­lected.

4. Patient should take 75 grams of glucose dissolved in 200 ml of distilled water.

5. Blood is taken every 30 minutes up to 2½ hour (5 samples are taken).

6. Urine is collected after taking glucose at the end of 1 hour and at the end of 2 hours (3 samples).

7. Blood sugar is estimated by blood sample and urine is examined in the presence of sugar; there is a graphical representation wherein time (in hours) is along base line and blood glucose or sugar (mg/dl) along vertical line.

Different Types of Curves in Glucose Tolerance Test:

A. Normal Response

B. Abnormal Response

A. Normal Response:

(a) Initial rise of blood sugar is normal and it never crosses renal thresh-hold.

(b) It returns to normal within 1 hours usu­ally, and almost always within 2 hours.

B. Abnormal Response:

(i) Diminished glucose tolerance.

(ii) Increased glucose tolerance.

(iii) Lag curve.

(iv) Decreased renal threshold.

(v) Increased renal threshold.

(i) Diminished glucose tolerance:

In this condition blood sugar is increased more than in normal patient, fasting blood sugar may be high and return to normal level is delayed. Urine shows glycosuria in most sample.

Two types of curves:

(a) Curve for true diabetes.

(b) Curve for potential diabetes.

(ii) Increased glucose tolerance:

(a) Blood sugar level is not very much in­creased and never crosses renal threshold.

(b) No glycosuria is found. This type of curve is found in hypo-function of pituitary, thy­roid, adrenal, and also in malabsorption syndrome, coeliac disease, and sprue.

(c) In the above-mentioned curve, fasting blood sugar is normal. Increase of blood sugar is not very much, always below re­nal threshold, and comes back to normal within half an hour. This curve is called flat type curve.

(iii) Log curve:

(a) Some apparently normal people are found to have tolerance curve in which return of blood sugar is normal but with certain ex­aggeration of blood sugar level. The re­turn to normal value is sharp and rapid.

(b) So peak rise is always above renal thresh­old and sugar is absent in fasting sample.

Cause:

Delayed insulin mechanism and there is in­creased absorption of glucose from intestine after rapid emptying of stomach. This is a harmless con­dition, these people do not develop diabetes later on.

This condition is found in:

1. Hyperthyroidism.

2. Pregnancy.

3. Gastro-enterostomy.

(iv) Decreased renal threshold:

(a) In some patient, blood sugar level is not very high. But sugar is present in urine. Blood sugar level does not exceed renal threshold but still there is glycosuria. So renal threshold is decreased. This is called renal glycosuria and it is accidently de­tected.

(b) It is found in late stage of pregnancy and Wilson’s disease.

(c) There is no defect in carbohydrate metabo­lism but defect in reabsorption of glucose by renal tubules ½ to 1 hour after taking glucose.

(v) Increased renal threshold:

(a) Blood sugar is always high. Fasting sugar is also high.

(b) It is increased ½ to 1 hour after taking glucose although blood sugar level is above 200 mg/dl. But there is no glycosu­ria.

(c) Urine examination of this group of patients does not give true picture of diabetes and this is a true diabetic curve without gly­cosuria. It is found in elderly diabetic pa­tient.