Glucose Tolerance Test

Professor (Dr.) Namrata ChhabraBiochemistry for Medics- Lecture noteswww.namrata.co

04/13/23 1Biochemistry for medics- Lecture notes

• The ability to utilize carbohydrates can be determined by Glucose tolerance test.

• Initially fasting blood glucose is estimated• A loading dose of glucose is given.• The blood glucose levels are estimated at

regular intervals after the glucose load• In conditions of insulin deficiency, blood

glucose levels get elevated due to impaired utilization of glucose.


A) Decreased Glucose toleranceDecreased carbohydrate tolerance

(non-utilization of carbohydrate load) is observed in conditions causing hyperglycemia, for example:

Diabetes mellitus Hyperactivity of anterior pituitary

and adrenal cortex Hyperthyroidism Stress04/13/23 3Biochemistry for medics- Lecture notes

B) Increased Glucose ToleranceIncreased carbohydrate tolerance is

observed in all conditions that cause hypoglycemia-

i) Hypopituitarism ii) Hyperinsulinism iii) Hypothyroidism iv) Adrenal cortical hypofunction v) Decreased gastro intestinal absorption

like sprue, celiac disease. 04/13/23 4Biochemistry for medics- Lecture notes

i) In asymptomatic persons with sustained or transient glycosuria

ii) In persons with symptoms of diabetes but no glycosuria or hyperglycemia

iii) Persons with family history but no symptoms or positive blood findings


iv)In persons with or without symptoms of diabetes mellitus showing one abnormal blood finding

v) In patients with neuropathies or retinopathies of unknown origin

vi) In women with H/o having delivered large babies.


a) In proven cases of diabetes mellitus the test is not required.

b) GTT is required only in doubtful cases, it is not recommended for follow up of patient.

c) The test should not be carried out in acutely ill patients


a) The patient is instructed to have good carbohydrate diet for 3 days prior to the test. Further , diet containing about 30-50 G of carbohydrate should be taken on the evening prior to the test.

b) The patient should avoid drugs likely to influence the blood glucose levels, for at least, 2 days prior to the test


c) Patient should abstain from smoking during the test.

d) Strenuous exercise on the previous day is to be avoided.

e) The exercise is also to be avoided on the same day prior to the test


Standard Oral glucose tolerance test

I/V Glucose tolerance test

Mini Glucose tolerance test


a) At about 8 a.m. the fasting blood and urine samples are collected. These are called zero samples.

b) A loading dose of 75 g. anhydrous glucose dissolved in 250-300 ml of water is given to the patient.


• In children 1.75 g of glucose /kg body weight is given.

• In the classical procedures, the blood and urine samples are collected at half hourly interval of the next two and a half hour or three hours.

• Glucose is estimated in all the blood samples.

• Urine is analyzed for the presence of glucose.


A curve is plotted with the blood glucose levels on the vertical axis against the time of collection on the horizontal axis.

The curve so obtained is called glucose tolerance curve.


Fasting (Zero sample)

30 minutes

60 minutes

90 minutes

120 minutes

150 minutes

180 minutes

Blood Glucose(mg/dl)

90 100 150 120 110 80 70

Urinary Glucose

nil nil nil nil nil nil nil



i) Fasting blood glucose (Zero hour sample)- is 90 mg /dl, which is well within the normal range(Normal 60-100 mg/dl).

ii) There is rise of blood glucose after glucose load and the peak value is observed at I hour. This is due to absorption of glucose from the intestine.


iii) Insulin is released upon increase of blood glucose level. There is fall in blood glucose with time due to glucose utilization promoted by insulin.

iv) The normal blood glucose level is achieved after 150 minutes.


1) Fasting blood glucose is higher than normal

2) The highest value is attained at 1 hour to 1 hour 30 minutes.

3) The highest value exceeds the renal threshold

4) Glucose is found in almost all the urine samples.

5) The blood glucose level does not return to the fasting level even within 2hour 30 minutes.



30 minutes

60 minutes

90 minutes

120 minutes

150 minutes

180 minutes


200 225 350 300 275 250 225

Urinary Glucose

+ + + + + + +


Time in minutes

Blo

od g

luco

se (

mg/

Dl)


Blood glucose levels are within the normal limits.

Glucose tolerance curve is normal.There is lowering of renal threshold.Thus glucose is found in some of the

samples depending upon the renal threshold.


Early diabetes mellitus,Pregnancy, Renal disease, Heavy metal poisoning Deficiency of carrier protein (SGLT-

2).Renal glycosuria can also be

observed in children of diabetic parents.



30 minutes

60 minutes

90 minutes

120 minutes

150 minutes

180 minutes


90 130 150 140 120 100 90

Urinary Glucose

nil + + + + ± nil


Time in minutes

Blo

od g

luco

se (

mg/

Dl)


Fasting blood glucose is normal.Sharp rise within 30 minutes to one

hourThe blood glucose levels exceed the

renal threshold.The decline is rapid and the normal

levels are attained back. Some of the urine samples contain

glucose, where the blood glucose is above the renal threshold.


HyperthyroidismPregnancyAfter gastro-enterostomyEarly diabetes mellitus



30 minutes

60 minutes

90 minutes

120 minutes

150 minutes

180 minutes


90 230 180 150 120 100 90

Urinary Glucose

nil + + nil nil nil nil


•This test is undertaken for patients with malabsorption (Celiac disease or enteropathies),•Under these conditions oral glucose load is not well absorbed and• the results of oral glucose tolerance test become inconclusive.


• I/V glucose tolerance test is carried out by giving 25 g of glucose dissolved in 100 ml distilled water as intravenous injection within 5 minutes.

• Completion of infusion is taken as 0 time.

• Blood samples are taken at 10 minutes interval for the next hour.

• The peak value is reached within a few minutes and the value touches to near normal in 45-60 minutes.


Interpretation• In normal individuals, blood glucose

level returns to normal within 60 minutes

• In diabetes mellitus, decline is slow• The initial values are attained in 120

minutes.


a) Acute infections- Cortisol is secreted, the curve is elevated and prolonged

b) Liver diseases- The curve is elevated and prolonged.

c) Hyperthyroidism- There is steep rise in curve.


d) Hypothyroidism-A flat curve is obtained in hypothyroidism. Thyroid hormone increases the absorption of glucose from the gut.

e) Starvation- There is rise of counter regulatory hormones, which show increased glucose tolerance


As per current WHO recommendations, in the mini or modern glucose tolerance test, only two samples are collected,

Fasting (zero hour) and 2 hour post glucose load.

Urine samples are also collected during the same time.

The diagnosis is made from the variations observed in these results.


Time of sample collection

Normal person

Criteria for diagnosing diabetes mellitus

Criteria for diagnosing IGT

Fasting

<110 mg/dl <(6.1m.mol/L)

> 126 mg/dl >(7.0m.mol/L)

110- 126 mg/dl

2 hour after glucose load

<140 mg/dl <(7.8mmol/L)

> 200 mg/dl

140-199 mg/dl


For proper evaluation of the test, the subjects should be normally active and free from acute illness.

Medications that may impair glucose tolerance include diuretics, contraceptive drugs, glucocorticoids, niacin, and phenytoin should be avoided on that day.


Cortisone stress test- used for detecting pre diabetes or Latent diabetes

Extended GTT- To diagnose the cause of hypoglycemia especially 2-3 hours after meals.


If the fasting plasma glucose level is 126 mg/dL or higher on more than one occasion, further evaluation of the patient with a glucose challenge is unnecessary. However, when fasting plasma glucose is less than 126 mg/dL in suspected cases, a standardized oral glucose tolerance test may be done .

04/13/23 Biochemistry for medics- Lecture notes 37

A random plasma glucose concentration >11.1 mmol/L (200 mg/dL) accompanied by classic symptoms of DM (polyuria, polydipsia, weight loss) is sufficient for the diagnosis of DM.

04/13/23 Biochemistry for medics- Lecture notes 38

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Glucose Tolerance Test