ORAL GLUCOSE TOLERANCE TEST (OGTT)
Glucose tolerance test is undertaken to evaluate the degree of tolerance to glucose
under standard conditions.
It essentially involves body’s ability to absorb glucose and metabolise it by secreting
adequate insulin. This response is sometimes altered and indicates abnormalities in
carbohydrate metabolism.
GTT can be performed in 2 ways
1) Oral GTT
2) IV GTT
Pre- requisites
1) Patient should be on normal unrestricted diet
2) Test should be preferably performed in morning after an overnight fast.
3) Patient should be relaxed, seated and not involved in physical activity.
4) All medications affecting carbohydrate metabolism should be stopped.
5) If fasting glucose is > 125 mg/ dL, do not perform OGTT
Indications:
1) In diagnosis of Diabetes Mellitus
2) Evaluation of non- diabetic glycosuria
3) Confirmation of gestational diabetes.
4) In patients with unexplained neuropathy, nephropathy, retinopathy with RBS <
140 mg/dL
5) As a part of epidemiological study.
Procedure:
After an overnight fast of 10-16 hrs, fasting blood and urine sample is collected.
A glucose load of 75 g in young adults is given dissolved in 300 ml of water. It is
ingested in about 5 min.
( Load for children= 1.75 / kg body weight )
(Pregnant ladies= 100 g)
Blood and urine samples are then collected after every half hour interval from the
time of taking the glucose load for 2 ½ hrs.
Plot a graph between glucose in mg% and time of sampling.
Normal curves:
Chart Title Fasting level within
160 normal limits
Peak is reached at 1hr. It
140
is below renal threshold.
120 It returns to fasting level
after 2 hours.
100
80
60
40
20
0
0 50 100 150
Diagnostic Criteria For Diabetes According to WHO
Characteristic Fasting glucose (mg%) 2 hr glucose (mg %)
Normal <110 <140
Impaired fasting glycemia >110 and < 126 <140
Impaired glucose tolerance <126 >140
Diabetes Mellitus >126 >200
Different types of GTT Curves
1) Impaired glucose tolerance test: The fasting level is between 100-125 mg%.
Peak value maybe more than the renal threshold value and the 2 hours value
remain between 140-200 mg%.
In impaired GTT , traces of glycosuria at the peak and 2 hr sample.
Chart Title Fasting glucose< 100 mg%
200 Peak at 1 hr < renal
150 threshold
100 After 2 hr level> 200 mg%
50
Renal glycosuria may be
present.
0
0 0.5 1 1.5 2
2) Diabetes Mellitus curve: This shows the maximum tolerance. The fasting
levels are more than 126 mg %. The level steadily rises to.significantly higher
values than the renal threshold level reaching a peak at 1 hr than gradually
decline slowly. But 2 hr level is always more than 200 mg % . Clearly there is
glycosuria in most of the sample of urine
Chart Title
300 Fasting is > 126 mg %
250 Maximum at 1 hr >= 250
200 mg/ dL
Renal glycosuria value >
150
200 mg % after 2 hr.
100
50
0
0 50 100 150
3) Lag curve or Alimentary glycosuria :
In this ,GTT curve is quite normal except that there is an abnormally high level at the
peak time accompanied by transient glycosuria during this period. The fasting
glucose level and return of the peak at 2 hr is well within the normal limits. This is
called lag curve where there is abnormally high peak.
Chart Title In this exaggerated response to
250 glucose loading
200 After 1 hr very high amount of
150 glucose levels ( renal glycosuria
100 present)
50 Reaches normal after 2 hr.
0
0 30 60 90
This is due to partial gastrectomy or hyperthyroidism due to rapid emptying of stomach.
4) Renal glycosuria:
In this type renal threshold level increases and decreases abnormally . This is seen
in elderly patients and also in many diabetics.
Lowered renal threshold – It’s curve is exactly same as normal curve except peak
reached is little lower than in normal curve . There is transient glycosuria at lowered
peak which is renal threshold of this subjects. This is due to reabsorption defect in
renal tubules so that appreciable.
Chart Title
140
120
100
80
60
40
20
0
0 50 100
6)Flat curve
In this patient show little or no effect on glucose loading on blood glucose even after
1 hr the level of plasma glucose remains near fasting values.
This may be due to malabsorption, hypothyroidism, hypopituitarism,
hypoadrenalism, etc.
Chart Title
85
80
75
0 50 100 150
5) Gestational Diabetes:
It is done to rule out gestational Diabetes Mellitus in pregnant women.These values
are taken as upper normal limits.
100g of anhydrous glucose is given.
Time ( hr) Plasma
glucose
Mg/dl
F 105
1 190
2 165
3 145
It is done in high risk pregnant women having
Family history of Diabetes Mellitus
Had Diabetes Mellitus in 1 pregnancy
More maternal age
If any of the 2 values are > the values mentioned here , it is indicative of gestational
Diabetes Mellitus.
