The glucose tolerance test (GTT) is a diagnostic procedure used to assess how well the body processes glucose and is particularly important for diagnosing diabetes mellitus. The test involves measuring blood glucose levels after fasting and following a glucose load, with interpretations indicating normal, impaired glucose tolerance, or diabetes based on fasting and post-load values. Various interpretations of the GTT curves help identify different conditions such as renal glycosuria or flat curves affecting glucose metabolism.

1. GLUCOSE TOLERANCE TEST

2. INTRODUCTION
 Blood glucose concentration in a normal adult
remains in the range of 60-100 mg/100 ml.
 When the value exceeds, it is called hyperglycemia.
 Persistent hyperglycemia may be a result of
diabetes mellitus, which is detected by estimating
blood glucose concentration during fasting.
 To confirm the diagnosis, fasting and postprandial
(after meals) samples are analyzed.
 Sometimes, it may also be essential to perform
glucose tolerance test (GTT).

3. ESTIMATION OF FASTING AND POSTPRANDIAL
BLOOD GLUCOSE LEVELS
 International Committee of Diabetologists has recommended
the estimation of blood glucose in the overnight fasting and
postprandial samples (2 hours after 75 g of a glucose load) to
confirm the diagnosis of diabetes mellitus
 Accordingly, an individual is said to be normal if fasting blood
glucose value is less than 100 mg/100 ml and the 2-hours'
value is below 140 mg/100 ml.
 A fasting value of 126 mg/100 ml or higher and the 2 hours'
value of more than 200 mg/100 ml is diagnostic of diabetes
mellitus.
 Fasting value between 100-125 mg/100 ml and/or the 2 hours'
value between 140-200 mg/100 ml is referred to as impaired
glucose tolerance

4. GLUCOSE TOLERANCE TEST
 Glucose tolerance test (GTT) is performed by giving
a load of glucose, orally.
 It is helpful in confirming the diagnosis of diabetes
in those patients whose fasting blood glucose
concentration is in the upper normal limit, with
transient or sustained glycosuria without the
symptoms of diabetes or with family history of the
disease.

5. WHAT IS IT
 The glucose tolerance test is a lab test to check
how your body moves sugar from the blood into
tissues like muscle and fat.
 The test is often used to diagnose diabetes. Tests
to screen for diabetes during pregnancy are similar.

7. PROCEDURE
 For GTT, fasting venous blood sample (after 12-16
hours of fast) is drawn, in a fluoride-oxalate tube
 Thereafter, 75 g of glucose (about 1 g/kg body
weight) is given in nearly 200-300 ml of water.
 Blood samples are again drawn after every 30
minutes, till 2 to 2½ hours.
 Urine specimen may also be collected
simultaneously.
 Each blood sample is analyzed and glucose
tolerance curve is plotted

9. INTERPRETATION
 Fasting blood glucose concentration, maximum blood glucose
value after a glucose load, and the rate at which it returns to
the fasting, gives an idea about the disease.
 For a normal individual, fasting blood glucose value should be
within the normal limit.
 It is increased by about 50% within 60 minutes and thereafter
returns towards fasting after two hours.
 Any deviation from the above is referred to as an abnormal
glucose tolerance curve.
 In a patient with diabetes mellitus, fasting blood glucose level is
above normal.
 In addition, the rise in blood glucose with time may be higher and
the level may not return towards fasting.
 In addition, glucose may be present in urine
 If glucose tolerance curve is normal but there is glycosuria, it is
referred to as renal glycosuria.

10. NORMAL CURVE
 The fasting level is within the normal limits (<100
mg%).
 The peak is reached at 1 hour and it is below the
renal threshold level of 180 mg%.
 However it is found to be usually below 150 mg%. It
returns to the fasting levels by 2 hours. Sometimes
there is a greater decrease at 2 hours. than the
original fasting level (dip), which usually returns to
fasting values by 2½ hours.
 All urine samples are negative for glucose innormal
response.

11. IMPAIRED GLUCOSE TOLERANCE CURVE (IGT)
 The fasting level is between 100-126 mg%, peak
value may be more than the renal threshold value
and the 2 hours values remain any where between
140-200 mg%. There are traces of glycosuria at the
peak and 2 hours samples.

12. DIABETES MELLITUS CURVE (DM)
 This shows the maximum intolerance.
 The fasting levels are more than 126 mg%, the level
steadily rises to significantly higher values than renal
threshold level, reaching a peak at 1 hour and then
gradually declining slowly, but the 2 hour level is always
more than 200 mg%.
 The values may be much higher, indicating a severe
diabetes mellitus.
 Clearly there is glycosuria in most of the samples of urine.
 Levels are usually above 200 mg% in newly diagnosed or
uncontrolled diabetics.

13. LAG CURVE OR ALIMENTARY GLYCOSURIA
 In this, the GTT curve is quite normal, there is an
abnormally high level at the peak time except that
of 1 hour accompanied by transient glycosuria
during this time. The fasting level, the return of
the peak and 2hour glucose level are well within
the normal limits.
 This is called the lag curve, on the assumption that
there is lag or delay in insulin secretion.
Consequently, there is an abnormally high peak.
The condition is benign and does not warrant any
treatment.

14. RENAL GLYCOSURIA CURVE
 Both lowered renal threshold level, as well as increased
renal threshold level are seen. The latter is seen in elderly
patients and also in many diabetics.
 (i) Lowered renal threshold: The GTT curve is very similar
to a normal curve except that the peak value is a little
lower than that in a normal curve. But there is transient
glycosuria at this lowered peak, which is the renal threshold
level for these subjects.
 This is usually due to reabsorption defect in renal tubules
so that an appreciable amount of glucose appears in the
urine. This condition is harmless and does not progress to
become diabetes mellitus. This is also sometimes seen in
later stages of pregnancy.

15.  (ii) Raised renal threshold:
 Here the glycosuria does not occur even at
significantly higher levels than the normal renal
threshold of 160-180 mg%.
 This is seen in elderly people and in many
diabetics. Probably the glomerular arteriosclerosis
leading to reduced GFR is responsible.
 At reduced glucose concentration may be reached
before GFR, a higher the tubular reabsorptive
capacity of glucose is saturated.
 Blood glucose levels of 200 mg% or more have
been found without any glycosuria in the elderly.

16. FLAT CURVE-INCREASED GLUCOSE TOLERANCE
 These patients show very little effect of glucose load
on their fasting level, which is well within the normal
limits.
 The peak values are also relatively very low and
there is no glycosuria at all.
 Such a curve is referred to as flat curve. For example
glucose levels at half hourly intervals, starting from
fasting level, could be 75, 90, 115, 95, 85, 80
(mg%).This is usually seen in hypoactivity of endocrinal
glands e.g. hypothyroidism, hypoadrenalism (Addison's
disease), hypopituitarism (Simmond's disease) etc.
 Steatorrhoea as a result of impaired absorption also
leads to flat curves.

17. GTT IN GESTATIONAL DIABETES MELLITUS
 GTT is done in pregnant women above 25 age with
risk factors like history of diabetes in the years of
previous pregnancy or in the family, or advanced
maternal age.
 75g of glucose load is given.
 Hourly samples are collected for 2 hours.
 The accepted upper normal limits are 125 mg% (F);
180 mg% (1 hour.); 199 mg% (2 hours) and 145
mg% (3 hours). At least two values should exceed
the above values.

18. INTRAVENOUS GTT (IV-GTT)
 This is done in situations where oral administration is not
feasible e.g. (a) some patients can't tolerate oral glucose
load, or (b) malabsorption syndrome of the intestine or (c)
partial gastrectomy etc. where glucose absoption is
inadequate.
 Glucose load at the rate of 0.5 g/kg body weight is given as
a 25 g/dL solution intravenuously.
 Blood samples are collected at 30, 60, 90 and 120 minutes
after infusion.
 Peak levels are reached at 30 minutes and it returns to
basal level in 60 minutes in a normal individual.
 In diabetics, basal level takes longer, usually 2 hours or
more.

19. EXTENDED GTT
 In this form, the test is extended upto 4-5 hours instead of
the usual 2½ hours.
 It is usually done in cases characterised by post prandial
hypoglycemia which is usually transient and is relieved by
taking glucose.
 It is commonly seen in patients who have undergone partial
gastrectomy or with B-cell islet tumor called insulinoma.
 Rapid emptying of the stomach and excessive insulin
secretion is the cause of hypoglycemia which is not
significantly low, but is usually accompanied by symptoms.