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Glucose Tolerance Test (GTT)
Glucose tolerance means the ability of the
body to utilize glucose in blood circulation. Glucose tolerance is
reduced in diabetes mellitus and in certain endocrine gland
(Endocrine Gland: a ductless body organ which produces hormones that
affect and help control various other organs. There are several
such glands -- thyroid, parathyroid, ovaries, testes, adrenals, pineal,
pituitary and pancreas islet cells) disorders like hyperthyroidism,
hyperpituitarism and hyperadrenalism.
Blood sugar in the case of a normal person
remains fairly constant throughout the day. There is temporary
rise in blood sugar levels after food which depends upon the type of food
consumed. This increase remains up to two to three hours and then returns to
normal.
Diminished glucose tolerance is
observed when the ability of the body to utilize glucose decreases. The rise
in blood sugar or glucose is greater than in a normal person.
Also, a return of blood glucose levels to the normal fasting level is slow.
This is observed in:
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Diabetes mellitus
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Hyperactivity of thyroid,
pituitary and adrenals
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Injection of cortisone like
hormone
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Increased secretion of the growth
hormone
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Very severe liver disease
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Glycogen storage disease of the liver
due to the limited capacity of a person to store excess glycogen.
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Severe infection of staphylococcal
bacilli or even common cold
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Time
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Fasting
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1/2 hour
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1 hour
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1 1/2 hours
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2 hours
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2 1/2 hours
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After taking glucose
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3 Blood Glucose gm/dl
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70
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130
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145
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105
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75
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80
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Urine Glucose ------>
Absent throughout the test <----------
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Normal renal threshold for
glucose = 150 -170 mg/dl.
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Raised renal threshold after ingestion
of glucose up to 250 - 300 mg/dl occurs with increasing age and prolonged
diabetes mellitus.
Lowered renal threshold (130 -150
gm/dl) is observed in abnormality of tubular re-absorption of glucose.
Extended glucose tolerance curve
(instead of two and a half hours goes up to four to five hours) is observed in
insulin secreting tumors of the pancreas. It is also observed in Simmond's
disease which causes hypoglycemia (means reduction of sugar or glucose
levels. It drops to below 60 mg/dl and affects brain cells).
Determination Of Glycosylated Hemoglobin
The red blood cells of a normal
person and children above six months old contain three genetically
determined hemoglobin species HbA - 90%, HbA2 - 2.5% and HbF - 0.5% of total
hemoglobin content of RBC. Besides these hemoglobins, other variants of
hemoglobin present are HbAla-1.6%, HbAlb - 0.8% and HbAlc - 4%. They
are products of non-enzymatic, post-synthetic transformation of HbA.
Their rate of formation depends upon the lifespan of RBCs and its mean
hexose concentration. These are collectively measured as HbA - abc or
HbAl. HbAl indicates the average blood sugar concentration for an
extended time period. It remains unaffected by the short term
fluctuations in blood sugar levels. HbAl levels term fluctuations in blood
sugar level. HbAl levels reflect carbohydrate imbalance better than
fasting glucose concentration or the GTT (Glucose Tolerance Test).
The determination of HbAl is a convenient and suitable test as it helps
evaluate the adequacy of
diabetic control in the prevention of various diabetic complications.
Normal Range: 4 - 7%
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