Special Blood Tests

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 1.

Diabetes mellitus.
Hyperactivity of thyroid, pituitary and adrenals.
Injection of cortisone like hormone.
Increased secretion of the growth hormone.
Very severe liver disease.
Glycogen storage disease of the liver due to the limited capacity of a person to store excess glycogen.
Severe infection of staphylococcal bacilli or even common cold.

Time	Fasting	1/2 hour	1 hour	1 1/2 hours	2 hours	2 1/2 hours
		After taking glucose
3 Blood Glucose gm/dl	70	130	145	105	75	80
Urine Glucose – Absent throughout the test
Normal renal threshold for glucose = 150 – 170 mg/dl.

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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