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Diabetes Mellitus: The Clinical Significance Of The Glucose Tolerance Test (GTT) And Prognosis

Updated on February 17, 2014

Glucose Tolerance Test (GTT)


The Glucose Tolerance Test (GTT)

It is ideal to perform the glucose tolerance test for establishing the diagnosis and assessing the severity of diabetes. Glucose tolerance is indicated in:

  1. Causal or fasting blood glucose in the equivocal range,
  2. Glycosuria in pregnancy
  3. Follow- up of patients with impaired glucose tolerance, and
  4. As part of a planned investigation.

The dose of glucose is 75g for adults and 1.75 g/Kg body weight for children. In normal subjects, the fasting blood sugar level is below 100 mg and the 2- hour value is below 120 mg/dl. The GTT helps to exclude renal glycosuria. The term “steeple curve” or “lag curve” is used if the fasting and 2 hour values are normal, but the intermediate values are high leading to glycosuria. This is generally a benign condition seen after gastrectomy and rarely even in normal subjects.

In subjects having symptoms of diabetes, a single fasting value above 120 mg/dl or a 2 hour plasma glucose, concentration value above 200 mg/dl (after 75g glucose), done after an overnight fast may be taken to be diagnostic. In asymptomatic subjects, at least two abnormal blood glucose values should be insisted upon to confirm the clinical diagnosis, e.g. 1- hour plasma glucose concentration value on a subsequent occasion.

For assessing the therapeutic response, fasting and post glucose or post prandial (1.5 hours after lunch or heavy breakfast) blood glucose is estimated. Any value below 140 mg/dl indicates adequate control. The glucose tolerance test may be abnormal in conditions other than the diabetes mellitus. Some of these conditions are listed below:

  1. Liver diseases like cirrhosis and hepatic failure.
  2. Chronic renal failure
  3. Chronic illnesses, prolonged immobility
  4. Starvation and under nutrition
  5. Acute emotional stress, fever, nausea, strokes, myocardial infarction
  6. Hypokalemia
  7. All other conditions producing secondary diabetes mellitus.

Cortisone-primed GTT: This is employed to unmask diabetes in borderline cases, prednisolone is administered in a dose of 0.4 mg/Kg body weight as two equal doses at midnight and on the morning of the test. This serves to unmask the diabetic state. This test is not universally accepted but in a pre-diabetic where clinical suspicion is high, this may add strength to the physician to adopt prophylactic measures.

Prognosis Of Diabetes Mellitus



Once established, the diabetic state tends to be lifelong. Modern management can achieve euglycemia during most part of the day and thereby confer clinical normalcy and prevent complications. Prognosis in the diabetic depends largely on the presence or absence of chronic vascular complications. Modern treatment has served to prevent or successfully overcome acute complications like ketoacidosis, infections, hypoglycemia and lactic acidosis. The long term complications to the eyes, kidneys, nerves, and blood vessels are mainly a consequence of inadequate diabetic control. These can also be prevented to a greate extent by rigid diet control and regular therapy. Once established, the degenerative complications relentlessly progress and lead to damage of vital organs like the kidney, heart and brain. Death is mainly due to complications like renal failure, myocardial infarction, cerebrovascular accident or acute diabetic complications such as diabetic coma, hypoglycemia or infections. Cumulative morbidity occurs due to blindness, peripheral vascular disease, and peripheral neuropathy which cripple the long term survivors. Proper therapy also helps to arrest the progress of established complications. Cardiovascular complications can be reduced further by elimination of other risk factors like hypertension, cigarette smoking and hyperlipidemia.

© 2014 Funom Theophilus Makama


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