Glucose Tolerance

The National Diabetes Data Group (NDDG) has defined criteria sufficient for the diagnosis of diabetes mellitus. These include any one the following:

  1. Sufficient clinical symptoms (polydipsia, polyuria, ketonuria, weight loss) plus random blood glucose >200 mg/dL.
  2. Elevated FBG >126 mg/dL on more than one occasion.
  3. A 2-hour blood glucose >200 mg/dL during oral GT testing.


These criteria should be reconfirmed by repeat testing on a different day in the absence of unequivocal hyperglycemia and metabolic decompensation.


The GT test, then, is used when diabetes is suspected (retinopathy, neuropathy, diabetic-type renal diseases), but the criteria for the diagnosis cannot be met without the data obtained by the GT test. This test is not part of routine screening for diabetes. The GT test may be used for the following:

  • Patients with a family history of diabetes
  • Patients who are massively obese
  • Patients with a history of recurrent infections
  • Patients with delayed healing of wounds (especially on the lower legs or feet)
  • Women who have a history of stillbirths or delivering large babies
  • Patients who have transient glycosuria or hyperglycemia during pregnancy or following myocardial infarction (MI), surgery, or stress


In the GT test, the patient’s ability to tolerate a standard oral glucose load (75 g) is evaluated by obtaining serum and urine specimens for glucose level determinations before glucose administration and then at 30 minutes, 1 hour, 2 hours, 3 hours, and sometimes 4 hours after the administration. Normally there is a rapid insulin response to the ingestion of a large oral glucose load. This response peaks in 30 to 60 minutes and returns to normal in about 3 hours. Patients with an appropriate insulin response are able to tolerate the dose quite easily, with only a minimal and transient rise in serum glucose levels within 1 to 2 hours after ingestion. Glucose will not spill over into the urine in normal patients.


Patients with diabetes will not be able to tolerate this load. As a result, their serum glucose levels will be greatly elevated from 1 to 5 hours. Also, glucose can be detected in their urine.


Gestational diabetes also can be diagnosed by the GT test. Generally the diagnosis of diabetes can be made if two or more of the results exceed the following:

  • Fasting: 105 mg/dL
  • 1 hour: 190 mg/dL
  • 2 hours: 165 mg/dL
  • 3 hours: 145 mg/dL


The American Diabetes Association recommends that pregnant women who have not previously had an abnormal GT result should be screened between 24 and 28 weeks of gestation with a 50-g dose of glucose. This is called the O’sullivan test. A glucose level of greater than 140 mg/100 mL warrants the GT test.


GI absorption can vary among individuals. For that reason, some centers prefer to administer an intravenous (IV) glucose load rather than depend on gastrointestinal (GI) absorption. Also, occasionally a patient is unable to tolerate the oral glucose load (e.g., patients with prior gastrectomy, short-bowel syndrome, malabsorption). In these instances an intravenous glucose tolerance (IV GT) test can be performed by administering the glucose load intravenously. The values for the IV GT test differ slightly from those of the oral GT test because IV glucose is absorbed faster.


Glucose intolerance also may exist in patients with oversecretion of hormones that have an ancillary affect on glucose, such as patients with Cushing syndrome, pheochromocytoma, acromegaly, aldosteronism, or hyperthyroidism. Patients with chronic renal failure, acute pancreatitis, myxedema, type IV lipoproteinemia, infection, or cirrhosis can also have an abnormal GT test. Certain drugs, as mentioned below, can cause abnormal GT results.


The GT test is also used to evaluate patients with hypoglycemia. This hypoglycemia may occur as late as 5 hours after the initial glucose load.


Glucose Tolerance test cannot be used with patients who have serious concurrent infections or endocrine disorders, because glucose intolerance will be observed even though these patients may not be diabetic. The test cannot be continued if the patient vomit part or all of the glucose meal, which invalidates the test.




Glucose Tolerance Side Effects

Glucose Tolerance test may lead to some side effects to the high glucose levels that occurs from the procedure. These side effects include dizziness, tremors, anxiety, sweating, euphoria, or fainting may occur during testing. If these symptoms occur, a blood specimen is obtained. If the glucose level is too high, the test may need to be stopped and insulin administered.




Causes of Glucose Tolerance False Results

  • Smoking during the testing period stimulates glucose production because of the nicotine.
  • Stress (e.g., from surgery, infection) can increase glucose levels.
  • Exercise during the test can affect glucose levels.
  • Fasting or reduced caloric intake before the GT test can cause glucose intolerance.


Drugs that may cause glucose intolerance include antihypertensives, antiinflammatory drugs, aspirin, beta blockers, furosemide, nicotine, oral contraceptives, phenothiazines, psychiatric drugs, steroids, and thiazide diuretics.




Normal Glucose Tolerance Levels

Normal Levels of glucose are found in patients who are not diabetic. These levels are:


Before Test:  Less than 110 mg/dL (6.1 mmol/L).

After 30 minutes: Less than 200 mg/dL (11.1 mmol/L).

After 1 hour: Less than 200 mg/dL (11.1 mmol/L).

After 2 hours: Less than 140 mg/dL (7.8 mmol/L).

After 3 hours: Less than 115 mg/dL (6.4 mmol/L).

After 4 hours: Less than 115 mg/dL (6.4 mmol/L).




Causes of High Glucose Tolerance Levels

Diabetes mellitus (DM): This disease is defined by glucose intolerance and hyperglycemia. A discussion of the many possible etiologies is beyond the scope of this manual.


Acute stress response: Severe stress, including infection, burns, and surgery, stimulates catecholamine release. This in turn stimulates glucagon secretion, which causes hyperglycemia and glucose intolerance.


Cushing syndrome: Blood cortisol levels are high. This causes hyperglycemia and glucose intolerance.


Pheochromocytoma: Catecholamines stimulate glucagon secretion, which causes hyperglycemia and glucose intolerance.


Chronic renal failure: Glucagon is metabolized by the kidney. With loss of that function, glucagon and glucose levels rise.


Glucagonoma: Glucagon is autonomously secreted, causing hyperglycemia.


Acute pancreatitis: The contents of the pancreatic cells (including glucagon) are spilled into the bloodstream as the cells are injured during the inflammation. The glucagon causes hyperglycemia.


Diuretic therapy: Certain diuretics cause hyperglycemia.


Corticosteroid therapy: Cortisol causes hyperglycemia and glucose intolerance.


Acromegaly: Growth hormone stimulates glucagon, which causes hyperglycemia and glucose intolerance.


Myxedema: Usually these patients have a flat GT curve, but they may have a “diabetic curve.”


Somogyi response to hypoglycemia: This is a reactive hyperglycemia following hypoglycemia that may occur from an exaggerated insulin response to the glucose load.


After gastrectomy: These patients can dump most of the glucose load into the small intestines in just minutes because the normal pylorus is absent. This can cause rapid absorption of glucose into the bloodstream and cause a false elevation in glucose level during the early part of the test.