Type of test

Blood; urine

Normal findings

Plasma test

Fasting: < 110 mg/dL or < 6.1 mmol/L (SI units)

1 hour: < 180 mg/dL or < 10 mmol/L

2 hours: < 140 mg/dL or < 7.8 mmol/L

Urine test : Negative

Test explanation and related physiology

The GTT is used when diabetes is suspected (retinopathy, neuropathy, diabetic-type renal diseases). It is also suggested for the following:

 • Patients with a family history of diabetes

 • Patients who are markedly obese

 • Patients with a history of recurrent infections

 • Patients with delayed healing of wounds

• Women who have a history of delivering large babies, still births, or neonatal births

 • Patients who have transient glycosuria or hyperglycemia during pregnancy or after myocardial infarction, surgery, or stress In the GTT, the patient’s ability to tolerate a standard oral glucose load (usually 75 g of glucose) is evaluated by obtaining plasma and urine specimens for glucose level determinations before glucose administration and then at 1 hour and 2 hours afterward. 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 glucose load quite easily, with only a minimal and transient rise in plasma glucose levels within 1 to 2 hours after ingestion. In normal patients, glucose does not spill over into the urine.

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 (Figure 1). Also, glucose can be detected in their urine. It is important to note that intestinal absorption may vary among individuals. For this reason, some centers prefer the glucose load to be administered intravenously.

Fig1. Glucose tolerance test curve for a diabetic and a prediabetic patient.

The American Diabetes Association recommends that pregnant women who have not previously had an abnormal GTT be tested between 24 and 28 weeks of gestation. A glucose level of more than 180 mg/100 mL 1 hour later is consistent with gestational diabetes.

Glucose intolerance also may exist in patients with oversecretion of hormones that have an ancillary effect on glucose, as in 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 GTT. The GTT also is used to evaluate patients with reactive hypoglycemia. This may occur as late as 5 hours after the initial glucose load.

Contraindications

 • Patients with serious concurrent infections or endocrine dis orders because glucose intolerance will be observed even though these patients may not have diabetes

Potential complications

• Dizziness, tremors, anxiety, sweating, euphoria, or fainting 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.

Interfering factors

 • Smoking during the testing period stimulates glucose production because of the nicotine.

• Stress (e.g., from surgery, infection) can increase levels.

• Exercise during the testing can affect glucose levels.

 • Fasting or reduced caloric intake before GTT can cause glucose intolerance.

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

Procedure and patient care

 Before

* Explain the procedure to the patient.

* Educate the patient about the importance of having adequate food intake with adequate carbohydrates (150 g) for at least 3 days before the test.

* Instruct the patient to fast for 12 hours before the test.

*  Instruct the patient to discontinue drugs (including tobacco) that could interfere with the test results.

* Give the patient written instructions explaining the pretest dietary requirement.

• Obtain the patient’s weight to determine the appropriate glucose loading dose (especially in children).

 During

 • Obtain fasting blood and urine specimens.

 • Administer the prescribed oral glucose solution, usually a 75- to 100-g carbohydrate load.

• Give pediatric patients a carbohydrate load based on their body weight.

* Instruct the patient to ingest the entire glucose load.

* Tell the patient that he or she cannot eat anything until the test is completed. However, encourage the patient to drink water.

*  Inform the patient that tobacco, coffee, and tea are not allowed because they cause physiologic stimulation.

 • Collect a venous blood sample in a gray-top tube at 30 minutes and at hourly periods. Collect urine specimens at hourly periods.

 • Mark on the tubes the time that the specimens are collected.

• Assess the patient for such reactions as dizziness, sweating, weakness, and giddiness. (These are usually transient.)

 • For the IV GTT, administer the glucose load intravenously over 3 to 4 minutes.

 After

• Send all specimens promptly to the laboratory.

 • Allow the patient to eat and drink normally.

• Administer insulin or oral hypoglycemics if ordered.

• Apply pressure to the venipuncture site.

Abnormal findings

- Diabetes mellitus

-Acute stress response

-Cushing syndrome

 -Pheochromocytoma

-Chronic renal failure

 -Glucagonoma

 -Acute pancreatitis

-Diuretic therapy

 -Corticosteroid therapy

-Acromegaly

-Myxedema

-Somogyi response to hypoglycemia

 -Postgastrectomy

مواضيع ذات صلة

Immunoglobulins

Complement

Ceruloplasmin

glucose, postprandial (2-Hour postprandial glucose [2-Hour PPG], 1-Hour glucose screen)

α1-antitrypsin

Albumin

Diagnostic Laboratory Tests for Mycobacterium Tuberculosis

Glucose, blood (Blood sugar, Fasting blood sugar [FBS])

Gliadin, endomysial, and tissue transglutaminase antibodies

Antimicrobial Susceptibility Testing and Therapy of Staphylococcus, Micrococcus, and Similar Organisms

Glucagon

genetic testing (Breast cancer [BRCA] and ovarian cancer, Colon cancer, Cardiovascular disease, Tay–Sachs disease, Cystic fibrosis, Melanoma, Hemochromatosis, Thyroid cancer, Paternity [parentage analysis], Forensic genetic testing)