Glucose Tolerance Tests

Glucose Tolerance Tests

Synonym/acronym: Standard oral tolerance test, standard gestational screen, standard gestational tolerance test, GTT.

Common use

To evaluate blood glucose levels to assist in diagnosing diseases such as diabetes.


Plasma (1 mL) collected in a gray-top (sodium fluoride) tube. Serum (1 mL) collected in a gold-, red-, or red/gray-top tube or plasma collected in a green-top (heparin) tube is also acceptable, but plasma is recommended for diagnosis. It is important to use the same type of collection container throughout the entire test.

Normal findings

(Method: Spectrophotometry)
Conventional UnitsSI Units (Conventional Units × 0.0555)
Standard Oral Glucose Tolerance (Up to 75-g Glucose Load)
Fasting sampleLess than 100 mg/dLLess than 5.6 mmol/L
Prediabetes or impaired fasting sample100–125 mg/dL5.6–6.9 mmol/L
2-hr sampleLess than 200 mg/dLLess than 11.1 mmol/L
Prediabetes or impaired 2-hr sample140–199 mg/dL7.8–11 mmol/L
Tolerance Tests for Gestational Diabetes
ACOG Standard gestational screen (50-g glucose load)Less than 141 mg/dLLess than 7.8 mmol/L
Standard gestational toleranceADA Threshold Recommendations for Gestational Diabetes (2012) (75-g glucose load)ACOG Threshold Recommendations for Gestational Diabetes (2011); either Carpenter and Coustan or National Diabetes Data Group (100-g glucose load)
Carpenter and CoustanNational Diabetes Data Group
Fasting sample Less than 93 mg/dL (SI: Less than 5.2 mmol/L)Less than 95 mg/dL (SI: Less than 5.3 mmol/L)Less than 105 mg/dL (SI: Less than 5.8 mmol/L)
 1-hr sampleLess than 181 mg/dL (SI: Less than 10 mmol/L)Less 180 mg/dL (SI: Less than 10 mmol/L)Less than 190 mg/dL (SI: Less than 10.5 mmol/L)
 2-hr sampleLess than 154 mg/dL (SI: Less than 8.5 mmol/L)Less than 155 mg/dL (SI: Less than 8.6 mmol/L)Less than 165 mg/dL (SI: Less than 9.2 mmol/L)
 3-hr sampleN/ALess than 140 mg/dL (SI: Less than 7.8 mmol/L)Less than 145 mg/dL (SI: Less than 8 mmol/L)
Plasma glucose values are reported to be 10% to 20% higher than serum values. According to recommendations of the ADA, the diagnosis of gestational diabetes is made if any of the four thresholds are met or exceeded. According to recommendations of the ACOG or National Diabetes Data Group, the diagnosis of gestational diabetes is made if any two of the four thresholds are met or exceeded. ACOG = American Congress of Obstetricians and Gynecologists; ADA = American Diabetes Association.


The glucose tolerance test (GTT) measures glucose levels after administration of an oral or IV carbohydrate challenge. Patients with diabetes are unable to metabolize glucose at a normal rate. The oral GTT is used for individuals who are able to eat and who are not known to have problems with gastrointestinal malabsorption. The IV GTT is used for individuals who are unable to tolerate oral glucose.

Diabetes is a group of diseases characterized by hyperglycemia or elevated glucose levels. Hyperglycemia results from a defect in insulin secretion (type 1 diabetes), a defect in insulin action, or a combination of dysfunction secretion and action (type 2 diabetes). The chronic hyperglycemia of diabetes over time results in damage, dysfunction, and eventually failure of the eyes, kidneys, nerves, heart, and blood vessels. The American Diabetes Association and National Institute of Diabetes and Digestive and Kidney Disease have established criteria for diagnosing diabetes to include any combination of the following findings or confirmation of any of the individual findings by repetition on a subsequent day:

  • Symptoms of diabetes (e.g., polyuria, polydipsia, and unexplained weight loss) in addition to a random glucose level greater than 200 mg/dL
  • Fasting blood glucose greater than 126 mg/dL after a minimum of an 8-hr fast
  • Glucose level greater than 200 mg/dL 2 hr after glucose challenge with standardized 75-mg load

The American Congress of Obstetricians and Gynecologists (ACOG) recommends screening for all pregnant women at 24 to 28 wk of gestation using patient history, clinical risk factors, or carbohydrate challenge testing. Protocol recommendations may vary among requesting health-care providers (HCPs). The ADA and International Association of Diabetes and Pregnancy Study Groups recommend that all women not previously diagnosed with diabetes undergo a 75-g OGTT at 24–28 wk of gestation because unrecognized glucose intolerance may have existed prior to the pregnancy, glucose intolerance identified during pregnancy may have continued unmonitored after pregnancy, and the frequency of diabetes in women of childbearing age has dramatically increased. The ADA also recommends a diagnosis of overt, rather than gestational, diabetes if test results meet the criteria for diabetes at the initial prenatal visit.

This procedure is contraindicated for



  • Evaluate abnormal fasting or postprandial blood glucose levels that do not clearly indicate diabetes
  • Evaluate glucose metabolism in women of childbearing age, especially women who are pregnant and have (1) a history of previous fetal loss or birth of infants weighing 9 lb or more and/or (2) a family history of diabetes
  • Identify abnormal renal tubular function if glycosuria occurs without hyperglycemia
  • Identify impaired glucose metabolism without overt diabetes
  • Support the diagnosis of hyperthyroidism and alcoholic liver disease, which are characterized by a sharp rise in blood glucose followed by a decline to subnormal levels

Potential diagnosis

Tolerance Increased in

Decreased absorption of glucose:
  • Adrenal insufficiency (Addison’s disease, hypopituitarism)
  • Hypothyroidism
  • Intestinal diseases, such as celiac disease and tropical sprue
  • Whipple’s disease
Increased insulin secretion:
  • Pancreatic islet cell tumor

Tolerance Impaired in

Increased absorption of glucose:
  • Excessive intake of glucose
  • Gastrectomy
  • Gastroenterostomy
  • Hyperthyroidism
  • Vagotomy
Decreased usage of glucose:
  • Central nervous system lesions
  • Cushing’s syndrome
  • Diabetes
  • Hemochromatosis
  • Hyperlipidemia
Decreased glycogenesis:
  • Hyperthyroidism
  • Infections
  • Liver disease (severe)
  • Pheochromocytoma
  • Pregnancy
  • Stress
  • Von Gierke disease

Critical findings


  • Adults & Children
  • Less than 40 mg/dL (SI: Less than 2.22 mmol/L)
  • Greater than 400 mg/dL (SI: Greater than 22.2 mmol/L)
  • Consideration may be given to verifying the critical findings before action is taken. Policies vary among facilities and may include requesting immediate recollection and retesting by the laboratory or retesting using a rapid Point of Care instrument at the bedside.

  • Note and immediately report to the HCP any critically increased or decreased values and related symptoms.

  • It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • Symptoms of decreased glucose levels include headache, confusion, hunger, irritability, nervousness, restlessness, sweating, and weakness. Possible interventions include oral or IV administration of glucose, IV or intramuscular injection of glucagon, and continuous glucose monitoring.

  • Symptoms of elevated glucose levels include abdominal pain, fatigue, muscle cramps, nausea, vomiting, polyuria, and thirst. Possible interventions include subcutaneous or IV injection of insulin with continuous glucose monitoring.

Interfering factors

  • Drugs and substances that may increase GTT values include acetylsalicylic acid, atenolol, bendroflumethiazide, caffeine, clofibrate, fenfluramine, fluoxymesterone, glyburide, guanethidine, lisinopril, methandrostenolone, metoprolol, nandrolone, niceritrol, nifedipine, nitrendipine, norethisterone, phenformin, phenobarbital, prazosin, and terazosin.
  • Drugs and substances that may decrease GTT values include acebutolol, beclomethasone, bendroflumethiazide, betamethasone, calcitonin, catecholamines, chlorothiazide, chlorpromazine, chlorthalidone, cimetidine, corticotropin, cortisone, danazol, deflazacort, dexamethasone, diapamide, diethylstilbestrol, ethacrynic acid, fludrocortisone, furosemide, glucagon, glucocorticosteroids, heroin, hydrochlorothiazide, mephenytoin, mestranol, methadone, methandrostenolone, methylprednisolone, muzolimine, niacin, nifedipine, norethindrone, norethynodrel, oral contraceptives, paramethasone, perphenazine, phenolphthalein, phenothiazine, phenytoin, pindolol, prednisolone, prednisone, propranolol, quinethazone, thiazides, triamcinolone, triamterene, and verapamil.
  • The test should be performed on ambulatory patients. Impaired physical activity can lead to falsely increased values.
  • Excessive physical activity before or during the test can lead to falsely decreased values.
  • Failure of the patient to ingest a diet with sufficient carbohydrate content (e.g., 150 g/day) for at least 3 days before the test can result in falsely decreased values.
  • The patient may have difficulty drinking the extremely sweet glucose beverage and become nauseous. Vomiting during the course of the test will cause the test to be canceled.
  • Smoking before or during the test can lead to falsely increased values.
  • The patient should not be under recent or current physiological stress during the test. If the patient has had recent surgery (less than 2 wk previously), an infectious disease, or a major illness (e.g., myocardial infarction), the test should be delayed or rescheduled.
  • Failure to follow dietary restrictions before the procedure may cause the procedure to be canceled or repeated.

Nursing Implications and Procedure

Potential nursing problems

ProblemSigns & SymptomsInterventions
Blood glucose (Related to sedentary lifestyle, circulating insulin deficiency secondary to pancreatic insufficiency; excessive dietary intake; insulin resistance; pregnancy)Excess: Fatigue; mild dehydration; elevated blood glucose; weight loss; weakness; polyuria; polydipsia; polyphagia; blurred vision; headache; paresthesia; poor skin turgor; dry mouth; nausea; vomiting; abdominal pain; Kussmaul respirations. Deficit: tremor, sweating, decreased concentration; diaphoresis; elevated blood pressure; palpitations; headache; hunger; restlessness; lethargy; altered mental status; combativeness; altered speech; altered coordinationCheck blood glucose before meals and at bedtime; administer prescribed insulin or oral agents; educate and encourage the patient to participate in glucose self-check and record results; assess readiness to learn and barriers to learning; collaborate with the health-care provider and dietician to support medical nutritional therapy; refer to dietician to assist the patient to select appropriate cultural foods; develop a plan of exercise commensurate with the patient’s physical abilities; discuss lifestyle alterations necessary to support positive health management secondary to disease process; teach good hygiene and infection prevention; monitor laboratory studies that may be impacted by altered glucose and trend results (HGB A1C; BUN; Cr; electrolytes; arterial pH; magnesium; urine ketones; urine microalbumin; WBC; amylase; HGB/HCT; WBC; C-reactive protein; liver enzymes); facilitate oral hydration; correlate blood glucose with other laboratory values and medical condition(s); address the psychosocial aspects of the disease; monitor serum insulin levels
Nutrition (Related to excessive dietary intake more than body requirements, insulin deficiency, stress; anxiety; depression; cultural lifestyle; unhealthy food sources; financial restrictions)Increased thirst, increased urination, weight loss; fatigue; elevated blood glucose levels; inadequate glucose management; increased hungerMonitor blood glucose results, refer to dietician for evaluation, administer insulin or oral agent; assess the cultural aspects of diet selection; correlate dietary intake with blood glucose and monitor trends; collaborate with a dietician to develop a cultural- and age-appropriate diet plan; correlate nutritional intake and exercise; ensure that the patient understands the relationship between caloric intake and medication (insulin, oral agent); refer to social services, dietitian as necessary
Infection risk (Related to altered blood glucose; exposure to opportunistic hosts; poor personal hygiene; broken skin; wound presence)Increased temperature; increased heart rate and respiratory rate; chills; change in mental status; fatigue; malaise; weakness; anorexia; headache; nausea; elevated blood glucose; hypotension; diminished oxygen saturation; elevated WBC; elevated C-reactive proteinProvide standard precautions in the provision of care; correlate symptoms with laboratory values and disease process; trend vital signs and laboratory values to monitor for improvement; administer prescribed antibiotics and medications for fever reduction; provide cooling measures; perform vigilant hand hygiene; educate patient and family regarding good hand hygiene; infuse ordered IV fluids to support adequate hydration; ensure implementation of infection prevention measures with consideration of age and culture such as adequate nutrition; provide aseptic wound care; ensure skin care; ensure oral care; ensure adequate rest; instruct patient to avoid exposure to opportunistic hosts; send cultures to the laboratory as ordered; correlate culture findings with selected antibiotics
Noncompliance (Related to refusal to accept new diagnosis; financial instability; cultural norms; complexity of the medical management; lack of knowledge)Insufficient disease management; alterations in blood glucose; poor self-management of medication administration; lack of supplies to support self-management; poor dietary control with inappropriate food selectionsAssess the patient’s ability to and prior efforts to manage the disease process; evaluate the ability to self-manage the disease including blood glucose screening, dietary management, exercise, and medication self-administration; assess for personal factors that may limit the patient’s ability to self-perform such as visual, cognitive, and hearing; assess the financial ability to purchase the medication and supplies necessary to provide self-care; assess the level of family support; ensure the patient has the adequate knowledge to perform self-care and if not provide the necessary training; ensure the patient knows the signs and symptoms related to the disease process; teach correct dietary selections meeting cultural- and age-appropriate needs; refer to social services or home health; discuss how to manage diabetes during travel


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in evaluating blood sugar levels.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s endocrine system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Inform the patient that multiple specimens may be required. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Instruct the patient to fast for at least 8 to 16 hr before the standard oral and standard gestational GTTs.
  • Note that there are no fluid or medication restrictions unless by medical direction prior to the gestational screen.


  • Potential complications:
  • Note that the patient may have difficulty drinking the extremely sweet glucose beverage and become nauseous. Vomiting during the course of the test will cause the test to be canceled.

  • Ensure that the patient has complied with dietary and activity restrictions as well as other pretesting preparations; ensure that food has been restricted for at least 8 to 12 hr prior to the procedure.
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis. Do not wait until all specimens have been collected to transport.
  • Standard Oral GTT

  • The standard oral GTT takes 2 hr. A fasting blood glucose is determined before administration of an oral glucose load. If the fasting blood glucose is less than 126 mg/dL, the patient is given an oral glucose load.
  • An oral glucose load should not be administered before the value of the fasting specimen has been received. If the fasting blood glucose is greater than 126 mg/dL, the standard glucose load is not administered and the test is canceled. The laboratory will follow its protocol as far as notifying the patient of his or her glucose level and the reason why the test was canceled. The requesting HCP will then be issued a report indicating the glucose level and the cancellation of the test. A fasting glucose greater than 126 mg/dL indicates diabetes; therefore, the glucose load would never be administered before allowing the requesting HCP to evaluate the clinical situation.
  • Adults receive 75 g and children receive 1.75 g/kg ideal weight, not to exceed 75 g. The glucose load should be consumed within 5 min, and time 0 begins as soon as the patient begins to ingest the glucose load. A second specimen is collected at 2 hr, concluding the test. The test is discontinued if the patient vomits before the second specimen has been collected.
  • Standard Gestational Screen

  • The standard gestational screen is performed on pregnant women. If results from the screen are abnormal, a full gestational GTT is performed. The gestational screen does not require a fast. The patient is given a 50-g oral glucose load. The glucose load should be consumed within 5 min, and time 0 begins as soon as the patient begins to ingest the glucose load. A specimen is collected 1 hr after ingestion. The test is discontinued if the patient vomits before the 1-hr specimen has been collected. If the result is normal, the test may be repeated between 24 and 28 wk gestation.
  • Standard Gestational GTT

  • The standard gestational GTT takes 3 hr. A fasting blood glucose is determined before administration of a 75-g or 100-g oral glucose load, depending on the order. If the fasting blood glucose is less than 126 mg/dL, the patient is given an oral glucose load.
  • An oral glucose load should not be administered before the value of the fasting specimen has been received. If the fasting blood glucose is greater than 126 mg/dL, the Glucola is not administered and the test is canceled (see previous explanation).
  • The glucose load should be consumed within 5 min, and time 0 begins as soon as the patient begins to ingest the glucose load. Subsequent specimens are collected at 1, 2, and 3 hr, concluding the test. The test is discontinued if the patient vomits before all specimens have been collected.


  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Instruct the patient to resume usual diet and activity, as directed by the HCP.
  • Nutritional Considerations: Impaired glucose tolerance may be associated with diabetes. There is no “diabetic diet”; however, many meal-planning approaches with nutritional goals are endorsed by the American Dietetic Association. Patients who adhere to dietary recommendations report a better general feeling of health, better weight management, greater control of glucose and lipid values, and improved use of insulin. Instruct the patient, as appropriate, in nutritional management of diabetes. The 2013 Guideline on Lifestyle Management to Reduce Cardiovascular Risk published by the American College of Cardiology (ACC) and the American Heart Association (AHA) in conjunction with the National Heart, Lung, and Blood Institute (NHLBI) recommends a “Mediterranean”-style diet rather than a low-fat diet. The new guideline emphasizes inclusion of vegetables, whole grains, fruits, low-fat dairy, nuts, legumes, and nontropical vegetable oils (e.g., olive, canola, peanut, sunflower, flaxseed) along with fish and lean poultry. A similar dietary pattern known as the Dietary Approaches to Stop Hypertension (DASH) diet makes additional recommendations for the reduction of dietary sodium. Both dietary styles emphasize a reduction in consumption of red meats, which are high in saturated fats and cholesterol, and other foods containing sugar, saturated fats, trans fats, and sodium. If triglycerides also are elevated, the patient should be advised to eliminate or reduce alcohol. The nutritional needs of each diabetic patient need to be determined individually (especially during pregnancy) with the appropriate HCPs, particularly professionals trained in nutrition.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.
  • Patient Education

    • Instruct the patient and caregiver to report signs and symptoms of hypoglycemia (weakness, confusion, diaphoresis, rapid pulse) or hyperglycemia (thirst, polyuria, hunger, lethargy).
    • Recognize anxiety related to test results, and be supportive of perceived loss of independence and fear of shortened life expectancy.
    • Discuss the implications of abnormal test results on the patient’s lifestyle.
    • Provide teaching and information regarding the clinical implications of the test results, as appropriate.
    • Emphasize, if indicated, that good glycemic control delays the onset and slows the progression of diabetic retinopathy, nephropathy, and neuropathy. Educate the patient regarding access to counseling services, as appropriate.
    • Provide contact information, if desired, for the American Diabetes Association ( or the AHA (
    • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP.
    • Instruct the patient in the use of home test kits approved by the U.S. Food and Drug Administration, if prescribed. Answer any questions or address any concerns voiced by the patient or family.
    • Teach the pregnant patient that untreated gestational diabetes can increase health risks to self and fetus.
    • Teach the patient that gestational diabetes may resolve after the pregnancy is over but that diabetes may reoccur later in life.
  • Expected Patient Outcomes

    • Knowledge
    • States the signs and symptoms of high and low blood sugar that should be reported to the health-care provider
    • Collaborates with the dietician to formulate a meal plan that will support fetal health and control blood glucose
    • Skills
    • Demonstrates proficiency in self-administration of prescribed medications to control blood glucose during pregnancy correctly
    • Demonstrates proficiency in the ability to perform a self-check glucose accurately
    • Attitude
    • Complies with the recommended medication and dietary plan to control blood glucose during pregnancy
    • Complies with the recommended follow-up glucose studies necessary to manage disease process

Related Monographs

  • Related tests include ACTH, ALP, antibodies gliadin, angiography adrenal, biopsy intestinal, biopsy thyroid, BUN, C-peptide, capsule endoscopy, catecholamines, cholesterol (total and HDL), cortisol, creatinine, DHEAS, fecal fat, fluorescein angiography, folate, fructosamine, fundus photography, gastric acid stimulation, gastrin stimulation, glucagon, glucose, glycated hemoglobin, gonioscopy, 5-HIAA, insulin, insulin antibodies, ketones, metanephrines, microalbumin, oxalate, RAIU, thyroid scan, TSH, thyroxine, triglycerides, VMA, and visual fields test.
  • Refer to the Endocrine System table at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners
References in periodicals archive ?
In our study, impaired oral glucose tolerance tests in lichen planus cases in comparison with general population were not statistically different.
(Glucose tolerance tests the body's ability to handle the rise in blood sugar after a meal.
Oral glucose tolerance tests were conducted at 24 to 28 weeks of gestation.
In its October 2015 recommendation, the USPSTF advocates screening with the hemoglobin Ale, fasting blood glucose, or oral glucose tolerance tests. Any abnormal findings should be confirmed with repeated testing using the same test on a different day.
First, just as we now have glucose tolerance tests for determining risk of diabetes, a "fructose tolerance test" could employ FGF21 level monitoring to look for a related but subspecified set of disease risks.
At baseline, statin users had higher mean two-hour plasma glucose levels than non-users on oral glucose tolerance tests. Statins were linked to both insulin resistance and reduced insulin secretion.
Based on a pilot study of 10 lean and obese adolescents, sleep duration was the primary predictor of abnormal 90-minute glucose values on oral glucose tolerance tests, Dr.
Participants underwent oral glucose tolerance tests, muscle biopsies, and blood testing for numerous factors.
Hemoglobin A1c has several advantages over traditional fasting plasma glucose and glucose tolerance tests: greater patient convenience, since fasting is not required; improved preanalytical stability; and fewer day to day variations caused by illness and stress.
The authors measured glucose and insulin levels in fasting blood and in blood collected during oral glucose tolerance tests to evaluate the diabetogenic effects of the treatment.
Glucose tolerance tests and fasting glucose measures have long been the standard tests for screening and diagnosis of diabetes mellitus.
Study limitations included the fact that the participants were volunteers and that the researchers used a single glucose tolerance test as their standard rather than two glucose tolerance tests.

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