Glucose Tolerance Tests

Synonym/Acronym

Standard oral tolerance test, standard gestational screen, standard gestational tolerance test, GTT.

Rationale

To evaluate blood glucose levels to assist in diagnosing diabetes.

Patient Preparation

There are no activity or medication restrictions unless by medical direction; additionally, there are no fluid restrictions prior to the gestational screen (unless by medical direction). Instruct the patient to fast for at least 8 hr before the standard oral and standard gestational GTTs and not to consume any caffeinated products or chew any type of gum before specimen collection for the test; these factors are known to elevate glucose levels. Note: The ADA recommends a diet with sufficient carbohydrate content (mixed diet of at least 150 gm carbohydrates/day) be consumed for at least 3 days before the test.

Normal Findings

Method: Spectrophotometry.

Standard Oral Glucose Tolerance (Up to 75-g Glucose Load)
	Conventional Units	SI Units (Conventional Units × 0.0555)
Nondiabetic, fasting sample	Less than 100 mg/dL	Less than 5.6 mmol/L
Nondiabetic, 2-hr sample	Less than 140 mg/dL	Less than 7.8 mmol/L
Prediabetes, fasting sample	100–125 mg/dL	5.6–6.9 mmol/L
Prediabetes, 2-hr sample	140–199 mg/dL	7.8–11 mmol/L

Plasma glucose values are reported to be 10% to 20% higher than serum values. A diagnosis of diabetes is made when the fasting glucose is equal to or greater than 126 mg/dL (7 mmol/L) or the 2-hr sample is equal to or greater than 200 mg/dL (11.1 mmol/L).

Tolerance Tests for Gestational Diabetes
One-Step Approach (75-g Glucose Load)		Conventional and SI Units (SI = Conventional Units × 0.0555)
Fasting sample		Less than 95 mg/dL (SI: Less than 5.3 mmol/L)
1-hr sample		Less than 140 mg/dL (SI: Less than 7.8 mmol/L)
2-hr sample		Less than 120 mg/dL (SI: Less than 6.7 mmol/L)

Plasma glucose values are reported to be 10% to 20% higher than serum values. A diagnosis of gestational diabetes is made when any of the three thresholds are met or exceeded.

Tolerance Tests for Gestational Diabetes
	Two-Step Approach	Conventional and SI Units (SI = Conventional Units × 0.0555)
Step 1	Standard Gestational Screen (50-g Glucose Load)
1-hr sample performed while the patient is not fasting	Less than 141 mg/dL	If results exceed 140 mg/dL, the 100-g GTT should be performed (American College of Obstetricians and Gynecologists [ACOG])
Step 2	Gestational GTT (100-g Glucose Load)	Carpenter and Coustan criteria
Fasting sample		Less than 95 mg/dL (SI: Less than 5.3 mmol/L)
1-hr sample		Less than 180 mg/dL (SI: Less than 10 mmol/L)
2-hr sample		Less than 155 mg/dL (SI: Less than 8.6 mmol/L)
3-hr sample		Less than 140 mg/dL (SI: Less than 7.8 mmol/L)

Plasma glucose values are reported to be 10% to 20% higher than serum values.

GTT = glucose tolerance test.

A diagnosis of gestational diabetes is made when two or more of the four thresholds are met or exceeded.

Critical Findings and Potential Interventions ⬆ ⬇

Glucose

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)

Newborns

Less than 32 mg/dL (SI: Less than 1.8 mmol/L)
Greater than 328 mg/dL (SI: Greater than 18.2 mmol/L)
Timely notification to the requesting health-care provider (HCP) of any critical findings and related symptoms is a role expectation of the professional nurse. A listing of these findings varies among facilities.

Consideration may be given to verification of 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 testing instrument at the bedside, if available.

Symptoms of decreased glucose levels include headache, confusion, polyphagia, irritability, nervousness, restlessness, diaphoresis, 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, polyphagia, and polydipsia. Possible interventions include fluid replacement in addition to subcutaneous or IV injection of insulin with continuous glucose monitoring.

Overview ⬆ ⬇

Study type: Blood collected in a gray- [sodium fluoride] or green-top [heparin] tube; plasma is recommended for diagnosis. Serum collected in a gold-, red-, or red/gray-top tube is also acceptable. It is important to use the same type of collection container throughout the entire test; related body system: Endocrine system.

The 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 glucose tolerance test (OGTT) 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.

Glucose, a simple six-carbon sugar (monosaccharide), enters the diet as part of the sugars sucrose, lactose, and maltose and from the complex polysaccharide, dietary starch. The body acquires most of its energy from the oxidative metabolism of glucose. Excess glucose is stored in the liver or in muscle tissue as glycogen. Glucose levels in plasma (one of the components of blood) are generally 10% to 15% higher than glucose measurements in whole blood (and even more after eating). This is important because home blood glucose meters measure the glucose in whole blood, whereas most laboratory tests measure the glucose in either plasma or serum.

Diabetes is a group of diseases characterized by hyperglycemia, or elevated glucose levels. Hyperglycemia can result from a defect in insulin secretion due to destruction of the beta cells of the pancreas (type 1 diabetes), a defect in insulin action, or a combination of defects in secretion and action (type 2 diabetes), or from a specific cause such as gestational diabetes, neonatal hyperglycemia, cystic fibrosis, post organ transplantation, or hyperglycemia induced by drugs used to treat other medical conditions (e.g., antiretrovirals, corticosteroids). The chronic hyperglycemia of diabetes over time may lead to damage, dysfunction, and eventually failure of the eyes (retinopathy), kidneys (nephropathy), nerves (neuropathy), heart (cardiovascular disease), and blood vessels (micro- and macrovascular conditions). The American Diabetes Association (ADA) and National Institute of Diabetes and Digestive and Kidney Disease (NIDDK) have established criteria for diagnosing diabetes. For additional information regarding screening guidelines and management of diabetes, refer to the study titled “Glucose Core Lab Study.”

The 2019 American College of Cardiology (ACC) and American Heart Association (AHA) guidelines for the prevention of cardiovascular disease and the 2022 American Diabetes Association (ADA) recommendations regarding diabetes self-management education and support suggest these topics be included in the patient-HCP conversation along with assessment of evidence-based risk factors in order to better and more realistically improve diabetes and cardiovascular disease health outcomes. Especially important are patient concerns that result in cost-related medication nonadherence to treatment.

Conditions Associated with the Development of Diabetes

CF related diabetes (CFRD) is a co-morbidity that occurs in about 20% of children and almost half of adults. CF is more common in Caucasians. Annual screening with an oral glucose tolerance test should begin by age 10 in CF patients who have not been previously diagnosed with CFRD. The hemoglobin A_1ctest is not recommended to screen for CFRD. Annual monitoring for complications of CFRD should begin 5 yr after diagnosis.

Screening for hyperglycemia should also be done after organ transplantation when the patient has achieved stable levels of immunosuppressant therapy and is free of an acute infection. The oral glucose tolerance test is the preferred method to diagnose posttransplantation diabetes (PTDM). The hemoglobin A_1ctest is not recommended to screen for PTDM.

The ACOG and ADA recommend screening for all pregnant women at 24 to 28 wk of gestation using patient history, clinical risk factors, and carbohydrate challenge testing. Protocol recommendations may vary among requesting 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 to 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. For more information regarding diagnostic criteria for unrecognized diabetes, refer to the study titled “Glucose Core Lab Study.” There are a number of differences in carbohydrate metabolism between pregnant and nonpregnant women. Glucose levels in the pregnant female with normal glucose metabolism are lower than in the nonpregnant female with normal glucose metabolism related to insulin-independent demand and uptake of glucose by the placenta and the developing fetus, insulin production by the fetal placenta (creating a higher demand for glucose), and the development of maternal insulin resistance induced by diabetogenic pregnancy hormones to meet the increasing demand for glucose by the fetus (e.g., progesterone, estrogens, and human placental lactogen). Hemoglobin A_1c goals are stricter for pregnant females especially in the second and third trimester related to hemodilution and increased RBC turnover, which has the effect of independently decreasing A_1c.

The Centers for Medicare and Medicaid Services (CMS) developed a screening tool in 2017 to assess areas unrelated to health that affect health outcomes which include access to housing, food, transportation, utilities, and interpersonal safety. Guidance for the prevention of diabetes and a related co-morbidity, cardiovascular disease, suggests these topics be included in the patient–HCP conversation along with assessment of evidence-based risk factors in order to better and more realistically improve diabetes related health outcomes. Especially important are patient concerns that result in cost-related medication nonadherence to treatment.

Indications ⬆ ⬇

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 liver disease related to substance use disorder (alcohol), which are characterized by a sharp rise in blood glucose followed by a decline to subnormal levels.

Interfering Factors ⬆ ⬇

Contraindications

N/A

Factors That May Alter the Results of the Study

Drugs and other substances that may increase GTT values include acarabose, acetylsalicylic acid, atenolol, caffeine, clofibrate, glyburide, guanethidine, lisinopril, metoprolol, nifedipine, nitrendipine, norethisterone, phenobarbital, prazosin, and terazosin.
Drugs and other substances that may decrease GTT values include acebutolol, beclomethasone, betamethasone, calcitonin, catecholamines, chlorothiazide, chlorpromazine, chlorthalidone, cimetidine, corticotropin, cortisone, danazol, deflazacort, dexamethasone, diapamide, diethylstilbestrol, ethacrynic acid, fludrocortisone, furosemide, glucagon, glucocorticosteroids, heroin, hydrochlorothiazide, mestranol, methadone, methylprednisolone, niacin, nifedipine, norethindrone, norethynodrel, oral contraceptives, paramethasone, perphenazine, phenolphthalein, phenothiazine, phenytoin, pindolol, prednisolone, prednisone, propranolol, quinethazone, 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 to follow dietary restrictions 8 hr before the fasting test can lead to falsely elevated glucose values.
Failure to ingest a diet with sufficient carbohydrate content (mixed diet of at least 150 gm carbohydrates/day) for at least 3 days before the test; fasting and carbohydrate restriction can falsely increase values.
Smoking before or during the test can lead to falsely increased values.

Other Considerations

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.
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.

Potential Medical Diagnosis: Clinical Significance of Results ⬆ ⬇

Tolerance Increased in

Decreased absorption of glucose:
- Adrenal insufficiency (Addison disease, hypopituitarism)
- Hypothyroidism
- Intestinal diseases, such as celiac disease and tropical sprue
- Whipple 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 syndrome
- Diabetes
- Hemochromatosis
- Hyperlipidemia
Decreased glycogenesis:
- Hyperthyroidism
- Infections
- Liver disease (severe)
- Pheochromocytoma
- Pregnancy
- Stress
- Von Gierke disease

Nursing Implications, Nursing Process, Clinical Judgement ⬆

Before the Study: Planning and Implementation

Teaching the Patient What to Expect

Discuss how this test can assist in evaluating blood sugar levels.
Explain that a blood sample is needed for the test and multiple samples may be required over the course of the test.

Standard OGTT

The standard OGTT takes 2 hr.
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 patient notification of glucose results and the reason the test was canceled.
The requesting HCP will 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.

Gestational Screen or Challenge Test

The gestational screen is performed on pregnant women, usually at their first prenatal visit. If results from the screen are abnormal, a gestational GTT is performed.
The gestational screen does not require fasting.
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 blood 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.

Gestational GTT

The gestational GTT takes 3 hr.
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.

After the Study: Implementation & Evaluation Potential Nursing Actions

Avoiding 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.

Treatment Considerations

Studies comparing risk for developing diabetes demonstrate that certain populations are disproportionately affected by type 2 diabetes and complications of type 2 diabetes as compared to the general population.
Non-modifiable risk factors include age, ethnicity, and family history of diabetes or metabolic syndrome.
The risk of developing type 2 diabetes increases with age. In the United States, affected populations include African Americans, Asian Americans, Hispanics, native Hawaiians and Pacific Islanders, American Indians, and Alaska natives.
Modifiable risk factors include lifestyle choices related to making healthy dietary choices, maintaining a healthy body weight, and meeting recommended levels of physical activity.
Prevention and medical management strategies for patients with type 2 diabetes should be provided through the coordinated efforts of multidisciplinary partners using strategies that are culturally appropriate.
Provide diabetic education and information regarding the clinical implications of the test results, as appropriate. For additional information regarding screening guidelines and management of diabetes, refer to the study titled “Glucose, Core Lab Study.”
Explain to the pregnant patient that untreated gestational diabetes can increase health risks to self and fetus and may resolve after the pregnancy is over but that diabetes may reoccur later in life.
Encourage collaboration between the patient and the HCP regarding the type and frequency of glucose monitoring; the plan that best fits the patient’s needs and that will be the most successful strategy.
Instruct the patient in the use of continuous glucose monitoring systems; home testing strips or meters approved for glucose, ketones, or A_1c by the U.S. Food and Drug Administration, if prescribed.
Answer any questions or address any concerns voiced by the patient or family.
Advise reporting signs and symptoms of hypoglycemia (weakness, confusion, diaphoresis, rapid pulse) or hyperglycemia (thirst, polyuria, hunger, lethargy).
Discuss how good glycemic management delays the onset and slows the progression of diabetic retinopathy, nephropathy, and neuropathy.

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 ADA.
Patients who adhere to dietary recommendations report a better general feeling of health, better weight management, better management of glucose and lipid values, and improved use of insulin.
Discuss the nutritional management of diabetes.
Variety of dietary patterns are beneficial for people with diabetes.
Encourage consultation with a registered dietitian who is a certified diabetes educator.

Clinical Judgement

Consider how to reinforce the importance of good glucose management to overall health and quality of life.

Follow-Up and Desired Outcomes

Adheres to the recommended medication and dietary plan to manage blood glucose during pregnancy.
Adheres to the recommended screening and monitoring regimen for other medical conditions associated with the development of diabetes.
Acknowledges contact information provided for the ADA (www.diabetes.org), American Heart Association (www.heart.org/HEARTORG), National Heart, Lung, and Blood Institute (www.nhlbi.nih.gov), NIDDK (www.niddk.nih.gov), and U.S. Department of Agriculture’s resource for nutrition (www.choosemy plate.gov).