A glucose tolerance test is used to determine a person's ability to handle a glucose load. The test can show whether a person can metabolize a standardized measured amount of glucose. The results can be classified as normal, impaired, or abnormal. A glucose tolerance test may be used to diagnose type 1 diabetes mellitus, type 2 diabetes mellitus, and gestational diabetes mellitus. It is a blood test that involves taking multiple blood samples over time, usually 2 hours.
Etiology and Epidemiology
Diabetes mellitus is a diverse set of conditions where high blood glucose levels (hyperglycemia) are the result.
In type 1 diabetes mellitus, antibodies are triggered in an auto-immune reaction, leading to the destruction of the beta cells in the pancreas. The pancreas consequently fails to produce adequate amounts of insulin to bind to the glucose. Since there is little available insulin, the blood sugar increases.
In type 2 diabetes mellitus, the cells in the liver become insulin-resistant, causing reduced absorption of glucose in the bloodstream. Often the pancreas overproduces insulin in response to the increased amount of glucose in the bloodstream, but with the liver's inability to absorb the glucose, the result is hyperglycemia.
Gestational diabetes mellitus is also a disease of insulin resistance. Signs of gestational diabetes usually appear around the second trimester of pregnancy, and frequently gestational diabetes will resolve upon completion of the pregnancy. Although gestational diabetes will resolve for some, it does carry a risk that type 2 diabetes may develop at a later time.
To perform the glucose tolerance test, the patient should be instructed to eat a normal carbohydrate diet of at least 150 grams of carbohydrates for at least 3 days prior to the test. On the day of the test typically a patient must arrive in a fasting state.
A fasting sample is taken either by phlebotomy or intravenous access to establish a baseline glucose level. Then, the patient will drink the glucose (comes in 2 formulas, either 75 grams or 100 grams). The amount is dosed by weight in pediatric patients at 1.75 g/kg of body weight, while the maximum dose for all nonpregnant patients is 75 grams.
Patients are asked to fast throughout the test except for drinking the glucose. Samples are then taken at various timepoints ending at either 60 or 120 minutes post-consumption of glucose. Throughout the test, patients should remain inactive, and excess hydration with water should be discouraged as these can impact the results of the test.
There are several ways one can order or perform a glucose tolerance test.
The first is the standard one-step glucose tolerance test, where a baseline sample is drawn, and a 60-minute sample is drawn. This is typically a non-fasting test, done in pregnant women around 24 to 28 weeks gestation to screen for gestational diabetes mellitus. If the test indicates impaired or abnormal glucose tolerance, then a 2- or 3-hour glucose tolerance test is ordered.
Another way to complete a glucose tolerance test is to collect a baseline sample in a fasting patient, and then collect a sample at 120 minutes after the consumption of glucola. This test can be used to confirm the diagnosis of diabetes if the blood glucose at the baseline or 120-minute time point is abnormal.
A glucose tolerance test can also be completed with multiple samples taken at baseline, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. This allows for review of impaired tolerance and practitioners will be able to determine if there is a delayed reaction in the excretion of insulin from the pancreas, or delayed absorption of glucose in the liver.
Specimen Requirements and Procedure
Specimens collected are typically insulin samples but, glucose samples and c-peptide samples can be ordered as well. Specimens should be processed in a centrifuge so that serum is separated from the platelets. Serum is then aliquotted into a transport tube, and the platelets can be discarded. Insulin samples must be frozen immediately upon separation to ensure test viability and validity.
It is extremely important to mark each sample with the time point of the draw and the actual time of the draw. For example, a baseline sample drawn at 9:13 AM, would be marked with "0 minute, 0913" or "Time 1, 0913."
For three days preceding the test, the patient should consume a normal diet or one that contains approximately 150 grams of carbohydrates per day. The patient should arrive fasting for a minimum of eight hours before the test (unless the patient is undergoing a non-fasting test). If the patient does not arrive fasting, the test should be rescheduled.
The test can be performed using multiple phlebotomies draws or by inserting a BC-shielded IV catheter.
The fasting sample should be taken and the time point should be notated. The patient should then consume the correct amount of glucose (established by weight, up to 75 grams) over a maximum of a 5-minute period.
Additional samples should be taken at 30-minute, 60-minute, 90-minute, and 120-minute time points or per provider orders. If using a BC-shielded IV catheter, the line should be flushed with a saline or heparin solution after each sample is taken to ensure patency of the line. After the 120-minute time point, the test is concluded, and the blood samples should be processed as required and sent to the lab for analysis.
Results, Reporting, Critical Findings
Normal Results for Type 1 Diabetes or Type 2 Diabetes
- Fasting glucose level 60 to 100 mg/dL
- One-hour glucose level less than 200 mg/dL
- Two-hour glucose level less than 140 mg/dL
Impaired Results for Type 1 Diabetes or Type 2 Diabetes
- Fasting glucose level: 100 to 125 mg/dL
- Two-hour glucose level 140 to 200 mg/dL
Abnormal (Diagnostic) Results for Type 1 Diabetes or Type 2 Diabetes
- Fasting glucose level greater than 126 mg/dL
- Two-hour glucose level greater than 200 mg/dL
Normal Results for Gestational Diabetes
- Fasting glucose level less than 90 mg/dL
- One-hour glucose level less than 130 to 140 mg/dL
- Two-hour glucose level less than 120 mg/dL
Abnormal Results for Gestational Diabetes
- Fasting glucose level greater than 95 mg/dL
- One-hour glucose level greater than 140 mg/dL
- Two-hour glucose level greater than 120 mg/dL
The glucose tolerance test is given to determine how quickly glucose is cleared from the blood. The test is used to test for diabetes, insulin resistance, impaired beta-cell function, reactive hypoglycemia, acromegaly, and other disorders of carbohydrate metabolism.
Enhancing Healthcare Team Outcomes
A glucose tolerance test is typically ordered by a medical doctor or advanced nurse practitioner. Interprofessional collaboration is required for the correct administration of the test. The provider or the nurse must ensure to give the patient adequate instructions to prepare for the test and for what to expect at the test.
The actual glucose tolerance test itself can be administered in several settings. A provider's office may administer the test if there are appropriate equipment and staffing. The glucose tolerance test can also be administered at a laboratory. A hospital, although the inpatient side is an atypical site for glucose tolerance tests, may have an outpatient or clinical research department where staff may have additional time set aside to complete the test.
Nurses, medical assistants, or phlebotomists may perform the test. There must be clear communication on order from the provider on the type of test, the length of time, and the number of samples ordered. The personnel administering the test should be aware of the requirements of the test including the fasting requirement and the pre-test dietary carbohydrate requirement. It is important to collaborate with laboratory personnel to ensure timely processing of the specimens, as well as proper storage and shipping (if necessary).
Laboratory personnel should work closely with providers to provide accurate results quickly.