Glucose Tolerance Test

Article Author:
Emily Eyth
Hajira Basit

Article Editor:
Carrie Smith

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James Hughes
Beata Beatty
Hajira Basit
Phillip Hynes

6/1/2019 3:27:45 PM


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.[1][2][3]

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.[4][5][6][7]


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 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."

Testing Procedures

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

Clinical Significance

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.[8][9]

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.

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Glucose Tolerance Test - Questions

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A 40-year-old obese man visits his primary care provider for the follow-up of his lab reports. On his previous visit, he came with the complaint of excessive thirst and increased urination. At that time, the PCP ordered a fasting glucose test which came out to be impaired. The provider wants to confirm the diagnosis of diabetes mellitus by proceeding with an oral glucose tolerance test. Which of the following levels after a 2-hour oral glucose tolerance test is indicative of diabetes mellitus?

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A 26-year-old G2P1 patient undergoes gestational diabetes screening at 24 weeks with a one-hour post 50 g glucose load. Her blood glucose level comes out to be 175 mg/dL which is considered impaired. What is the next best step in the management of this patient?

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A 66-year-old man presents to the clinic complaining of frequent urination and slightly blurry vision over the past few months. He has a BMI of 31 and a history of hypertension for which he takes lisinopril regularly. He has tried diets in the past without success. His most recent labs completed two months ago show a hemoglobin A1c of 5.4%. An order for a glucose tolerance test is given. When the nurse goes to administer the test, the patient's fasting blood glucose level is 132 mg/dL. Which of the following is the next best step in the management of this patient?

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Glucose Tolerance Test - References


Wei YM,Liu XY,Shou C,Liu XH,Meng WY,Wang ZL,Wang YF,Wang YQ,Cai ZY,Shang LX,Sun Y,Yang HX, Value of fasting plasma glucose to screen gestational diabetes mellitus before the 24th gestational week in women with different pre-pregnancy body mass index. Chinese medical journal. 2019 Apr 20;     [PubMed]
Li LJ,Yu Q,Tan KH, Clinical practice of diabetic pregnancy screening in Asia-Pacific Countries: a survey review. Acta diabetologica. 2019 Apr 6;     [PubMed]
Maldonado-Hernández J,Martínez-Basila A,Rendón-Macías ME,López-Alarcón M, Accuracy of the {sup}13{/sup}C-glucose breath test to identify insulin resistance in non-diabetic adults. Acta diabetologica. 2019 Apr 6;     [PubMed]
Inoue M,Shiramoto M,Oura T,Nasu R,Nakano M,Takeuchi M, Effect of Once-Weekly Dulaglutide on Glucose Levels in Japanese Patients with Type 2 Diabetes: Findings from a Phase 4, Randomized Controlled Trial. Diabetes therapy : research, treatment and education of diabetes and related disorders. 2019 Apr 4;     [PubMed]
Renz PB,Chume FC,Timm JRT,Pimentel AL,Camargo JL, Diagnostic accuracy of glycated hemoglobin for gestational diabetes mellitus: a systematic review and meta-analysis. Clinical chemistry and laboratory medicine. 2019 Mar 20;     [PubMed]
Fayyaz B,Rehman HJ,Minn H, Interpretation of hemoglobin A1C in primary care setting. Journal of community hospital internal medicine perspectives. 2019;     [PubMed]
Shi L,Feng L,Yang Y,Li X,Zhang M,Zhang Y,Ni Q, Prevention of type 2 diabetes mellitus with acupuncture: Protocol for a systematic review and meta-analysis. Medicine. 2018 Nov;     [PubMed]
Huhn EA,Rossi SW,Hoesli I,Göbl CS, Controversies in Screening and Diagnostic Criteria for Gestational Diabetes in Early and Late Pregnancy. Frontiers in endocrinology. 2018;     [PubMed]
Benhalima K,Minschart C,Ceulemans D,Bogaerts A,Van Der Schueren B,Mathieu C,Devlieger R, Screening and Management of Gestational Diabetes Mellitus after Bariatric Surgery. Nutrients. 2018 Oct 11;     [PubMed]


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