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Glucose Tolerance Test (Outpatient Only)

2 hr Glucose Tolerance Test, GTT

Test Codes

2 Hour GTT: EPIC: LAB5054, Beaker: GTT2

Department

Chemistry

Instructions

• This test must be scheduled through the Beaumont Appointment Center at 800-328-8542.
• If the patient is pregnant, this test should be ordered as a Pregnancy Glucose Tolerance test. Please refer to Glucose Tolerance 2 Hour, Pregnancy (75 g Glucola) and Glucose Tolerance 3-Hour, Pregnancy (100 g Glucola).

Patient Preparation

The patient should be on a full, well-balanced diet (containing AT LEAST 150 grams carbohydrates) for 3 days prior to the test. If a patient has been given a special diet by their physician, that diet should be followed prior to the test. If a patient has not been given any instructions on diet, then the following diet should be used.

Three Day Diet for Glucose Tolerance Test

Continue to follow a regular, unrestricted weight maintenance diet for three consecutive days before the glucose tolerance test. The diet must contain AT LEAST 150 grams of carbohydrates each day. Failure to consume at least 150 grams of carbohydrates (CHO) each day may lead to inaccurate test results. Recommendations for providing adequate amounts of carbohydrates are below:

Choose the number of servings from the Food Groups of your choice to equal or exceed 150 grams of carbohydrates.

Food Groups
Each food listed equals one serving
Fruit (15 grams CHO/serving) -1 fresh medium fruit -1/2 cup canned fruit -1/2 cup fruit juice -1/4 cup dried fruit	Starch (15 grams CHO/serving) -1/2 cup cooked pasta, cereal, potato or starchy vegetable, (e.g. corn, lima beans) -1/3 cup cooked rice or dried beans -3/4 cup dried cereal -1 slice bread -1/2 English muffin, bagel or bun
Vegetable (5 grams CHO/serving) -1/2 cup cooked vegetable -1/2 cup raw vegetable -1/2 cup tomato or vegetable juice	Milk (12 grams CHO/serving) -1 cup milk -8 ounces yogurt
Meat (as desired) poultry, meat, cheese, cottage cheese, tuna, egg or egg substitute & peanut butter.	Fat (as desired) oil, margarine, mayo or salad dressing

Example:
Food Group	Minimum Servings/Daily
Fruit	3 servings	= 45 grams carbohydrates
Starch	6 servings	= 90 grams carbohydrates
Vegetable	1 serving	= 5 grams carbohydrates
Milk*	1 serving	= 12 grams carbohydrates
Meat	as desired	= 0
Fat	as desired	= 0
		152 grams of carbohydrates

*NO If milk is not tolerated, choose at least 1 more serving from either the fruit or starch group.

Specimen Collection Criteria

The patient should fast after the evening meal the night before the test. (Fasting for more than 10 hours, but less than 16 hours)

The test should be done in the morning before the patient has exercised.

The patient should not exercise or smoke during the test.

A few small sips of water are acceptable.

Collect:

One Gray-top Potassium Oxalate/Sodium Fluoride tube prior to administration of the oral glucose load, and at each hour interval, as outlined below.

Also acceptable: One Gold-top SST tube or Microtainer^® drawn under the same guidelines as listed above and below. SST tubes and Microtainers^® must be centrifuged within 1 hour of collection. Contact the Laboratory for acceptibility of other tube types. 

• All specimens must be collected with the same tube type. Example: If a Tolerance Test is started with an SST tube, all following draws must also be SST tubes.
• After the fasting glucose is drawn, the specimen is assayed by a glucose meter. If the value of the fasting glucose for a non-pregnant patient is 150 mg/dL or greater, the glucose tolerance test will be canceled and only the fasting glucose value will be reported. This policy will prevent the unnecessary administration of an oral glucose load to a non-fasting or overtly diabetic patient. If the value is less than the above cutoff value the appropriate dose of glucose will be given.
• Administer Dose: The proper oral glucose dose for adults is 75 grams. Please refer to the Pediatric Glucose Tolerance Dosage Chart for patients under 18 years of age.

• Unless the physician requests alternative times, draw specimens at 1 and 2 hours after glucose load. Record the exact time of collection on the tube. Specify if specimen is fasting, 1 hour or 2 hours. 

Physician Office/Draw Specimen Preparation

Maintain Gray-top tubes refrigerated (2-8°C or 36-46°F) prior to courier transport. (Minimum: 0.5 mL)

Let SST specimens clot 30-60 minutes then immediately centrifuge to separate serum from cells. Refrigerate (2-8°C or 36-46°F) the centrifuged SST tube within two hours of collection. (Minimum: 0.5 mL)

Preparation for Courier Transport

Transport: Centrifuged SST tubes or Gray-top tubes, refrigerated (2-8°C or 36-46°F). (Minimum: 0.5 mL each tube)

Rejection Criteria

• Grossly hemolyzed specimens.
• Inadequately centrifuged Microtainers^® or Gold-top tubes.
• Specimens collected in any tube other than an SST or Gray-top tube.
• SST samples in which serum has not been separated from cells within 1 hour of collection.
• Gray-top tubes that are not received within 24 hours of the time of collection.
• Specimens not collected and processed as indicated.

In-Lab Processing

Let SST specimens clot 30-60 minutes. Centrifuge SST, Gray-top tubes, or Microtainers^® to separate serum or plasma from cells. Deliver immediately to the appropriate testing station.

Storage

Specimen Stability for Testing:

Centrifuged SST Tubes and Microtainers^® with Separator Gel
Room Temperature (20-26°C or 68-78.8°F): 2-4 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): Unacceptable

Gray-top Tubes
Room Temperature (20-26°C or 68-78.8°F): 24 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): Unacceptable

Serum Specimens (Pour-Overs)
Room Temperature (20-26°C or 68-78.8°F): 2-4 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): 7 days

Specimen Storage in Department Prior to Disposal:

Refrigerated (2-8°C or 36-46°F): 7 days

Laboratory

Dearborn Chemistry Laboratory
Farmington Hills Chemistry Laboratory 
Grosse Pointe Chemistry Laboratory 
Royal Oak Automated Chemistry Laboratory 
Troy Chemistry Laboratory
Taylor Chemistry Laboratory
Trenton Chemistry Laboratory
Wayne Chemistry Laboratory 

Performed

Monday – Saturday.
Results available within 24 hours of receipt in the Laboratory.

Regular scheduled appointment times are Monday – Friday, at 8:00 am and 9:00 am.

Patients with  2-hour GTT orders will be required to call the Beaumont Appointment Center at 1-800-328-8542, to schedule their GTT test.

An Access Center representative will review the fasting instructions and schedule the GTT appointment. The Access Center is open Monday – Friday, from 7:30 am – 7:00 pm. 

Reference Range

Fasting: 60 – 99 mg/dL.
1-Hour: 60 – 199 mg/dL.
2-Hour: 60 – 139 mg/dL.

Test Methodology

Enzymatic.

Interpretation

Diagnostic Criteria for Diabetes Mellitus in a Non-Pregnant Patient:

Three possible ways to diagnose diabetes are listed below. An abnormal fasting or 2-hour post Glucola glucose must be confirmed on a subsequent day, preferably by another fasting glucose. 

1. Symptoms of diabetes plus casual plasma glucose of greater than or equal to 200 mg/dL. Casual is defined as any time of day without regard to time since last meal. The classic symptoms of diabetes include polyuria, polydipsia, and unexplained weight loss.
2. Fasting plasma glucose of greater than or equal to 126 mg/dL. Fasting is defined as no caloric intake for at least 8 hours.

3. 2-hour plasma glucose of greater than or equal to 200 mg/dL during an oral glucose tolerance test using 75 gram Glucola or anhydrous glucose dissolved in water.

Impaired Glucose Tolerance and Impaired Fasting Glucose:

1. Impaired fasting glucose is defined as a fasting glucose greater than or equal to 100 mg/dL, but  less than 126 mg/dL.
2. Impaired glucose tolerance is defined as a 2-hour plasma glucose greater than or equal to 140 mg/dL and less than 200 mg/dL during an oral glucose tolerance test using 75 gram Glucola or anhydrous glucose dissolved in water.

Clinical Utility

The glucose tolerance test establishes the presence of glucose intolerance. This assay aids in the diagnosis of diabetes mellitus in patients with borderline fasting and postprandial glucose levels.

CPT Codes

2 Hour GTT: 82947, 82951. 

Contacts

Last Updated

10/22/2023