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The University of Chicago Medicine Laboratories

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Mayo Clinic Laboratories

Test Code OGTT Glucose Tolerance Test (GTT) 

Specimen and Container/Tube

Specimen Type: Plasma

Preferred Specimen: Grey Top, NaF/K-Oxalate

Other Acceptable specimens: None 

Volume

Preferred Volume: 0.5 mL

Minimum Volume: 0.2 mL

Specimen Stability

Storage Temperature

Stability

Room temperature

3 days

Refrigerated

3 days

Frozen

Not acceptable 

Collection Instructions

  • Collect specimen per standard laboratory collection procedures. 

 

  • Fasting Draw: a fasting blood sample should be drawn into a grey top Vacutainer tube before Glucocrush® 75 ingestion. Sample is labeled GGT-FASTING. 

 

  • Glucose Beverage Ingestion: Patient drinks the entire bottle of refrigerator-temperature Glucocrush® 75 solution within 5 minutes. Timing begins the moment the patient starts drinking. Patient is to remain seated as much of the time as possible and is not to chew gum, smoke, drink or eat during the test. Ingestion of small amounts of water is acceptable

  •  

  • Prepare for Blood Draw
    • A copy of the time schedule will be provided to the patient and any differences in collection times will be noted on specimen’s label.
    • If the patient feels ill, or if they regurgitates, the phlebotomy staff should be notified immediately in order to contact the patient's physician for further instructions. If the physician instructs to have the test completed, this will be noted in the information system. 

  •  

  • Specimen Collection 
    • Samples are drawn using a Gray Top Tube, exactly at defined intervals after the patient started drinking the Glucocrush® 75.
    • For example, after 30 minutes, sample is collection and tube is labeled “30 minutes GTT”. 
    • All blood samples are brought immediately to the lab for testing.

Specimen Handling and Transport Instructions

UCMC Onsite Instructions: 

  • Collect specimen per standard laboratory collection procedures. 

 

UCMC Offsite and UC MedLab Outreach Instructions: 

  • Collect specimen per standard laboratory collection procedures. 
  • Centrifuge specimens at >2500 x g for 10 minutes at room temperature within 2 hours of collection.
  • Aliquot removed plasma into a plastic tubes containing a minimum of 0.2 mL of specimen.
  • Plasma samples must then be sent room temperature at 20 - 25oC

Rejection Criteria

  • Common specimen rejected situations include: incorrect specimen type, insufficient volume, missing or incomplete specimen identifiers, incorrect specimen transport, or specimens outside stability limits. 
  • Specimens that are clotted or grossly hemolyzed.
  • Specimens that were transported unspun/whole blood and draw time is greater than 6 hours

Reference Values

 

Test

Age

Sex

Reference Range

Glucose fasting

All

All

< 100 mg/dL

Glucose 2 hours

All

All

<140 mg/dL

Critical Values

Analyte

Unit

Low

High

Age

Glucose

mg/dL

<40

>500

>14 days

Turnaround Time

Turnaround times are relative to the time the specimen is received in the test laboratory. 

 

STAT Turnaround Time: Not Available as STAT test

Routine Turnaround Time: 4 hours

Testing Schedule:  24/7

Clinical Indications

This test is used primarily to diagnose diabetes mellitus. The test involves measuring venous plasma glucose in samples taken before and at varying intervals after a 75 gram dose of oral glucose given to an adult patient who is fasted 8-14 hours before the test.

Doses for children are based on patient weight (1.75 g/kg ideal body weight, not to exceed 75 grams). However, GTTs on children are ordinarily performed by the Department of Pediatrics in the University of Chicago Medicine. 

Test Components

Glucose

Methodology

Enzymatic Method with hexokinase (Roche Diagnostics)

CPT Codes

82947

LOINC Codes

TBD

Synonyms/Keyword Search

GDS, DM, GDM, IFG, IGT

Additional Information

  • Results should always be assessed in conjunction with patient's medical history, clinical examination and other findings.
  • Interpretation of Results:
    • In a position statement of the American Diabetes Association (ADA) on the Diagnosis and Classification of Diabetes Mellitus [Ref 2], criteria for the provisional diagnosis of diabetes mellitus may be made when:
      • Fasting plasma glucose level of ≥ 126 mg/dL  OR
      • A 2 hour post load glucose level of ≥ 200 mg/dL during an oral glucose tolerance test (using a 75 g glucose load)  OR
      • Any casual (random) glucose level or ≥ 200 mg/dL with symptoms of hyperglycemia or hyperglycemic crisis.
          1.  
    • Intermediate states indicative of impaired glucose homeostasis that do not meet the diagnostic criteria for diabetes mellitus include:
      • Impaired fasting glucose (IFG), where fasting plasma glucose levels are 100-125 mg/dL
      • Impaired glucose tolerance test (IGT), where 2 hour post 75 g glucose load glucose levels are 140-199 mg/dL during an oral glucose tolerance test.

Last Review Date

6/4/2025