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The University of Chicago Medicine Laboratories

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Mayo Clinic Laboratories

Test Code OBGTT Gestational Glucose Tolerance (GTT) Testing 

Specimen and Container/Tube

Specimen Type: Plasma

Preferred Specimen: Grey Top, NaF/K-Oxalate

Other Acceptable specimens: None 

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 gray top Vacutainer tube before Glucocrush® 100 ingestion. Sample is labeled GGTT Fasting. 

 

  • Glucose Beverage Ingestion: Patient drinks the entire bottle of refrigerator-temperature Glucocrush® 100 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
    • If the patient feels ill, or if she 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 on the chemistry requisition.
    • Patient will return for blood draw no more than 55 minutes after the ingestion. 

 

  • Specimen Collection 
    • Sample is drawn using a Gray Top Tube, exactly 60 minutes after the patient started drinking the solution. Sample tube is labeled "GGTT60" and sent immediately to the laboratory. 
    • At 2 hours (120 minutes after glucose ingestion began) label GGTT120.
    • At 3 hours (180 minutes after glucose ingestion began) label GGTT180. 
    • 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 hour

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

The Gestational Glucose Tolerance Tests (GGTT) is a diagnostic test for diabetes in pregnant patients usually between 24 and 28 weeks of pregnancy. Venous plasma glucose is measured in a fasting patient before and at 1, 2, and 3 hour intervals after an oral dose of 100 g of glucose.

Test Components

Glucose fasting, Glucose each hour of test time period 

Methodology

Enzymatic Method with hexokinase (Roche Diagnostics)

CPT Codes

82951 - up to 3 specimens

82952 - each additional specimen beyond 3

LOINC Codes

TBD

Synonyms/Keyword Search

GGTT, OBGTT

Additional Information

  • Results should always be assessed in conjunction with patient's medical history, clinical examination and other findings.
  • Interpretation of Results: 
    1.  
    • Any two results reaching or exceeding the following limits for a 100g oral glucose load are generally considered definitive for the diagnosis of gestational diabetes mellitus (GDM); however, exceptions can occur (see references below.):
      • Fasting level 95 mg/dL
      • 1 hour level of 180 mg/dL
      • 2 hour level of 155 mg/dL
      • 3 hour level of 140 mg/dL

Last Review Date

6/3/2025

Volume

Preferred Volume: 0.5 mL

Minimum Volume: 0.2 mL