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Test Code LAB169 - Draw only @ 2 HOUR GTT

Important Note

Draw only at: TidalHealth Peninsula Regional

Additional Codes

CPT Code: 82951

Methodology

Includes fasting and 2-hour glucose levels.

Performing Laboratory

TidalHealth Peninsula Regional - Core Laboratory: Chemistry

Specimen Requirements

 Patient Preparation
1. Patient should be on unrestricted physical activity and on a “normal” diet for 3 days prior to test.

2. Patient should be instructed to eat and drink nothing except for water for 10 to 16 hours prior to test.

3. Patient should be instructed to refrain from smoking and drinking coffee just before and during test.

 

Specimen Collection: Plasma
Draw blood in a green-top (lithium heparin) tube from a fasting patient (10-16 hours). (Hemolyzed specimen is not acceptable.) Spin down and send 2 mL of lithium heparin plasma as follows:

1. Draw a fasting glucose.

2. Obtain fasting glucose level. 

3. If fasting glucose is >126 mg/dL, do not continue test. (Do not administer glucose solution.)  
4. Patient is given a glucose solution orally of:

    A. 75 g for adults

    B. 1.75 g/kg (up to 75 g) for children.

5. Start timing when patient begins to drink. Instruct patient to drink solution at rate of 15 g per 5 mins.

6. Have patient remain quiet for duration of test.

7. Draw specimen at 2-hours post glucose administration.
Note: Label specimens appropriately (corresponding draw time).

Reference Values

Fasting:

Cut-off for diabetes mellitus is 126 mg/dL.

Cut-off for “impaired fasting” or “impaired tolerance” is 110 mg/dL.

Note: A fasting glucose of 100-125 mg/dL is termed “impaired fasting glucose.”

 

Pediatric Critical value (automatic call-back) < 16 years:  <50 mg/dL or >400 mg/dL

 

Adult Critical value (automatic call-back) ≥ 16 years:  <50 mg/dL or >500 mg/dL

 

2-HOUR

Cut-off for diabetes mellitus is 200 mg/dL.

Cut-off for “impaired fasting” or “impaired tolerance” is 140 mg/dL.

 

 

Oral glucose tolerance test is no longer recommended by the American Diabetes Association (ADA) for routine clinical use. There are alternative tests, which are preferred, based on their reduced cost, reduced inconvenience, and better reproducibility. According to ADA guidelines, criteria for diagnosis of diabetes mellitus are:

1. Symptoms of diabetes (including polyuria, polydipsia, and unexplained weight loss), plus any glucose concentration ≥200 mg/dL (measured at any time of day without regard to time since last meal).

2. Fasting glucose ≥126 mg/dL. Fasting is defined as no caloric intake for at least 8 hours.

3. 2-hour glucose ≥200 mg/dL during an oral glucose tolerance test (not recommended).

 

In absence of unequivocal hyperglycemia with acute metabolic decompensation, these criteria should be confirmed by repeat testing on a different day.