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Test Code LAB167 - Done only at PRMC 5 HOUR GTT

Important Note

This test CANNOT be performed in satellite labs.

 

Additional Codes

  

Methodology

Includes fasting, 30-minute, 1-, 2-, 3-, 4-, and 5-hour glucose levels.

Performing Laboratory

Peninsula Regional Medical Center - 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.

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 specimens at 30-minutes, 1-, 2-, 3-, 4-, and 5-hours post glucose administration.
Note: Label specimens appropriately (corresponding draw time).

 

Reference Values

FASTING

0-4 days: 30-115 mg/dL

>4 days-13 years: 65-130 mg/dL

>13 years: 70-100 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

 

30 minutes:  65-200 mg/dL

1 hour: 65-200 mg/dL

2 hours: 70-139 mg/dL

3 hours: 70-100 mg/dL

4 hours: 70-100 mg/dL

5 hours: 70-100 mg/dL

 

 

Use of the Glucose Tolerance Test, 5 Hour is discouraged. The 5-hour glucose tolerance test has been used to diagnose hypoglycemia. “Reactive hypoglycemia” is the condition that can be diagnosed with 5-hour tolerance test. Measurement of glucose levels when patient is symptomatic following a meal is a more reliable test for “true” hypoglycemia. Fifty percent of females have abnormally low glucose concentrations at some point during 5-hour tolerance test. Males also have abnormally low glucose concentrations during 5-hour test, but at a lower rate. A diagnosis of hypoglycemia requires a glucose result of ≤50 mg/dL that occurs spontaneously and is accompanied by symptoms. “Hypoglycemic” glucose levels can also be caused by episodic low blood pressure and seizure disorders.