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Glucose Tolerance

Source Blood
Mnemonic non-pregnant: GTTNP
pregnant: GTTP
Specimen Requirements

This test requires collection of a fasting blood specimen, followed by collection of several blood specimens after the patient is given a glucose drink. The concentration of glucose in the drink is dependent on the patient's weight. The procedure takes 2 or more hours.

Test cannot be collected by Mobile Collection Service (MCS).

By appointment only:
Community patients: Contact CLS Patient Appointment Line at 403-777-5136 to book an appointment at any Patient Service Centre.

Patients less than 18 years of age will be required to provide their body weight. Patients weighing less than 42 kg (92 pounds) must be referred to the Alberta Children's Hospital Outpatient Laboratory with advanced notification given to the lab.

Hospital patients: Nursing unit to book appointment with on-site lab.

CLS staff:
Refer to PX-SC740 Timed Interval Tests for instructions.

Glucose drink is administered and specimens collected ONLY IF fasting glucose result meets criteria defined below:
 Site  GTTP  GTTNP
 CRL and RRL  Less than or equal to 5.2 mmol/L  Less than or equal to 7.8 mmol/L
 PSC  Less than or equal to 5.9 mmol/L

Administer drink regardless of fasting glucose IF 'Give glucose drink regardless of fasting glucose' is written on the requisition. Clinicians may do this for certain patients, e.g. mother at high risk of GDM or early in pregnancy (< 20 weeks gestation).

Each collection requires:
1.x.3 mL or 4.5 mL light green (mint) PST tube. Gold top SST tube also acceptable.
0.2 mL plasma or serum.

Minimum Whole Blood Volumes for Microcollections

Patient MUST be fasting a minimum of 8 hours, but not more than 16 hours.

Specimen Handling
Additional Information

Patient instruction sheet (form CH3002) may be given to patient.

The drink provided for this test includes the following ingredients:

 Flavoured Drink

 Unflavoured Drink 

Patient Service Centre and Calgary Hospital Sites

  • Dextrose/Glucose (derived from corn)
  • Citric acid
  • Artificial flavours
  • Sodium benzoate
  • FD&C Yellow #6
  • Purified water

 Calgary Rural Lab Sites

  • Dextrose/Glucose (derived from corn)
  • Citric acid
  • Natural orange flavouring
  • Sodium benzoate
  • FD&C Yellow #6
  • FD&C Red #40
  • Purified water

 All Sites

  • Dextrose/Glucose (derived from corn)
  • Potassium sorbate (0.2%)
  • Purified water

For non-pregnant patients:
The test consists of a fasting glucose and a 2 hour collection.

For pregnant patients:
The Glucose Tolerance Test for Gestational Diabetes Mellitus (GDM) consists of a fasting blood sample, as well as samples collected at 1 hour and 2 hours after consuming a glucose drink. The glucose drink is not given if the fasting glucose result is high enough to suggest GDM.

Write 'Give glucose drink regardless of fasting glucose' on the requisition if you wish to override this procedure, e.g. mother at high risk of GDM or early in pregnancy (< 20 weeks gestation).

Pregnant patients being tested for GDM should first undergo the Gestational Diabetes Screen. If the screen result is between 7.8 mmol/L up to and including 11.0 mmol/L, then the Glucose Tolerance Test is warranted. Screen results of 11.1 mmol/L or greater are diagnostic of GDM; therefore a glucose tolerance test is not needed.

For Diabetes Clinical Practice Guidelines, please see: Canadian Diabetes Association Clinical Practice Guidelines

Testing Location Chemistry-ACH, DSC, FMC, PLC, RGH, SHC
Rural: CRL
Testing Frequency Weekdays
Alternate Name(s) 75g Oral Glucose Tolerance Test
Reference Interval

Reference Interval

Fasting Specimen
-  less than 30 days: 2.5 - 5.5 mmol/L
-  30 days - 150 years: 3.3 - 6.0 mmol/L


Interpretation


Non-Pregnant Adults and Children
 Diagnosis of Diabetes Mellitus  Fasting Plasma glucose 7.0 mmol/L or greater

OR

Plasma glucose 2 hours post administration of the 75 g glucose drink 11.1 mmol/L or greater
 Diagnosis of impaired fasting glucose (Prediabetes)  Fasting plasma glucose of 6.1 mmol/L up to and including 6.9 mmol/L;

AND

Plasma glucose 2 hours post administration of the 75 g glucose drink of less than 7.8 mmol/L
 Diagnosis of impaired glucose tolerance (Prediabetes)  Fasting plasma glucose less than 6.1 mmol/L;

AND

Plasma glucose 2 hours post administration of the 75 g glucose drink of 7.8 mmol/L up to and including 11.0 mmol/L 
 Diagnosis of impaired fasting glucose and impaired glucose tolerance (Prediabetes)  Fasting plasma glucose of 6.1 mmol/L up to and including 6.9 mmol/L

AND

Plasma glucose 2 hours post administration of the 75 g glucose drink of 7.8 mmol/L up to and including 11.0 mmol/L


Pregnant patients:
Note: The 75 g Oral Glucose Tolerance Test for GDM should be performed if the Gestational Diabetes Screen result is 7.8 mmol/L up to and including 11.0 mmol/L.

      

Diagnosis of Gestational Diabetes Mellitus   GDM is diagnosed if one or more of the following three values are elevated during the 75 g Oral Glucose Tolerance Test:

Fasting plasma glucose 5.3 mmol/L or greater

Plasma glucose 1 hour post administration of glucose drink of 10.6 mmol/L or greater

Plasma glucose 2 hours post administration of glucose drink of 9.0 mmol/L or greater

Hypoglycemia:

Documentation of reactive hypoglycemia requires demonstration of Whipple’s Triad:

  •  Symptoms
  •  Alleviation by glucose
  •  Plasma glucose less than 4.0 mmol/L for patients treated with insulin or an insulin secretagogue

Severe hypoglycemia is typically defined at plasma glucose levels less than 2.8 mmol/L.