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25383 Glucose Tolerance Test - Cystic Fibrosis

Glucose Tolerance Test - Cystic Fibrosis
Test Code: OGTTCF
Useful For
To diagnose and manage cystic fibrosis related diabetes.
Specimen Requirements
Fasting Required Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)		 Pediatric Minimum Volume
(no repeat)
​Yes
8 hours

Plasma/Serum

​Lithium-Heparin Plasma Separator Tube (PST), Serum Separator Tube (SST)

Lithium/Sodium-Heparin Green Top Tube (GTT), 
Red Top Tube (RTT), Grey Top Tube (GYTT)​

 1.0 mL​ ​0.3 mL ​0.1 mL
Collection Processing Instructions
The test should be performed in the morning during the period of stable baseline health (at least six weeks since an acute exacerbation). Patient should consume a minimum of 150 g of carbohydrate daily for the preceding 3 days. Fasting for a minimum of 8 hours not exceeding 12 hours before the test is required. Water is permitted during this fasting period. Patients should also be advised to discontinue, whenever possible, all nonessential medication that can affect glucose metabolism at least three days before testing.

Glucola (oral glucose load, containing 1.75g/Kg glucose; maximum dose 75g) will be administered to patient by phlebotomist and must be consumed within a 5 minute interval; sipping of water is permitted. After consuming the glucose load the patient will be directed not to smoke, chew gum, eat and consume caffeine-containing beverages and have minimal activity during the testing phase. Patient will be permitted to drink small sips of water. Note: The patient will be advised by the phlebotomist that if any adverse symptoms develop (i.e. nausea, headache, dizziness or vomiting) the patient should report immediately back to the laboratory. At that point it will be determined if testing will continue or if medical attention is needed.

Draw a fasting blood sample before administering glucose load dose. Administer oral glucose load and draw blood after 1 and 2 hours. Separate serum or plasma from cells within 30 minutes of venipuncture. Refrigerate if specimen are collected in tube other than gray top.
Specimen Stability Information
Specimen Type Temperature Time
​Plasma/Serum ​ ​ ​Ambient ​8 hours
​Refrigerated ​72 hours
​Frozen at -20 deg Celsius ​14 days
Whole blood specimens in gray top tubes are stable for up to 24 hours at 250C and 72 hours in the refrigerator. Glucose concentration decreases ex vivo in whole blood because of glycolysis by approximately 5 to 7% per hour but varies with glucose concentration, temperature, white blood cell count and other factors. ​ ​ ​
Rejection Criteria

 

​Non-fasting patient
​Whole blood specimens collected in any other collection tube
​Plasma/Serum not separated from cells within 60 minutes after collection
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Marshfield ​Monday through Sunday ​Less than 2 hours Hexokinase-UV/NAD Endpoint Assay/Beckman AU
Test Information

Cystic fibrosis related diabetes (CFRD) is most common morbidity in patients with cystic fibrosis (CF). Though it shares the features of type 1 and type 2 diabetes, CFRD is distinct clinical entity. Primarily caused by insulin insufficiency as well as fluctuating levels of insulin resistance related to acute & chronic illness also plays a role.

Presence of CFRD has negative impact on pulmonary function and survival in CF. The majority of patients with CFRD do not have fasting hyperglycemia, thus in the absence of classic symptoms, the diagnosis of CFRD frequently relies on the oral glucose tolerance test (OGTT).

The OGTT is the screening test of choice for CFRD. Although it is an imperfect test due to inherent variability observed in individuals over time, longitudinal studies demonstrate that a diabetes diagnosis by OGTT correlates with clinically important CF outcome including the rate of lung function decline over next 4 years, risk of microvascular complications and risk of early death.

As of 2010 annual screening for CFRD starting at age of 10 years is recommended by US Cystic Fibrosis Foundation (CFF) & International Society of Pediatric & Adolescent Diabetes (ISPAD) & American Diabetes Association (ADA) using 2 hour 75 g OGTT (ADA consensus). OGTT should also be performed prior to transplant and pregnancy. (Clinical Care Guidelines for CFRD, Diabetes Care. 2010; 33(12): 2697, J Cystic Fibrosis. 2013; 12: 318).

Diagnosis recommendations: During a period of stable baseline health the diagnosis of CFRD can be made in CF patients according to standard ADA criteria. Testing should be done on 2 separate days to rule out laboratory error unless there are unequivocal symptoms of hyperglycemia (polyuria and polydipsia).
Positive fasting plasma glucose (FPG) or HbA1C can be used as a confirmatory test, but if FPG is normal the OGTT should be performed.
If diagnosis of diabetes is not confirmed, the patient resumes routine annual testing.
• 2-h OGTT plasma glucose ≥200 mg/dl
• FPG ≥126 mg/dl
• A1C >6.5% (A1C >6.5% does not rule out CFRD because this value is often spuriously low in CF.)
• Classical symptoms of diabetes (polyuria and polydipsia) in the presence of a casual glucose level >200 mg/dl.

Reference Range Information
Performing Location Reference Range
​Marshfield
Normal Glucose Tolerance
Fasting Plasma Glucose: <126 mg/dl
1-Hour Post-dose Plasma Glucose: <200 mg/dl
2-Hour Post-dose Plasma Glucose: <140 mg/dl
Impaired Glucose Tolerance (IGT)
Fasting Plasma Glucose: <126 mg/dl
2-Hour Post-dose Plasma Glucose: ≥140 mg/dl and <200 mg/dl.
Cystic Fibrosis Related Diabetes (CFRD)
Fasting Plasma Glucose: ≥126 mg/dl
2-Hour Post-dose Plasma Glucose: ≥200 mg/dl
Outreach CPTs
CPT Modifier
(if needed)		 Quantity Description Comments
​82951 ​ ​ ​ ​
Ordering Applications
Ordering Application Description
If the ordering application you are looking for is not listed, contact your local laboratory for assistance.
Specimen Requirements
Fasting Required Specimen Type Preferred Container/Tube Acceptable Container/Tube Specimen Volume Specimen Minimum Volume
(allows for 1 repeat)		 Pediatric Minimum Volume
(no repeat)
​Yes
8 hours

Plasma/Serum

​Lithium-Heparin Plasma Separator Tube (PST), Serum Separator Tube (SST)

Lithium/Sodium-Heparin Green Top Tube (GTT), 
Red Top Tube (RTT), Grey Top Tube (GYTT)​

 1.0 mL​ ​0.3 mL ​0.1 mL
Collection Processing
The test should be performed in the morning during the period of stable baseline health (at least six weeks since an acute exacerbation). Patient should consume a minimum of 150 g of carbohydrate daily for the preceding 3 days. Fasting for a minimum of 8 hours not exceeding 12 hours before the test is required. Water is permitted during this fasting period. Patients should also be advised to discontinue, whenever possible, all nonessential medication that can affect glucose metabolism at least three days before testing.

Glucola (oral glucose load, containing 1.75g/Kg glucose; maximum dose 75g) will be administered to patient by phlebotomist and must be consumed within a 5 minute interval; sipping of water is permitted. After consuming the glucose load the patient will be directed not to smoke, chew gum, eat and consume caffeine-containing beverages and have minimal activity during the testing phase. Patient will be permitted to drink small sips of water. Note: The patient will be advised by the phlebotomist that if any adverse symptoms develop (i.e. nausea, headache, dizziness or vomiting) the patient should report immediately back to the laboratory. At that point it will be determined if testing will continue or if medical attention is needed.

Draw a fasting blood sample before administering glucose load dose. Administer oral glucose load and draw blood after 1 and 2 hours. Separate serum or plasma from cells within 30 minutes of venipuncture. Refrigerate if specimen are collected in tube other than gray top.
Specimen Stability Information
Specimen Type Temperature Time
​Plasma/Serum ​ ​ ​Ambient ​8 hours
​Refrigerated ​72 hours
​Frozen at -20 deg Celsius ​14 days
Whole blood specimens in gray top tubes are stable for up to 24 hours at 250C and 72 hours in the refrigerator. Glucose concentration decreases ex vivo in whole blood because of glycolysis by approximately 5 to 7% per hour but varies with glucose concentration, temperature, white blood cell count and other factors. ​ ​ ​
Rejection Criteria

 

​Non-fasting patient
​Whole blood specimens collected in any other collection tube
​Plasma/Serum not separated from cells within 60 minutes after collection
Useful For
To diagnose and manage cystic fibrosis related diabetes.
Reference Range Information
Performing Location Reference Range
​Marshfield
Normal Glucose Tolerance
Fasting Plasma Glucose: <126 mg/dl
1-Hour Post-dose Plasma Glucose: <200 mg/dl
2-Hour Post-dose Plasma Glucose: <140 mg/dl
Impaired Glucose Tolerance (IGT)
Fasting Plasma Glucose: <126 mg/dl
2-Hour Post-dose Plasma Glucose: ≥140 mg/dl and <200 mg/dl.
Cystic Fibrosis Related Diabetes (CFRD)
Fasting Plasma Glucose: ≥126 mg/dl
2-Hour Post-dose Plasma Glucose: ≥200 mg/dl
For more information visit:
Performing Laboratory Information
Performing Location Day(s) Test Performed Analytical Time Methodology/Instrumentation
​Marshfield ​Monday through Sunday ​Less than 2 hours Hexokinase-UV/NAD Endpoint Assay/Beckman AU
For billing questions, see Contacts
Outreach CPTs
CPT Modifier
(if needed)		 Quantity Description Comments
​82951 ​ ​ ​ ​
For most current information refer to the Marshfield Laboratory online reference manual.