Sign in →

Test Code LAB5922 Glucose Tolerance Testing (GTT) 75gram Cystic Fibrosis

Important Note

TEST MUST BE SCHEDULED WITH OUTPATIENT LAB AT LEAST ONE DAY IN ADVANCE

Additional Codes

test includes: GTT 75g Fasting CF, GTT 75g 1 Hour CF & GTT 75g 2 hour CF

Performing Laboratory

St. Louis Children's Hospital Laboratory - Chemistry

Specimen Requirements

Patient Preparation: Eating affects results. Patient needs to be fasting for first specimen collection. 

Collection Container/Tube:

Preferred: Mint green-top lithium heparin (MNTGRN LiHep) tube

Acceptable: Green samplette/microtainer, plain red-top tube, Gold SST gel tube, or Lavender-top (EDTA) tube

Specimen Collection Volume: 1.0 mL (minimum 0.6 mL) for each timed specimen collected

Special Instructions: Indicate time of collection on each specimen. Send to lab promptly.

 

Sample Rejection

Mislabled or unlabeled specimens

Logistics

Test Availability: Monday-Sunday

Cut-off time: None, performed as received

Turnaround Time: Same Day; 2 hours from receipt in lab

Reference Values

Reference Interval: (all ages)

Fasting: <100 mg/dL
1-Hour: <200 mg/dL
2-Hour: <140 mg/dL

Interpretive Data:

Cystic Fibrosis-Related Diabetes

FASTING

Normal: <100 mg/dL

Impaired Glucose Tolerance: 100-125 mg/dL

Possible CF-related Diabetes: >125 mg/dL

1-HOUR* 

Normal: <200 mg/dL

Abnormal: ≥200 mg/dL

2-HOUR

Normal: <140 mg/dL

Impaired Glucose Tolerance: 140-199 mg/dL

Possible Diabetes: ≥200 mg/dL

*Elevated glucose at 1-hour during OGTT has been associated with impaired pulmonary function and may be a marker for worsening disease in patients with possible CF-related diabetes. The 1-hour timepoint is not currently recommended by the American Diabetes Association for diagnosis.

Reference Interval Info: Diabetes Care 2020; 43(Suppl 1):S14-31. Diabetes Care 2010; 33 (12). Clinical Care Guidelines for Cystic Fibrosis-Related Diabetes. Brosky et al. Diabetes Care 2011; (34) 292-295.

Critical Value: (any specimen)

<3 days: <45 mg/dL OR >or= 450 mg/dL
≥ 3 days: <55 mg/dL OR >or= 450 mg/dL

All Critical results will be called back automatically 

 

Methodology

UV (Enzymatic reference method with hexokinase)

Limitations

  • In very rare cases gammopathy, in particular type IgM (Waldenstrom's macroglobulinemia) may cause unreliable results.

Additional Information

FOR SLCH CORE LABORATORY USE ONLY

Lab Processing Instructions: Centrifuge to separate within 2 hours of collection. If specimen is collected in microtainer or plain red-top tube aliquot sample into a Roche false-bottom tube for testing (labeling aliquot with specimen type).

Processed Volume: 0.3 mL plasma/serum

Analyte Stability:

Specimen Type Temperature Time
Whole Blood (unprocessed)  Ambient 2 hours (12 hours if in Sodium Fluoride tube) 
Plasma/Serum (processed) Refrigerated (preferred) 3 days
  Ambient  24 hours
  Frozen  1 year

Test Classification and CPT Code

82951

LOINC

20436-2

Last Reviewed

06/2025