Glucose Tolerance Testing (GTT) 1-, 2- or 3-Hour

Additional Information:

  • GTT, 1-hour: 28084
  • GTT, 2-hour: 28085
  • GTT, 3-hour: 28086

CPT Code(s):

  • GLU1P: 82950
  • GESTT: 82951
  • GTT3P: 82951, 82952

Specimen Requirements:

Type: Plasma
Container/Tube: Fluoride/Oxalate gray-top tubes

Preparation: Submit specimens in their original tubes.

Sample Volume: Submit a full tube.

Minimum Volume: 0.5 mLof plasma

Stability (collection to time of analysis/testing):

  • Ambient: 24 hours at 15 – 25°C
  • Refrigerated: 72 hours at 2 – 8°C
Rejection Criteria:

  • Samples with moderate or marked hemolysis will be rejected.


The oral glucose tolerance test evaluates glucose clearance from the circulation after glucose loading under defined and controlled conditions. Serum glucose levels may be abnormally high (hyperglycemia) or abnormally low (hypoglycemia). Glucose measurements are used in the diagnosisand treatment of carbohydrate metabolism disorders including diabetes mellitus, idiopathic hypoglycemia, and detection of gestational diabetes.


Glucose is phosphorylated by hexokinase (HK) in the presence of adenosine triphosphate (ATP) and magnesium ions to produce glucose-6-phosphate (G-6-P) and adenosine diphosphate (ADP). Glucose-6-phosphate dehydrogenase (G6P-DH) specifically oxidizes G-6-P to 6-phosphogluconate with the concurrent reduction of nicotinamide adenine dinucleotide (NAD+) to nicotinamide adenine dinucleotide, reduced (NADH). The change in absorbance at 340/660 nm is proportional to the amount of glucose present in the sample. (Package Insert: OSR General Chemistry BAOSR6x21.04, 20143-04)


Within 24 hours

Reference Values:

  • Fasting: 65 – 99 mg/dL
  • 1HR: 65 – 179 mg/dL
  • 2HR: 65 – 154 mg/dL
  • 3HR: 65 – 139 mg/dL