Glucose Tolerance: Diabetes Mellitus

Rationale: The American Diabetes Association (ADA) 2011 Standards of Medical Care provide the following recommendations regarding the diagnosis of diabetes.^1,2

Criteria for the diagnosis of diabetes:¹

• A_1c ≥6.5%.* This test should be performed in a laboratory using a method that is NGSP certified and standardized to the DCCT assay, or
• Fasting blood glucose ≥126 mg/dL.* Fasting is defined as no caloric intake for at least eight hours, or
• Two-hour plasma glucose ≥200 mg/dL during an oral glucose tolerance test.* This test should be performed as described by the World Health Organization (WHO) using a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water. The LabCorp test number for the WHO glucose tolerance test is 101200, or
• In a patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose ≥200 mg/dL.

*In the absence of unequivocal hyperglycemia, the result should be confirmed by repeat testing.

There is an intermediate group of subjects whose glucose levels, although not meeting criteria for diabetes, are nevertheless too high to be considered normal. These individuals have been referred to as having prediabetes, indicating the relative high risk for the future development of diabetes. Categories of increased risk for diabetes include impaired fasting glucose, impaired glucose tolerance, and elevated A_1c.

• Fasting plasma glucose of 100−125 mg/dL = impaired fasting glucose, or
• Two-hour plasma glucose in the 75 g OGTT 140−199 mg/dL = impaired glucose tolerance, or
• A_1c of 5.7% to 6.4%

*For all of these tests, risk is continuous, extending below the lower limit of the range and becoming disproportionately greater at higher ends of the range.

ADA guidelines refer to only two oral glucose tolerance tests (OGTT). These are the World Health Organization two-hour OGTT and the two-hour gestational OGTT. Patient preparation is the same for both of these tests.

• Patient should be active and eat a regular diet that includes at least 150 grams of carbohydrate daily for three days prior to the test.
• Patients should be instructed not to eat or drink anything except water for at least eight hours and not more than 14 hours before the test.
• Patients should also be advised to discontinue, whenever possible, all nonessential medication that can affect glucose metabolism at least three days before testing.

The World Health Organization OGTT guidelines (test 101200) for nonpregnant patients, pregnant patients at the first prenatal visit, and for screening for persistent diabetes six to 12 weeks postpartum are given below.

• Draw a fasting blood sample before administering glucose.
• Administer 75 g glucose and draw blood after two hours. The subject should remain seated throughout the test.
• Draw a second sample after two hours.
• Result interpretation is given in sections A and B above.

LabCorp Diabetes Mellitus-related Testing

Test Name	Test Number
Glucose, Plasma	001818
Glucose Tolerance Test (GTT), Two-hour (Oral WHO Protocol)	101200
Gestational Glucose Tolerance Screening and Diagnostic Test (Two-hour, ADA Recommendations)	101000
Hemoglobin (Hb) A1c	001453

References

1. American Diabetes Association. Standards of Medical Care in Diabetes−2015: summary of revisions. Diabetes Care. 2015 Jan; 38(Suppl 1):S1-S93. PubMed 25537706

2. World Health Organization. Definition, Diagnosis and Classification of Diabetes Mellitus and its Complications. Part 1: Diagnosis and a WHO Consultation. Geneva, Switzerland: WHO;1999. WHO/NCD/NCS/99.2.