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Oral Glucose Tolerance Test 75 Gram Dose

Blood glucose concentrations are tightly regulated through the coordinated actions of insulin and counter-regulatory hormones, such as glucagon and epinephrine. Diabetes mellitus is characterized by elevated plasma glucose concentrations resulting from insufficient insulin, insulin resistance, or both. Type 1diabetes accounts for less than 10% of all diabetes, while type 2 diabetes accounts for the remaining 90%. Gestational diabetes occurs in 3% of all pregnancies.

The American Diabetes Association’s criteria for the diagnosis of diabetes mellitus in nonpregnant adults include:

  • A fasting glucose of 126 mg/dL or greater. Fasting is defined as no caloric intake for at least 8 hours or
  • A casual plasma glucose of 200 mg/dL or greater plus symptoms (thirst, polyuria, weight loss, blurry vision) of diabetes. Casual is defined as any time of day without regard to time since last meal or
  • A 2 hour post-load value of 200 mg/dL or greater after a 75 g oral glucose tolerance test.

Any of these 3 criteria can be used to diagnose diabetes. In the absence of unequivocal hyperglycemia, each must be confirmed on a subsequent day by any one of these methods.

Although the 75 g oral glucose tolerance test (OGTT) is more sensitive and modestly more specific than the fasting plasma glucose to diagnose diabetes, it is poorly reproducible and difficult to perform in practice. Because of ease of use, acceptability to patients, and lower cost, the fasting plasma glucose is the preferred diagnostic test. OGTT may be useful for further evaluation of patients in whom diabetes is still strongly suspected but who have normal fasting plasma glucose or impaired fasting glucose (100 – 125 mg/dL).

The National Diabetes Data Group guidelines for the oral glucose tolerance test are:

  • Test should be done in the morning after a 10 to16 hour fast, preceded by 3 days of diet containing at least 150 g of carbohydrate, and unrestricted physical activity.
  • After obtaining a fasting specimen, a 75 g glucose loading dose is consumed over 5 minutes.
  • Patient should remain seated throughout the test & may drink as much water as desired.
  • Blood is drawn at 2 hours after the glucose load.
  • Specimens should be refrigerated after collection.

The National Diabetes Data Group (NDDG) in the United States and the World Health Organization (WHO) established diagnostic criteria in 1979 for normal glucose tolerance and diabetes based upon an oral glucose tolerance test (OGTT). They also suggested that a category between normality and diabetes should be used called impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) because subjects with IFG or IGT are at increased risk of developing overt diabetes and atherosclerotic vascular disease. IFG and IGT have been officially termed pre-diabetes. Although the natural history of individuals with IFG and IGT is variable, approximately 25 percent of subjects will progress to diabetes over three to five years. Subjects with additional diabetes risk factors, including obesity and family history, are more likely to develop diabetes.

The following definitions are from ADA reports:

FPG

2h OGTT

Random glucose

Normal

<100 mg/dL

(5.6 mmol/L)

<140mg/dL

(7.8 mmol/L)

IFG

100-125 mg/dL

(5.6 to 6.9 mmol/L)

IGT

140-199 mg/dL

(7.8 to 11.0 mmol/L)

Diabetes Mellitus

126 mg/dL or >

(7.0 mmol/L or >)

200 mg/dL or >

(11.1 mmol/L or >)

200 mg/dL or > with symptoms

Neither the American Diabetes Association or the World Health Organization has specific recommendations for measuring fasting glucose before administering glucola. There is no evidence that it is harmful to give glucola to someone with increased fasting glucose, but it seems prudent to avoid doing so. Some laboratories perform a finger stick glucose measurement before administering glucola and do not perform the OGTT if the fasting glucose level is >200 mg/dL.

Specimen requirement is one gray top (potassium oxalate-sodium fluoride) tube of blood drawn before the glucose dose and at 2hours after the glucose challenge.

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