The Oral Glucose Tolerance Test

Because CF can destroy the pancreatic beta-cells and disrupt their normal insulin secretion, individuals with CF are at high risk for developing CFRD. The decrease in insulin secretion, and eventual insulin deficiency, prevents the body from taking up glucose. As a result, blood sugar increases and the excess sugar is dumped into urine. Weight loss and poor weight gain can occur. The higher blood sugars and worse nutrition can result in poorer lung function. CFRD often does not present with common signs or symptoms of diabetes such as polydipsia (excessive thirst), polyuria (increased urination), and polyphagia (increased appetite). Thus, individuals with CF are encouraged to undergo a yearly screening with an Oral Glucose Tolerance Test (OGTT) to determine their risk for the disease. This regular screening is important in detecting CFRD as early as possible in order to minimize potential problems with weight and lung function.

The Oral Glucose Tolerance Test

  • Performed after an overnight fast
  • Fasting blood is obtained for glucose
  • A dextrose (sugar) containing drink is consumed over 5 minutes
  • Blood is obtained at 60 min and 120 min for glucose (and insulin)
CF Foundation recognizes several glucose tolerance categories, based on one and two-hour plasma glucose concentrations during an OGTT. OGTT Graph
  • Normal glucose tolerance (NGT)–> one hour glucose < 200 mg/dL and two hour glucose < 140 mg/dL
  • Indeterminate glucose tolerance (Ind-GT)–> one hour glucose ≥ 200 mg/dL but two hour glucose <140 mg/dL
  • Impaired glucose tolerance (IGT)–> two hour glucose ≥140 but < 200 mg/dL
  • CFRD–> two hour glucose ≥ 200 mg/dL; Fasting hyperglycemia (FH) , is present when fasting BG is > 126 mg/dL