Differential Diagnosis

Type I diabetes is an absolute deficiency of insulin secretion.

Type II diabetes represents various degrees of insulin resistance.

Gestational diabetes may be from unrecognized type I diabetes or subclinical and incipient type II diabetes. Diabetes may result from heterogeneous causes, such as exocrine pancreatic disease, toxins to islet cells, or use of drugs that antagonize the insulin effect.

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Suggested Additional Lab Testing

Fasting plasma glucose

Standard oral glucose tolerance test with plasma glucose values at 2 hours after a 75-g glucose bolus

  • Patients without disease: less than 140 mg/dL

  • Patients with impaired glucose tolerance: 140-200 mg/dL

  • Patients with diabetes mellitus: greater than or equal to 200 mg/dL plus fasting plasma glucose greater than or equal to 125 mg/dL

Gestational diabetes

  • Screening oral glucose tolerance test involves ingestion of a 50-g glucose bolus with no need for fasting. Positive test for gestational diabetes: 1-hour post-bolus value greater than or equal to 140 mg/dL

  • Confirmatory oral glucose tolerance test in gestational diabetes is ingestion of a 100-g glucose bolus with fasting. A positive test is produced if any two of the following four are met: plasma glucose value prior to bolus greater than or equal to 95 mg/dL, 1-hour post-bolus value greater than 180 mg/dL, 2-hour post-bolus value greater than or equal to 155 mg/dL, or 3-hour post-bolus value greater than 140 mg/dL.


Other causes of diabetes associated with decreased insulin production

  • Exocrine pancreatic disease: amylase and lipase level

  • Cushing syndrome: 24-hour urine cortisol

  • Glucagonoma: plasma glucagon

  • Hyperthyroidism: TSH level

  • Hemochromatosis: iron, ferritin, and total iron binding capacity

  • Separates normal from those with impaired fasting glucose or an impaired glucose tolerance and from those who have diabetes mellitus

  • If fasting plasma glucose is not definitive, a standard oral glucose tolerance that measures the plasma glucose at 2 hours after a 75-g glucose bolus helps identify patients in these three categories.

  • Normal range for fasting plasma glucose: less than 100 mg/dL

  • Patient with impaired fasting glucose: 100-125 mg/dL

  • Diagnosis of diabetes mellitus: plasma glucose greater than 125 mg/dL after a fast of greater or equal to 8 hours, glucose greater than 200 mg/dL 2 hours after the start of a 75-g oral glucose tolerance test, or a random glucose greater than 200 mg/dL with symptoms consistent with diabetes.