Diabetes Mellitus: Diagnosis

Endocrinopathies, such as Cushing’s disease, acromegaly, pheochromocytoma, and hyperthyroidism, may impair glucose tolerance and should be ruled out.

Type 1 and Type 2 Diabetes

Diabetes can be diagnosed by any of the following:

• A fasting plasma glucose concentration greater than 125 mg/dL.
• A 2–hour oral glucose tolerance test (GTT) result of 200 mg/dL or greater.
• A random plasma glucose of 200 mg/dL or greater when diabetes symptoms are present.

Normal fasting plasma glucose is less than 100 mg/dL, and the 2–hour GTT is less than 140 mg/dL. Intermediate results reflect impaired fasting glucose (impaired glucose tolerance in the case of GTT), which often precedes an eventual diabetes diagnosis. Hemoglobin A1c (A1c) is not currently accepted for diagnosis of diabetes because it is too insensitive, but it is crucial for clinical management.

Type 1 DM often presents with ketoacidosis, which is caused by partial or total insulin deficiency and normally requires hospital admission and intensive care. Type 1 DM can be confirmed by the identification of islet–cell antibodies or other autoantibodies (eg, anti–glutamic acid dehydrogenase (GAD) or anti–insulin antibodies (ICA). However, some patients have no identifiable cause of islet cell destruction.

Patients initially thought to have type 2 DM but with autoantibodies most likely have late–onset type 1 diabetes or late–onset autoimmune diabetes. They are unlikely to respond adequately to oral hypoglycemics and will likely require insulin.

Measure of glycemia is the only specific diagnostic test to confirm type 2 diabetes.

Gestational Diabetes

Screening for GDM is a routine part of prenatal examinations. Several strategies may be used for diagnosis.

Screening typically occurs between 24 and 28 weeks of gestation and involves a 1–hour challenge with a 50–g carbohydrate load. A venous serum or plasma glucose greater than 129 or 139 (either threshold may be used),⁵ with sensitivity of 90% and 80%, respectively, is abnormal and necessitates a 3–hour GTT (with 100–g carbohydrate load).

The Fourth International Workshop–Conference on Gestational Diabetes Mellitus established more stringent diagnostic criteria, which are based on the 3–hour GTT and require that 2 of the following 4 criteria to be met:

• Fasting serum glucose concentration >95 mg/dL (5.3 mmol/L).
• One–hour serum glucose concentration >180 mg/dL (10 mmol/L).
• Two–hour serum glucose concentration >155 mg/dL (8.6 mmol/L).
• Three–hour serum glucose concentration >140 mg/dL (7.8 mmol/L).

In pregnancy, mildly abnormal glucose levels can lead to fetal complications, which is why fasting serum glucose greater than 95 mg/dL is considered abnormal and treatment is started very early. Treatment should be considered in women with fasting glucose greater than 90 mg/dL due to the increased risk of macrosomia, which can occur even with a normal GTT.⁶