According to the ADA, criteria for the diagnosis of diabetes includes:
A fasting plasma glucose level ≥ 126 mg/dL (7 mmol/L); fasting is defined as no caloric intake for at least 8 hours
A 2-hour plasma glucose level ≥ 200 mg/dL (11.1 mmol/L) or during a 75-g oral glucose tolerance test
An A1c level ≥ 6.5% (≥ 48 mmol/mol)
A random plasma glucose level ≥ 200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia or hyperglycemic crisis
Measuring concentrations of insulin or C-peptide (a fragment of proinsulin that serves as a marker for insulin secretion) is rarely necessary to diagnose T2D or differentiate T2D from T1D. Insulin levels generally are high early in the course of T2D and gradually wane over time. A fasting C-peptide level > 1 ng/dL in a patient who has had diabetes for more than 1-2 years is suggestive of T2D (ie, residual beta-cell function).
Learn more about the workup of T2D.
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Cite this: Romesh Khardori, Anne L. Peters. Fast Five Quiz: Type 2 Diabetes Key Aspects - Medscape - Dec 15, 2022.