Chemistry

A fasting plasma glucose (FPG) of 126 mg/dL (7.0 mmol/L) or greater, confirmed on a second occasion, meets the diagnostic threshold for diabetes. Fasting is defined as no caloric intake for at least 8 hours.

A 2-hour plasma glucose of 200 mg/dL (11.1 mmol/L) or greater during a 75 g oral glucose tolerance test (OGTT) is diagnostic for diabetes mellitus.

An HbA1c of 6.5% or higher is diagnostic for diabetes. HbA1c reflects average glycemia over approximately the prior 2-3 months because glucose irreversibly binds the N-terminal valine of the hemoglobin beta chain.

An HbA1c of 5.7% to 6.4% defines prediabetes. The corresponding fasting glucose range (impaired fasting glucose) is 100-125 mg/dL.

HbA1c reflects the average blood glucose over the preceding 2-3 months, corresponding to the roughly 120-day lifespan of red blood cells.

A random plasma glucose of 200 mg/dL or greater in a patient with classic hyperglycemic symptoms (polyuria, polydipsia, unexplained weight loss) is diagnostic. The symptomatic plus random ≥200 mg/dL combination does not require confirmation by fasting.