What does the Glucose, Serum (Fasting) test measure?

Why is Glucose, Serum (Fasting) measured?

Fasting glucose is the primary first-line screen for diabetes and prediabetes alongside HbA1c in clinical practice — a fasting glucose of 100–125 mg/dL on two occasions confirms impaired fasting glucose (IFG, a prediabetes state), and ≥126 mg/dL on two occasions confirms type 2 diabetes per ADA and WHO diagnostic criteria. In the CMP, fasting glucose also reflects glucocorticoid status (cortisol excess elevates fasting glucose), renal gluconeogenic capacity, and the metabolic risk context for interpreting other panel parameters — particularly eGFR, albumin, and liver enzymes in the context of metabolic syndrome.

Reference ranges

Standard reference

Fasting: 70–99 mg/dL (3.89–5.49 mmol/L); Impaired fasting glucose (prediabetes): 100–125 mg/dL; Diabetes: ≥126 mg/dL on two occasions # REVIEWER FLAG: Confirm Quest Diagnostics reference range for fasting serum glucose and whether they report ADA diagnostic category labels (normal, prediabetes, diabetes) alongside the numerical value, or provide a single numerical reference range.

Functional medicine optimal

70–85 mg/dL (functional medicine and preventive cardiology optimal for minimal glycation-related risk and optimal insulin sensitivity; metabolic syndrome risk increases progressively above 85 mg/dL even within the 'normal' range). # REVIEWER FLAG: Not a formal clinical guideline intervention threshold — reviewer to confirm appropriate framing.

Units

mg/dL, mmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.