Fasting Blood Glucose
The primary test for diagnosing diabetes — and the first number to rise in insulin resistance
Fasting glucose is maintained in a tight range by insulin and glucagon. A single elevated result doesn't confirm diabetes — but a fasting level ≥ 7.0 mmol/L on two separate occasions does, and the 5.6–6.9 mmol/L range signals prediabetes with 25% progression to type 2 within 5 years.
Blood glucose is maintained between 3.9–5.6 mmol/L during fasting by the reciprocal action of insulin (suppresses hepatic glucose output, promotes cellular uptake) and glucagon (promotes glycogenolysis and gluconeogenesis). Fasting glucose above 7.0 mmol/L on two tests, or any random glucose ≥ 11.1 mmol/L with symptoms, is diagnostic of diabetes mellitus.
Reference Ranges
How clinicians interpret Fasting Blood Glucose results — from optimal to concerning.
⚠ Reference ranges vary by laboratory and assay. Always interpret your result in context of your laboratory's own reference intervals and your clinical presentation.
What raises Fasting Blood Glucose
Insulin resistance + progressive beta-cell failure. Fasting glucose rises before postprandial glucose in T2DM.
Acute illness, surgery, MI: catecholamine and cortisol release transiently raises glucose — does not diagnose diabetes.
Glucocorticoids promote gluconeogenesis and insulin resistance — steroid-induced diabetes requires monitoring.
Excess cortisol causes hepatic glucose overproduction — fasting glucose typically 7–14 mmol/L.
Chronic pancreatitis or pancreatectomy — endocrine insufficiency causes glucose elevation with malabsorption.
What lowers Fasting Blood Glucose
Most potent glucose-lowering agent. Hypoglycaemia risk requires careful dose titration.
First-line oral agent — reduces hepatic gluconeogenesis and improves insulin sensitivity.
Muscle GLUT4 glucose uptake during exercise lowers glucose — 30 min aerobic exercise reduces glucose 2–4 mmol/L.
Force urinary glucose excretion independent of insulin. Also reduce CV and renal outcomes in T2DM.
Conditions this biomarker signals
When Fasting Blood Glucose is outside normal range, these are the most clinically significant possibilities.
FPG ≥ 7.0 mmol/L on two tests, or ≥ 11.1 with symptoms. Triggers full metabolic workup: HbA1c, lipids, kidney function, urine ACR.
FPG 5.6–6.9 mmol/L: 25% T2DM progression in 5 years. Lifestyle intervention (5–7% weight loss + 150 min/week exercise) reduces progression 58%.
FPG > 14 mmol/L with ketones + acidosis = DKA emergency. Usually type 1 or insulin-deficient type 2.
Recurrent fasting hypoglycaemia < 2.5 mmol/L with high insulin:glucose ratio suggests insulin-secreting tumour.
Which tests measure this biomarker
Fasting Blood Glucose may be included in or ordered alongside these panels.
Standard diagnostic test for diabetes. Even black coffee affects results.
3-month average glucose — more convenient but misses acute hyperglycaemia.
Gold standard — diagnoses impaired glucose tolerance and GDM missed by FPG alone.
How the liver and pancreas maintain fasting glucose
During an overnight fast, hepatic glucose output — first from glycogenolysis, then from gluconeogenesis — is finely controlled by insulin and glucagon. In type 2 diabetes, hepatic insulin resistance causes inappropriate continued glucose output despite rising blood glucose — fasting hyperglycaemia is often the earliest metabolic abnormality, appearing years before postprandial glucose becomes abnormal. The DPP trial proved that 5–7% weight loss + 150 min/week moderate exercise reduces T2DM progression from prediabetes by 58% — more than metformin alone (31%).
422 million people have diabetes globally
WHO estimates 422 million people have diabetes worldwide, 90% type 2. An estimated 240 million (57%) are undiagnosed — because glucose rises slowly and symptoms only appear at very high levels. Fasting glucose screening is the primary tool for identifying undiagnosed cases.
Lifestyle cuts T2DM progression by 58%
The Diabetes Prevention Programme (DPP) RCT showed intensive lifestyle intervention (5–7% weight loss + 150 min/week moderate exercise) reduced T2DM progression from prediabetes by 58% vs control — more effective than metformin (31%). Establishing lifestyle modification as the cornerstone of prediabetes management.
First-phase insulin response is lost early in T2DM
In healthy individuals, glucose peaks at 30–60 minutes post-meal and returns to fasting within 2 hours — driven by a rapid "first phase" insulin release (within 2 minutes). This first-phase response is the earliest loss in type 2 diabetes — causing the characteristic postprandial glucose spike that precedes fasting glucose elevation.
Clinical use — when and why this is ordered
How clinicians use Fasting Blood Glucose in practice — the real-world scenarios where it changes decisions.
Diabetes screening (high-risk adults)
Screen every 3 years from age 45 (earlier if overweight, family history, hypertension, or prior GDM). Earlier if symptomatic.
Gestational diabetes mellitus (GDM)
FPG ≥ 5.1 mmol/L at booking, or ≥ 7.0 at 24–28 weeks OGTT, diagnoses GDM — universal screening at 24–28 weeks in most guidelines.
Inpatient glucose monitoring
Inpatient glucose target 6–10 mmol/L. Persistent hyperglycaemia predicts surgical site infection, poor wound healing, and ICU mortality.
Prediabetes action trigger
FPG 5.6–6.9 mmol/L prompts: HbA1c, lipid panel, BP target, dietitian referral, and structured lifestyle programme enrolment.