Diabetes Screening: HbA1c, Fasting Glucose, and Catching Prediabetes Early

Why screen when there are no symptoms

Type 2 diabetes typically develops silently over 5 to 10 years. During that pre-clinical phase — prediabetes — blood glucose is elevated, insulin resistance is building, and low-grade damage to blood vessels, nerves, and kidneys has already begun. There is no symptom to warn you until glucose is high enough to cause classic thirst, urination, and fatigue — usually well after the diagnosis becomes irreversible.

The good news: prediabetes reverts to normal in roughly 40 to 60% of adults who receive structured lifestyle intervention. That is why screening matters.

The three tests that matter

HbA1c (glycated hemoglobin): reflects average blood glucose over the last 2 to 3 months. No fasting required. Normal < 6.0%; prediabetes 6.0–6.4%; diabetes ≥ 6.5%.

Fasting plasma glucose: measured after at least 8 hours without food. Normal < 5.6 mmol/L; prediabetes 5.6–6.9 mmol/L; diabetes ≥ 7.0 mmol/L on two occasions.

Oral glucose tolerance test: a 75g glucose drink followed by a 2-hour measurement — used selectively, especially in pregnancy and in patients with normal HbA1c but strong clinical suspicion.

A modern preventive workup usually pairs HbA1c with fasting glucose.

Who should be screened, and how often

Diabetes Canada recommends screening every 3 years starting at age 40, or earlier and more often for adults with any of: overweight/obesity (BMI ≥ 25), a first-degree relative with type 2 diabetes, ethnicity at higher risk (South Asian, Indigenous, African, Hispanic, Asian), history of gestational diabetes or PCOS, hypertension, dyslipidemia, or a cardiovascular event.

If any of those apply, annual HbA1c is reasonable — the cost of catching it is low, and the cost of missing it is high.

Prediabetes: the window of opportunity

Prediabetes is not a mild version of diabetes — it is a red flag that the metabolic system is under stress. Landmark trials (DPP, Finnish Diabetes Prevention Study) show that a structured lifestyle program — 7% body weight loss and 150 minutes of activity a week — reduces progression to diabetes by 58%. Metformin adds a further modest benefit for those unable to sustain the lifestyle change.

For patients with prediabetes and significant weight to lose, GLP-1 receptor agonists have transformed outcomes — see our GLP-1 medications article and our Weight Management program.

Screen your diabetes risk

HbA1c and fasting glucose are included in every Essential Checkup. Book through the contact form or read our related diabetes blood tests guide.