⚕️ Educational content only. This article explains how blood glucose tests work and what the results generally indicate. It is not medical advice. Speak with your doctor or healthcare provider for interpretation in the context of your personal health.
Blood glucose tests measure the concentration of glucose (sugar) in the blood. Glucose is the body’s primary energy source, and its regulation involves a complex interaction between insulin, glucagon, and other hormones. Abnormal blood glucose — whether chronically high (hyperglycaemia) or occasionally low (hypoglycaemia) — is associated with serious health conditions, most notably diabetes mellitus. Blood glucose testing is fundamental to the diagnosis, monitoring, and management of diabetes and prediabetes.
Key Takeaways
- Fasting plasma glucose measures blood glucose after at least 8 hours without food or drink (other than water).
- A fasting glucose ≥11.1 mmol/L (200 mg/dL) with symptoms, or ≥7.0 mmol/L (126 mg/dL) on two separate occasions, is diagnostic of diabetes.
- The oral glucose tolerance test (OGTT) is the most sensitive test for diagnosing type 2 diabetes and gestational diabetes.
- HbA1c measures average blood glucose over approximately 2–3 months and is used both to diagnose diabetes and to monitor long-term glucose control.
- Prediabetes (impaired fasting glucose or impaired glucose tolerance) indicates elevated glucose below the diabetes threshold — an important risk state for intervention.
Fasting Plasma Glucose
Fasting plasma glucose (FPG) is measured after at least 8 hours of fasting (no food or caloric drinks). It reflects the baseline glucose level when insulin demand is at its lowest and gives a standardised snapshot of glucose regulation. Diagnostic thresholds (WHO 2006 and updated guidelines): normal fasting glucose is below 6.0 mmol/L (108 mg/dL); impaired fasting glucose (IFG, prediabetes) is 6.0–6.9 mmol/L (108–125 mg/dL); diabetes is ≥7.0 mmol/L (126 mg/dL) on two separate occasions (or once if symptoms are present).
Random Plasma Glucose
Random plasma glucose is measured at any time of day, regardless of when the person last ate. It is less standardised than fasting glucose but is useful for detecting markedly elevated glucose in symptomatic patients. A random glucose ≥11.1 mmol/L (200 mg/dL) in the presence of classic diabetes symptoms (polyuria, polydipsia, unexplained weight loss) is sufficient to diagnose diabetes without a second confirmatory test. Random glucose is also measured in hospitalised patients to monitor for hyperglycaemia.
Oral Glucose Tolerance Test (OGTT)
The OGTT is the most sensitive test for diagnosing diabetes and glucose intolerance. The patient fasts overnight, a fasting blood glucose is measured, then they consume a standard glucose load (75 g dissolved in water), and blood glucose is measured again 2 hours later. The 2-hour post-load glucose result identifies people whose glucose regulation is impaired under challenge, even if fasting glucose is normal. Diagnostic thresholds for the 2-hour post-load result: normal is below 7.8 mmol/L; impaired glucose tolerance (IGT, prediabetes) is 7.8–11.0 mmol/L; diabetes is ≥11.1 mmol/L. The OGTT is also used to screen for gestational diabetes mellitus (GDM) in pregnancy, typically performed between 24 and 28 weeks of gestation, using a 75 g load with glucose measured at fasting, 1 hour, and 2 hours (WHO 2013 criteria).
HbA1c for Diagnosis and Monitoring
HbA1c (glycated haemoglobin) measures the percentage of haemoglobin that has been glycated (bound to glucose) and reflects average blood glucose over approximately 2–3 months. It is now widely used both for diabetes diagnosis and for long-term monitoring. For diagnosis: HbA1c ≥48 mmol/mol (6.5%) is diagnostic of diabetes; HbA1c 42–47 mmol/mol (6.0–6.4%) indicates high risk (prediabetes). For monitoring: in people with diagnosed diabetes, HbA1c is typically measured every 3–6 months. For more detail on HbA1c, see the dedicated HbA1c article on this site.
Limitations and Conditions Affecting Results
HbA1c is unreliable in conditions affecting red blood cell turnover, including haemolytic anaemia, iron deficiency anaemia, and haemoglobin variants — in these situations, fasting glucose or OGTT is preferred for diagnosis. Stress hyperglycaemia occurs during acute illness or surgery when blood glucose rises temporarily due to stress hormones; this does not indicate diabetes. Certain medications (particularly corticosteroids, antipsychotics, and some immunosuppressants) can elevate blood glucose. Glucose results from capillary blood (fingerstick) and venous plasma differ slightly — diagnostic criteria use venous plasma glucose.
Hypoglycaemia
Low blood glucose (hypoglycaemia) is typically defined as a plasma glucose below 4.0 mmol/L (72 mg/dL), though the threshold for symptoms varies between individuals. In people taking insulin or certain diabetes medications, hypoglycaemia is a significant clinical risk. Symptoms include sweating, tremor, confusion, and in severe cases, loss of consciousness. Non-diabetic hypoglycaemia can occur in fasting states, alcohol excess, insulin-secreting tumours (insulinomas), and adrenal insufficiency.
References
- National Library of Medicine. Blood Glucose Test. MedlinePlus. https://medlineplus.gov/lab-tests/blood-glucose-test/
- NHS. Type 2 diabetes — Diagnosis. https://www.nhs.uk/conditions/type-2-diabetes/getting-diagnosed/
- World Health Organization. Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus. WHO. 2011.
- American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Diabetes Care. 2024;47(Suppl 1).
- NICE Guideline NG28. Type 2 diabetes in adults: management. National Institute for Health and Care Excellence. 2022.