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Blood Glucose Test
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Blood Glucose Test

A blood glucose test measures the sugar levels in your blood to screen for, diagnose, or monitor conditions like prediabetes and diabetes.

What is a blood glucose test?

A blood glucose test measures the glucose levels in your blood. Glucose is your body's main source of energy, derived from the food you eat. A hormone called insulin helps move glucose into your cells. If you have too much or too little glucose, it may be a sign of a medical condition.

High blood glucose (hyperglycemia) may indicate prediabetes or diabetes. Low blood glucose (hypoglycemia) is often caused by diabetes medications, though it can also result from certain diseases or other medications.

Why do I need a blood glucose test?

Your provider may order this test as part of a routine checkup or if you have symptoms of high or low glucose. You are at higher risk for type 2 diabetes if you are over 35, overweight, have a family history of diabetes, or have conditions like high blood pressure or non-alcoholic fatty liver disease.

Symptoms of high blood glucose include increased thirst, frequent urination, blurred vision, fatigue, and slow-healing sores. Symptoms of low blood glucose include shakiness, hunger, dizziness, confusion, headache, and a fast heartbeat.

Types of blood glucose tests

  • Fasting blood glucose test: Measures glucose after at least 8 hours of fasting.
  • Random blood glucose test: Can be done at any time, regardless of when you last ate.
  • Glucose challenge test: Used during pregnancy to screen for gestational diabetes.
  • Oral glucose tolerance test (OGTT): Used to diagnose gestational diabetes or confirm prediabetes/type 2 diabetes.
  • Hemoglobin A1C (HbA1c) test: Provides an average of your blood glucose levels over the past 3 months.

Managing glucose at home

If you have diabetes, your provider may recommend home testing using blood glucose meters (finger pricks) or continuous glucose monitors (CGM), which use a sensor under the skin to track glucose levels automatically.

Frequently Asked Questions

You should not eat or drink anything except water for at least 8 hours before the test, usually overnight. Avoid heavy exercise, alcohol, and smoking the night before, as they can affect glucose levels. Take any prescribed diabetes medications as directed by your doctor, unless you’re told to skip them.
Most guidelines recommend measuring HbA1c every 3‑6 months while you’re working on lifestyle changes or medication. If your levels stay stable and you’re maintaining weight, diet, and activity goals, your doctor may extend the interval to once a year.
fasting glucose below 100 mg/dL (5.6 mmol/L) is considered normal. Levels between 100‑125 mg/dL (5.6‑6.9 mmol/L) suggest prediabetes. A result of 126 mg/dL (7.0 mmol/L) or higher on two separate occasions meets the diagnostic criteria for diabetes.
random glucose of 180 mg/dL (10 mmol/L) or higher, especially if you have symptoms of hyperglycemia (excessive thirst, frequent urination, blurred vision), warrants a prompt call to your clinician. They may order a repeat test, an HbA1c, or an oral glucose tolerance test to confirm a diagnosis.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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