Blood test
Blood Glucose Test
A blood glucose test measures the amount of sugar in your blood. It is used to diagnose and monitor diabetes, and can be taken fasting or as part of a glucose-tolerance test.
Quick answer
Blood Glucose Test: what it is and what the results mean
A blood glucose test measures the level of sugar (glucose) in the blood at a given time. It may be done fasting, at random, or as an oral glucose tolerance test (OGTT) where sugar is measured before and after a sugary drink.
- Why it is done: It is used to diagnose diabetes and prediabetes, to screen for gestational diabetes in pregnancy, and to help people with diabetes monitor and manage their blood sugar, often alongside the HbA1c test.
- Understanding results: Higher glucose levels, especially fasting, can indicate prediabetes or diabetes according to defined thresholds.
What it is
A blood glucose test measures the level of sugar (glucose) in the blood at a given time. It may be done fasting, at random, or as an oral glucose tolerance test (OGTT) where sugar is measured before and after a sugary drink.
Why it is done
It is used to diagnose diabetes and prediabetes, to screen for gestational diabetes in pregnancy, and to help people with diabetes monitor and manage their blood sugar, often alongside the HbA1c test.
What to expect
For a fasting test or an OGTT you are asked not to eat for a period beforehand. Blood is taken from the arm, and for an OGTT a second sample is taken about two hours after a measured sugary drink. Some people also self-test with a fingerprick monitor.
Understanding the results
Higher glucose levels, especially fasting, can indicate prediabetes or diabetes according to defined thresholds. A single high random reading is usually confirmed with a fasting test, OGTT or HbA1c before a diagnosis is made.
Good to know
Risks and limitations
Risks are those of a routine blood test. A single glucose reading captures one moment and can be affected by recent food, stress or illness, so diagnosis usually relies on more than one measurement.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
Blood Glucose Test: frequently asked questions
What is a normal fasting blood sugar?
A normal fasting glucose is generally below the diabetes and prediabetes thresholds your clinician uses; higher values may indicate prediabetes or diabetes and are usually confirmed with a repeat or an HbA1c test.
What is an oral glucose tolerance test?
An OGTT measures blood sugar before and about two hours after a measured sugary drink. It is used to diagnose diabetes and, commonly, gestational diabetes in pregnancy.
Related tests
Other blood test investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Royal College of Pathologists / relevant professional body
Building patient-education content for tests or procedures?
We create clear, accurate, referenced medical explainers and decision aids for teams.