A blood test is the only way to know for sure that you have diabetes. Anyone can develop diabetes, and diabetes symptoms can be tricky to spot. Finding out you have diabetes before it gets worse is the best way to control it and prevent problems with your eyes, nerves, and feet, as well as other complications.
This guide explains why diabetes testing takes place, which tests a doctor will use, what the results mean, how to access a diabetes test, what happens next, and the difference between testing and screening.
Three key takeaways
Diabetes is diagnosed using blood tests. Repeat testing is crucial for accurately tracking how your blood sugar rises and falls.
Different tests measure blood sugar in different ways. While home tests can give a picture of your blood sugar levels, only a test given by your doctor can diagnose diabetes.
Early testing can help identify diabetes or prediabetes sooner and help you prevent diabetes from getting worse.
How do you test for diabetes?
A diabetes test involves a blood test that measures your blood sugar. If the test results show that your blood sugar is high over a long period, your doctor will diagnose diabetes. Home testing kits can give a picture of your blood sugar levels, but only tests sent to a lab can diagnose diabetes.
Diabetes testing might be recommended if you have symptoms that could result from high blood sugar. These can include:
Extreme thirst
The frequent need to wee
Losing weight without trying or feeling constantly tired
Itchy genitals or repeated urinary infections
Infections that keep coming back
Blurry eyesight
However, many people with diabetes don’t have any symptoms despite being at high risk of diabetes. A doctor might suggest screening tests to check whether further testing is needed. If the following sounds like you, you might benefit from diabetes screening:
You already have high blood pressure or heart disease.
You have obesity or a body mass index (BMI) of 30 or higher.
You’re from a Black African, African Caribbean, or South Asian background.
Your siblings, parents, or grandparents have diabetes.
You take anti-psychotic medications like Olanzapine or Clozapine long-term.
You have polycystic ovary syndrome.
You’ve already had diabetes due to pregnancy, known as gestational diabetes.
You have prediabetes, meaning you’re at risk of developing diabetes
Also, people with learning disabilities have higher rates of obesity and heart disease, which increases diabetes risk.
People with a high diabetes risk should generally get screened once yearly.
Why test for diabetes?
Testing for diabetes can help get the right treatment for your type of diabetes and prevent complications. A diabetes test is a quick, simple way to start getting the care you need.
Testing for diabetes can help you spot it early. Uncontrolled type 2 diabetes can affect a whole range of organs, including your nerves, eyes, heart, blood vessels, and kidneys. By getting help before diabetes causes these problems.
And while type 1 diabetes causes more obvious symptoms, like unwanted or unexpected weight loss, frequent urination, and extreme thirst and tiredness, ignoring these symptoms can lead to a dangerous condition called diabetic ketoacidosis.
A diabetes test can also show you whether you have prediabetes, which suggests a high risk of developing type 2 diabetes. Getting a prediabetes diagnosis can help you understand what lifestyle changes you can make to control your blood sugar and prevent type 2 diabetes.
“One of the biggest reasons to get tested for diabetes is simply knowing where you stand. Not knowing can feel really worrying - you might have symptoms and spend ages wondering 'what if?' or convincing yourself not to bother getting checked. A test gives you clear answers, and that means you can stop worrying and take action.
“Testing is also really straightforward - it's usually just a finger-prick blood test or a blood sample from your arm. The few minutes it takes could literally add years to your life and help you avoid serious health problems down the line. Think of it as looking after future you.”
Ian Budd, Prescribing Pharmacist at C4U
Blood tests used to diagnose diabetes
The main tests for diagnosing diabetes include the HbA1c test, a non-fasting blood sugar test, and a fasting blood sugar test.
HbA1c test
An HbA1c test shows your average blood sugar level over the last 8 to 12 weeks. It tests a substance called glycated haemoglobin (HbA1c), which forms when blood sugar sticks to your red blood cells. After 8 to 12 weeks, the body replaces red blood cells with new ones.
The more blood sugar you have over that time, the higher the HbA1c count will be. You don’t need to fast, as the test gives a snapshot of your average glucose levels over time, so what you eat on the day wouldn’t make a difference to your results.
Results usually take a few days to come through, and your doctor can diagnose diabetes based on them.
Non fasting blood tests
Another test you don’t need to fast before taking is the random plasma glucose test. If you’ve got severe diabetes symptoms, this can occur at any time of day. Unlike the HbA1c test, this only shows you your blood glucose at that moment in time.
A random plasma glucose test is useful if you already know you have diabetes and are keeping an eye on daily blood glucose changes, or if you’re confirming the results of an HbA1c test. However, if you’re trying to find out about diabetes for the first time, only the HbA1c shows long-term, raised blood sugar.
Home random glucose testing is available using a finger-prick monitor that gives you a reading there and then. However, you’ll need to have another test using blood collected from the arm that your doctor sends to a laboratory after a finger-prick test. This can confirm your readings.
Fasting blood sugar test
For a fasting plasma glucose test, you’ll need to avoid eating or drinking anything except water for eight hours before a nurse or doctor collects blood from your arm. The easiest way to get this test is in the morning, after fasting overnight.
What you eat and drink can affect your blood sugar reading. Fasting is needed so your doctor can see how your blood sugar changes on its own.
If you have high blood glucose after a fasting period, you have a high risk of diabetes. In healthy people, blood sugar levels will stay much lower after a fasting period.
Testing for type 1 and type 2 diabetes
Tests for type 1 and type 2 diabetes can differ, and doctors use more than just a single blood sugar result to make a diagnosis. While blood tests can confirm high blood sugar in both types, doctors also look at other factors to decide which type of diabetes someone has. These include:
How quickly symptoms developed
Age at diagnosis
Body weight
Family history, particularly of type 1 diabetes
If a doctor suspects type 1 diabetes, they will usually request additional blood and urine tests. These may include tests for ketones, substances that build up in the blood when the body does not have enough insulin. High ketone levels can be a sign that the body is breaking down fat for energy and may indicate a medical emergency.
If ketone levels are raised, your doctor may refer you urgently to hospital for further tests and treatment.
Doctors may also use other specialist tests to help confirm the type of diabetes. These can include tests for diabetes-related autoantibodies and C-peptide, which shows how much insulin the body is producing. These tests help distinguish between type 1 and type 2 diabetes when the diagnosis is unclear.
Blood sugar test results and what they mean
Your diabetes test results can help you work out whether you have diabetes, how severe it is, and what to do next.
What if my blood sugar levels are too high?
If an initial blood test shows high blood sugar and you do not have clear symptoms, your doctor will usually repeat the test to confirm the result. A raised result on 2 separate tests is needed to diagnose diabetes.
One common test used is the HbA1c test, which measures average blood sugar levels over the past 2–3 months. HbA1c results are measured in millimoles per mole (mmol/mol).
An HbA1c result of 48 mmol/mol or higher means diabetes is diagnosed
A result between 42 and 47 mmol/mol indicates prediabetes
Prediabetes means blood sugar levels are higher than normal but not high enough for a diabetes diagnosis. It is an important warning sign, as early changes to diet, activity, and weight can help prevent or delay type 2 diabetes.
What if my blood sugar levels are too low?
Low blood sugar, also known as hypoglycaemia or a “hypo,” happens when blood glucose drops too low. It is most common in people who take insulin or certain diabetes medications.
Low blood sugar can happen if you take too much medication, miss a meal, exercise more than usual, or inject insulin into the same area repeatedly. Drinking alcohol without eating can also increase the risk.
Symptoms of low blood sugar can include:
Dizziness or shaking
Extreme tiredness
Blurred vision
A fast or pounding heartbeat
Confusion or difficulty concentrating
If blood sugar drops very low, it can lead to seizures or loss of consciousness and needs urgent treatment.
Taking your medication exactly as prescribed and checking your blood sugar regularly can help prevent hypos. Learning your early warning signs is also important so you can treat low blood sugar quickly.
Speak to a doctor if you manage diabetes and have frequent hypos, if you do not always recognise when your blood sugar is low, or if you experience symptoms of low blood sugar without having a diabetes diagnosis.
How to test for diabetes at home
Home blood sugar tests are available. While they can be helpful for people who already know they have diabetes, they can’t replace a diagnosis from your doctor.
Some tests, known as fingerprick tests, involve using a tiny needle called a lancet to collect a drop of blood. Then you apply this to a testing strip, which is inserted into a digital monitor. These can be useful for people who already have diabetes or are managing their blood glucose day to day.
Others, known as continuous glucose monitors (CGM), check your blood sugar constantly. Both of these can show your blood sugar level in the moment, but don’t give a clear and accurate picture of your long-term blood sugar control. As such, they can’t diagnose diabetes.
Home HbA1c tests are available, but research has shown that accuracy varies depending on which test you use. They vary in quality because some use capillary blood and may not meet the same lab standards used in NHS testing.
Can an eye test detect diabetes?
Opticians can’t diagnose diabetes. But if undiagnosed diabetes causes eye changes, known as diabetic retinopathy, they might notice these in a standard eye test and refer you to your GP.
A diabetic eye screening can show eye changes linked to diabetes, known as diabetic retinopathy. If your doctor says you have diabetes, the NHS will write to you inviting you to an eye screening every year. This will be reduced to every two years if you have two tests in a row showing no eye changes.
In these, you’ll get eye drops, an eye specialist will check your ability to read letters of different sizes, and they’ll take pictures of the back of your eye, where diabetes-related changes often occur.
Where can I get a free diabetes test in the UK?
In the UK, you can test for diabetes in several ways:
As part of a yearly NHS Health Check
On request from your GP
From a Community Pharmacy
You can get an NHS Health Check if you’re aged between 40 and 74 years and don’t already have a diagnosed health problem like diabetes. Your GP or local council should invite you for a health check every five years, but if they don’t, chat to your clinic for information on local services. Community pharmacies can check blood sugar, but diagnosis must still be confirmed by a GP with laboratory testing.
What happens after a diabetes test?
If your HbA1c test shows high blood sugar but you don’t have symptoms, your doctor will repeat the test to confirm the result. A diagnosis of diabetes is made when two separate HbA1c results are 48 mmol/mol or higher.
If your second result is still raised, but below 48 mmol/mol — usually between 42 and 47 mmol/mol — this means you have prediabetes. In this case, your doctor will usually recommend another HbA1c test in six to twelve months to monitor your risk.
If the repeat result returns to normal, you won’t be diagnosed with diabetes or prediabetes, but your doctor may still suggest future checks if you have risk factors.
If you are diagnosed with diabetes, help is available. Your GP will guide you through lifestyle changes, diet advice, blood sugar monitoring options, regular check‑ins, and medication if needed.
Frequently asked questions about diabetes tests
What is the most accurate test for diabetes?
HbA1c is the preferred test for most people, but your doctor may use other tests in certain conditions.
Can diabetes be diagnosed with one test?
A doctor can diagnose type 2 diabetes after an HbA1c test but may request further testing if they suspect type 1 diabetes based on other factors like age, weight, and
Do I need to fast before a diabetes blood test?
You don’t need to fast before an HbA1c test. However, some types of tests require fasting, but you can do this overnight while you sleep and test in the morning.
Can you test for diabetes without a blood test?
No. Only a blood test can confirm that you have diabetes.
How long does it take to get diabetes test results?
You’ll get results back in a few days.
Should I test for diabetes even if I feel fine?
Yes. Diabetes doesn’t always cause symptoms. If you feel well but have high blood sugar, your GP may ask for a second test to confirm.
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