Who should be tested for diabetes? Screening and diagnosis

Many people with diabetes or prediabetes feel fine, so symptoms alone are not a reliable guide. Blood tests are how doctors find out. This article explains which tests are used, who should be tested and what to expect, using information from health agencies.

Why testing matters

The NHS notes that not everyone with diabetes has symptoms, and recommends seeing a GP if you are at higher risk. The NIDDK says home glucose meters cannot diagnose diabetes, so people should not try to diagnose themselves. Only a doctor can diagnose diabetes, prediabetes or gestational diabetes using blood tests.

Who should be tested?

The NIDDK says testing is appropriate for:

• Anyone with symptoms of diabetes
• People with risk factors for type 2 diabetes, even without symptoms
• Most pregnant women, to check for gestational diabetes

It also says adults with normal results should be retested every 3 years, and adults and children with prediabetes should be tested yearly. For children and teens aged 10 to 18 who have overweight or obesity plus another risk factor, the NIDDK says testing for type 2 diabetes is recommended.

The main tests

• Fasting plasma glucose: measured in the morning after at least 8 hours without food or drink, except sips of water.
• A1C: shows average blood glucose over about 3 months. No fasting is needed. The NIDDK says it may be unreliable in the second or third trimester of pregnancy, with certain anaemias or with some haemoglobin variants.
• Random plasma glucose: can be done at any time and is used mainly when symptoms are present and fasting is not practical.
• Oral glucose tolerance test: needs 8 hours of fasting, then a sugary drink, with blood drawn at 2 hours. The NIDDK says it is more expensive and less convenient than other tests.

What do the numbers mean?

For non-pregnant adults, the NIDDK, citing the American Diabetes Association, gives these cut-offs:

• Normal: A1C below 5.7%, fasting glucose 99 mg/dL or below, 2-hour result 139 mg/dL or below
• Prediabetes: A1C 5.7% to 6.4%, fasting glucose 100 to 125 mg/dL, 2-hour result 140 to 199 mg/dL
• Diabetes: A1C 6.5% or above, fasting glucose 126 mg/dL or above, 2-hour result or random result with symptoms 200 mg/dL or above

The NIDDK says pregnancy uses different cut-offs and that a doctor usually confirms diabetes with a second test.

Testing in pregnancy

The NIDDK says women planning pregnancy who have risk factors should be tested for type 2 diabetes, and women who are already pregnant may be tested at the first prenatal visit or within the first 15 weeks. Pregnant women without a prior diagnosis are tested for gestational diabetes at 24 to 28 weeks. A glucose challenge test uses a sweet drink with blood drawn an hour later. After gestational diabetes, testing for type 2 diabetes is recommended, usually within 12 weeks after delivery. The NHS says to expect a test 6 to 13 weeks after birth.

Finding out the type

Blood sugar tests show that diabetes is present but not which type. The NIDDK says autoantibody tests can suggest type 1 diabetes, and genetic testing can identify most forms of monogenic diabetes, which are more common in babies diagnosed before 6 months of age and in some teens and young adults.

When to see a doctor

See a doctor and ask about testing if you have risk factors or signs such as strong thirst, frequent urination, tiredness, blurred vision, slow-healing sores or weight loss you cannot explain. For suspected type 1 diabetes in a child or adult, the NHS advises an urgent appointment or a call to NHS 111.

Seek urgent care for severe symptoms such as trouble breathing, fruity-smelling breath, confusion, fainting, or stomach pain with vomiting. In an emergency, call your local emergency number (112 in India, 911 in the United States).

This article is for education only and does not replace advice from your own doctor.