Hyperglycaemia (a high blood sugar level) occurs when the body can't remove glucose from the blood and turn it into energy. It usually only happens in people with diabetes.

The symptoms of hyperglycaemia tend to develop over several days or weeks, and can include:

• increased thirst
• a dry mouth
• needing to urinate frequently, particularly at night
• tiredness
• recurrent infections, such as thrush

Over time there may be further symptoms including weight loss and blurred vision.

If you haven't been diagnosed with diabetes, visit your GP if you or your child experience symptoms of hyperglycaemia. Your GP will usually be able to diagnose the condition based on your symptoms. They may confirm the diagnosis by testing the level of glucose (sugar) in your blood.

What causes hyperglycaemia?

Hyperglycaemia usually occurs in people with diabetes because people with the condition have problems with insulin – the hormone that helps remove glucose from the blood and converts it to energy.

There are two types of diabetes, described below.

• Type 1 diabetes (insulin-dependent) – where the body produces little or no insulin. People with type 1 diabetes require lifelong treatment to replace the insulin. They also need to check their blood glucose level regularly to prevent complications developing.
• Type 2 diabetes (non-insulin dependent) – where either the body does not make enough insulin, or it cannot use the insulin properly (insulin resistance). This type of diabetes is often linked to obesity or being overweight and mostly occurs in people aged over 40.

If you have diabetes, there are some situations that can trigger an increase in blood glucose, including stress, missing a dose of insulin, eating too much or being ill.

Read more about the causes of hyperglycaemia.

Preventing and treating hyperglycaemia

If you have diabetes, your diabetes care team will explain how to monitor and manage your blood glucose levels. If you have type 1 diabetes, it is important not to miss or alter your dose of insulin and to maintain your fluid and food intake.

If hyperglycaemia occurs, your blood glucose levels will need to be lowered again following the advice of your care team. Increasing your dose of insulin or making changes to your diet are two ways of doing this.

It is important that you test your blood glucose levels regularly. You will be advised about when and how often your blood should be tested.

Contact your diabetes care team for further advice if you regularly experience episodes of hyperglycaemia.

Read more about how hyperglycaemia is treated.

When to seek urgent medical attention

You should seek immediate medical attention if you, or a friend or family member with diabetes, experience the following symptoms:

• nausea or vomiting 
• stomach pain
• a fruity smell on your breath, which may smell like pear drops or nail varnish
• drowsiness or confusion
• rapid breathing (hyperventilation)
• dehydration (signs of which include a headache, dry skin, and a weak, rapid heartbeat)
• loss of consciousness

These symptoms may be a sign of diabetic ketoacidosis, a serious and potentially life-threatening complication of hyperglycaemia.

Read more about the complications of hyperglycaemia.