High blood sugar (hyperglycemia): causes, signs and what to do

High blood sugar mainly affects people living with diabetes. This article explains what it is, why it happens, what you can check yourself and when to get help. It draws on MedlinePlus and the NHS.

1. Introduction

High blood sugar is also called high blood glucose or hyperglycemia (the medical word). Glucose is the sugar that gives the body energy. MedlinePlus explains that insulin, a hormone, normally moves glucose out of the blood and into muscle and fat cells ^[1]. When the body makes too little insulin, or the cells do not respond to it well, glucose stays in the blood and levels rise ^[1].

The NHS says high blood sugar mainly affects people with diabetes and can become serious if it is not dealt with ^[2]. The NHS also describes diabetes as a condition in which the body makes too little insulin, none at all, or cannot use it properly, which is called insulin resistance ^[3].

2. Signs and symptoms

The NHS notes that symptoms often develop slowly and may only show up when blood sugar is very high ^[2]. Common signs listed by the NHS and MedlinePlus include ^{[1, 2]}:

• Feeling very thirsty or having a dry mouth
• Passing urine a lot
• Feeling weak or tired
• Blurred vision
• Dry skin
• Weight loss

MedlinePlus adds that very high or long-lasting high blood sugar can weaken the immune system, which raises the chance of infections ^[1]. Because symptoms can be easy to miss, checking blood sugar as your care team advises is the only sure way to know your level.

3. Causes and triggers

In people with diabetes, the NHS lists several common triggers ^[2]:

• Being unwell
• Stress
• Eating too much sugary or starchy food
• Being less active than usual
• Missing diabetes medicine

The NHS also mentions diabetes medicine that is not working well, some other medicines such as steroids, and recent surgery ^[2]. MedlinePlus agrees that stress from surgery, infection or trauma can push blood sugar up for a while, and that it usually settles once the stress passes ^[1].

What to check when readings are high

MedlinePlus suggests thinking through a short checklist ^[1]:

• Food: have you eaten more than planned, or more carbohydrate (starch and sugar) than usual?
• Medicines: has your doctor changed them, and are you taking them as directed? If you use insulin, check it has not expired and has been stored correctly.
• Injection sites: injecting into a scar, a lump or a numb area can affect how insulin works, so rotate the sites.
• Other changes: less activity, illness with fever, dehydration, stress, weight gain or not checking your blood sugar regularly.

4. Blood sugar numbers

The NHS gives a guide, and says levels can differ by age and type of diabetes ^[2]. In general, a home reading above 7 mmol/L on waking or before eating, or above 11 mmol/L at least 90 minutes after eating, is described as high. Your own targets are set with your care team, so use their numbers first.

A blood test called HbA1c shows average blood sugar over about two to three months. The NHS lists 48 mmol/mol (6.5%) or over as a high level ^[2].

5. Why repeated high levels matter

The NHS says occasional, slight and short-lived highs are usually not serious. Long periods of high blood sugar can damage nerves in the hands and feet, harm the eyes (diabetic retinopathy) and raise the risk of heart disease and stroke ^[2]. A very serious problem called diabetic ketoacidosis can also develop ^[2]. Ketones are acids the body makes when it breaks down fat for energy because it lacks insulin. The NHS says your care team may ask you to test your blood or urine for ketones, and a high level is a sign of ketoacidosis ^[2].

MedlinePlus gives a general guide for testing urine ketones when high readings have no clear reason: after about 3 days above goal in type 2 diabetes, or about 6 hours above goal in type 1 diabetes. If ketones are present, it advises contacting your provider or getting emergency care ^[1]. Ask your own doctor which rule applies to you.

6. Lowering the risk

Steps named by MedlinePlus and the NHS include ^{[1, 2]}:

• Take your diabetes medicines as your doctor directs, and do not skip or change them unless your care team tells you to.
• Follow your meal plan and avoid eating too much sugary or starchy food.
• Stay physically active.
• Manage stress.
• Aim for a healthy weight.
• Agree target blood sugar levels, and how often to check, with your provider.
• Follow your "sick day rules" when you are unwell.

7. When to see a doctor

Contact your care team or GP surgery if your blood sugar stays high, or symptoms continue after you have tried the steps above ^[2]. MedlinePlus says to call your provider when readings are too high or too low and you do not understand why ^[1]. The NHS also advises seeing a doctor if you have symptoms of high blood sugar but have not been told you have diabetes ^[2].

Seek urgent help if high blood sugar comes with any of these, according to the NHS ^[2]:

• Feeling sick, being sick or stomach pain
• Breathing faster, or a faster heartbeat than usual
• Feeling drowsy or finding it hard to stay awake
• Breath that smells fruity
• Confusion or difficulty concentrating
• High ketone levels in blood or urine

The NHS advises not to drive yourself to hospital: ask someone to take you or call an ambulance, and bring your medicines ^[2]. In an emergency, call your local emergency number (112 in India, 911 in the United States).

8. Conclusion

High blood sugar is common in diabetes and often has an identifiable trigger, such as illness, stress, missed medicine or a change in food or activity. Short, mild rises are usually not dangerous, but readings that stay high, or high readings with vomiting, drowsiness, confusion or fast breathing, need medical attention. Your doctor decides any change to your treatment.