Did you know that you have sugar in your bloodstream at all times? In fact, your blood sugar levels fluctuate over the course of the day.
Levels are higher right after meals (as carbohydrates are broken down into glucose and enter the bloodstream through the small intestine) and lower after exercise (when glucose has been burned to fuel the activity).
In someone who doesn't have diabetes, blood sugar levels stay within a narrow range. Between meals, the concentration of sugar in the blood ranges from about 60 to 100 mg/dl (milligrams per deciliter).
After meals it may reach 120 to 130 mg/dl, but rarely goes higher than 140 mg/dl.
But if you have type 2 diabetes, blood sugar levels can go much higher — to 200, 300, or even 400 mg/dl and beyond — and will go much higher unless you take the necessary steps to bring them down.

Diagnosing Type 2 Diabetes

All types of diabetes are diagnosed on the basis of hyperglycemia. Several different types of blood tests may be used to make the diagnosis, including:
  • fasting plasma glucose test
  • random glucose test
  • HbA1c test
  • oral glucose tolerance test
When screening for diabetes in a person with risk factors for type 2 diabetes, doctors generally order either a fasting plasma glucose test (which requires fasting for 8 to 10 hours) or an HbA1c test (which does not require fasting).
A fasting plasma glucose test result of 126 mg/dl or higher indicates diabetes, as does an HbA1c test result of 6.5 percent or higher.
A fasting plasma glucose test result between 100 and 125 mg/dl and an HbA1c test result between 5.7 percent and 6.4 percent indicate prediabetes, which is associated with a high risk of developing type 2 diabetes in the future.
A random glucose test may be done if you have signs or symptoms of diabetes. A test result of 200 mg/dl or higher suggests diabetes and should be confirmed with another diagnostic test on another day. (In fact, all the tests noted above should be confirmed on another day before making a diagnosis of diabetes or prediabetes).

Causes of Hyperglycemia

Once you've been diagnosed with type 2 diabetes, a treatment plan is put in place to lower blood sugar and keep it as close to the normal range as possible.
But even after you start treatment, you may still develop hyperglycemia at times.
When you have diabetes, it's almost impossible not to have hyperglycemia — and high blood sugar can happen for no identifiable reason. Luckily, with enough detective work, a likely cause for high blood sugar can be found.
Some of the reasons blood sugar may go too high include:
  • Missing prescribed medicines or taking medication at the wrong times or in the wrong amounts
  • High food intake or larger consumptions of carbohydrate than expected or intended
  • Lack of sleep
  • Emotional stress
  • Intense exercise
Illness is another important — and common — cause of hyperglycemia. The stress of an illness (cold or flu) or infection can cause your body to release stress hormones, which raise blood sugar.
So even if you aren't eating much because you feel sick, it's still necessary to check blood sugar levels and take diabetes medicines.
You should also ask your doctor about how to take your diabetes medications if you aren't eating properly. You may develop hypoglycemia (low blood sugar) if you keep taking your medication without eating right — especially sulfonylureas such as glipizide (Glucotrol), glyburide (Micronase, Glynase, and Diabeta), and glimepiride (Amaryl).
You may need to use insulin temporarily during an illness, even if it's not part of your usual diabetes regimen.
This is why it's especially important to work with your diabetes care team to develop a sick-day plan, which includes instructions and supplies for checking blood sugar levels, taking medicines and calling for help when necessary.

When Hyperglycemia Is a Medical Emergency

Extreme hyperglycemia can lead to hyperosmolar hyperglycemic state (HHS), which is a medical emergency that must be treated in a hospital.
In HHS, the blood sugar level exceeds 600 mg/dl and may reach as high as 2,000 mg/dl. The very high blood sugar level is accompanied by dehydration, and the blood literally becomes thick, as the sugar concentration rises and the water content decreases.

HHS usually develops over several days or even weeks. It can be prevented by checking your blood sugar level regularly and by treating very high levels by taking rapid-acting insulin and drinking water.