A blood glucose test measures the amount of glucose in your blood. Glucose, a type of simple sugar, is your body’s main source of energy. Your body converts the carbohydrates you eat into glucose.
The amount of sugar in your blood is usually controlled by a hormone called insulin. However, if you have diabetes, your body either doesn’t make enough insulin or the insulin produced doesn’t work properly. This causes sugar to build up in your blood. Increased levels of blood sugar can lead to severe organ damage if left untreated.
In some cases, blood glucose testing may also be used to test for hypoglycemia. This condition occurs when the levels of glucose in your blood are too low.
Type 1 diabetes is usually diagnosed in children and teenagers whose bodies aren’t able to produce enough insulin. It’s a chronic, or long-term, condition that requires continuous treatment. Late-onset type 1 diabetes has been shown to affect people between the ages of 30 and 40.
Type 2 diabetes is usually diagnosed in overweight and obese adults, but it can develop in younger people as well. This condition occurs when your body doesn’t make enough insulin or when the insulin you produce doesn’t work properly. The impact of type 2 diabetes may be reduced through weight loss and healthy eating.
Gestational diabetes occurs if you develop diabetes while you’re pregnant. Gestational diabetes usually goes away after you give birth.
After receiving a diagnosis of diabetes, you may have to get blood glucose tests to determine if your condition is being managed well. A high glucose level in a person with diabetes may mean that your diabetes isn’t being managed correctly.
Other possible causes of high blood glucose levels include:
- hyperthyroidism, or overactive thyroid
- kidney problems
- pancreatitis, or inflammation of your pancreas
- pancreatic cancer
- prediabetes, which happens when you’re at increased risk of developing type 2 diabetes
- stress to the body from illness, trauma, or surgery
It’s also possible to have levels of blood glucose that are too low. However, this isn’t as common. Low blood glucose levels, or hypoglycemia, may be caused by:
- insulin overuse
- hypopituitarism, or underactive pituitary gland
- hypothyroidism, or underactive thyroid
- Addison’s disease, which is characterized by low levels of cortisol
- alcohol abuse
- liver disease
- insulinoma, which is a type of pancreatic tumor
Blood glucose tests are either random or fasting tests.
For a fasting blood glucose test, you can’t eat or drink anything but water for eight hours before your test. You may want to schedule a fasting glucose test first thing in the morning so you don’t have to fast during the day.
You may eat and drink before a random glucose test.
Fasting tests are more common because they provide more accurate results and are easier to interpret.
Before your test, tell your doctor about the medications you’re taking, including prescriptions, over-the-counter drugs, and herbal supplements. Certain medications can affect blood glucose levels. Your doctor may ask you to stop taking a particular medication or to change the dosage before your test temporarily.
Medications that can affect your blood glucose levels include:
- acetaminophen (Tylenol)
- birth control pills
- hormone therapy
- aspirin (Bufferin)
- atypical antipsychotics
- epinephrine (Adrenalin)
- tricyclic antidepressants
- monoamine oxidase inhibitors
- sulfonylurea medications
Severe stress can also cause a temporary increase in your blood glucose and is usually due to one or more of these factors:
You should tell your doctor if you’ve recently had any of these.
This simple test involves giving a small sample of blood.
The sample can most likely be collected with a very simple prick to a finger. If you need other tests, your doctor may require a blood draw from a vein.
Before drawing blood, the healthcare provider performing the draw cleans the area with an antiseptic to kill any germs. They next tie an elastic band around your upper arm, causing your veins to swell with blood. Once a vein is found, they insert a sterile needle into it. Your blood is then drawn into a tube attached to the needle.
You may feel slight to moderate pain when the needle goes in, but you can reduce the pain by relaxing your arm.
When they’re finished drawing blood, the healthcare provider removes the needle and places a bandage over the puncture site. Pressure will be applied to the puncture site for a few minutes to prevent bruising.
The sample of blood is then sent to a lab for testing. Your doctor will follow up with you to discuss the results.
There’s a very low chance that you’ll experience a problem during or after a blood test. The possible risks are the same as those associated with all blood tests. These risks include:
- multiple puncture wounds if it’s difficult to find a vein
- excessive bleeding
- lightheadedness or fainting
- hematoma, or blood collecting under your skin
The implications of your results will depend on the type of blood glucose test used. For a fasting test, a normal blood glucose level is between 70 and 100 milligrams per deciliter (mg/dL). For a random blood glucose test, a normal level is usually under 125 mg/dL. However, the exact level will depend on when you last ate.
If you had a fasting blood glucose test, the following results are abnormal and indicate you have either prediabetes or diabetes:
- A blood glucose level of 100–125 mg/dL indicates that you have prediabetes.
- A blood glucose level of 126 mg/dL and higher indicates that you have diabetes.
If you had a random blood glucose test, the following results are abnormal and indicate you may have either prediabetes or diabetes:
- A blood glucose level of 140–199 mg/dL indicates that you may have prediabetes.
- A blood glucose level of 200 mg/dL and higher indicates that you likely have diabetes.
If your random blood glucose test results are abnormal, your doctor will probably order a fasting blood glucose test to confirm the diagnosis.
If you’re diagnosed with prediabetes or diabetes, you can find more information and additional resources at http://healthline.com/health/diabetes.