A lab test called the glucose tolerance test measures how quickly your body transports sugar from the blood to tissues like muscle and fat. The exam is frequently used to identify diabetes.
The tests used to screen for diabetes during pregnancy are same, but they are performed differently.
Why is the test done?
The sugar that the body utilizes as fuel is called glucose. Untreated diabetic patients have elevated blood glucose levels.
The following first tests are frequently used to identify diabetes in adults who are not pregnant:
- If the fasting blood glucose level is more than 126 mg/dL (7 mmol/L) on two independent tests, diabetes is diagnosed.
- Diabetes is identified if the hemoglobin A1c test result is 6.5% or above.
Diabetes can also be identified via glucose tolerance testing. When a person's fasting blood glucose level is high but not high enough (over 125 mg/dL or 7 mmol/L) to fulfill the diagnosis for diabetes, the OGTT is used to screen for or diagnose diabetes.
An early indication of diabetes than an abnormal fasting glucose is aberrant glucose tolerance, which occurs when blood sugar levels rise excessively following a glucose challenge.
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Get A Second OpinionHow the test is Performed
The oral glucose tolerance test (OGTT) is the most common glucose tolerance test used. Here's how it works:
- Before the test: You must fast for at least 8 hours (no food or drink, except water). A blood sample will be taken first.
- During the test: You will drink a solution containing about 75 grams of glucose. After that, your blood will be drawn every 30 to 60 minutes to check how your body is processing the glucose.
- Duration: The test can take up to 3 hours.
Another variation is the intravenous (IV) glucose tolerance test (IGTT), but it’s rarely used and never for diagnosing diabetes. In the IGTT:
- Glucose is injected into your vein over 3 minutes.
- Blood insulin levels are measured before the injection, and again 1 and 3 minutes after.
What to Do Before the Test
Following are the points to remember before the test:
- Make sure you eat normally for some days before the test.
- At least eight hours before the test, refrain from eating or drinking anything. You cannot eat while taking the exam.
- If any of the medications you take have the potential to alter the outcome of the test, ask your doctor.
How to Prepare for a Glucose Tolerance Test
The glucose solution is equivalent to a highly sweet drink when consumed.
It is extremely rare for this test to have serious negative effects. After consuming the glucose, some patients experience nausea, dizziness, lightheadedness, or even fainting symptoms during the blood test. If you have a history of these symptoms following blood tests or other medical procedures, let your healthcare professional know.
- Inform your provider about all medications and supplements you take, as some can affect the results. Your provider may ask you to stop certain medications before the test.
- Eat normally for at least three days prior to the test, ensuring you consume at least 150 grams of carbohydrates each day.
- Maintain your typical activity level for at least three days before the test and avoid excessive exercise.
- Fast (don’t eat or drink anything except water) for at least eight hours before the test.
- Do not eat or smoke during the test.
- Your healthcare provider will give specific instructions based on your situation and the type of glucose tolerance test you're doing. Be sure to follow them and ask questions if you need clarification.
Risks associated with the test
There is minimal risk when having blood drawn. However, veins and arteries can vary in size from person to person, and from one side of the body to the other, so drawing blood may be easier for some people than others.
Possible, but rare, risks include:
- Excessive bleeding
- Multiple attempts to find a vein
- Feeling lightheaded or fainting
- A hematoma (blood collecting under the skin)
- Infection (a slight risk whenever the skin is punctured)
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Book an AppointmentUnderstanding the results
- Normal Results :
Normal blood values for a 75-gram OGTT used to check for type 2 diabetes in those who are not pregnant:
Fasting - 60 to 100 mg/dL (3.3 to 5.5 mmol/L)
1 hour - Less than 200 mg/dL (11.1 mmol/L)
2 hours - This value is used to make the diagnosis of diabetes.
- Less than 140 mg/dL (7.8 mmol/L) is normal.
- Between 141 mg/dL and 200 mg/dL (7.8 to 11.1 mmol/L) is considered impaired glucose tolerance.
- Above 200 mg/dl (11.1mmol/L) is diagnostic of diabetes.
The mentioned levels serve as standard benchmarks for these tests' outcomes. Different laboratories may have somewhat different normal value ranges. Several laboratories test various samples or use various metrics. Ask your physician what your particular test findings signify.
- GTT Fasting: ≥92 mg/dL (5.1 mmol/L),
- One hour: ≥180 mg/dL (10 mmol/L),
- Two hour: ≥153 mg/dL (8.5 mmol/L)
If your blood glucose level is greater than usual, you may have diabetes or pre-diabetes:
- Impaired glucose tolerance refers to a 2-hour result between 140 and 200 mg/dL (7.8 and 11.1 mmol/L). You are now more likely to eventually get diabetes or you are pre-diabetic.
- Diagnosing diabetes requires a blood glucose level of 200 mg/dL (11.1 mmol/L) or greater.
Your blood glucose level may increase if your body experiences significant stress, such as that brought on by trauma, stroke, heart attack, or surgery. Your blood glucose level can be lowered by vigorous exercise.
Your blood sugar level can be increased or decreased by several medications. Inform your physician of any medications you are taking before the test.
