Difference Between an Oral Glucose Tolerance Test and Fasting Glucose Test

Introduction

Blood sugar is the primary source of energy from the food eaten. The food consumed is broken down to glucose and released into the bloodstream. When the glucose level in the blood increases, the pancreas is stimulated to produce insulin. When the body is not able to produce the required amount of insulin or the insulin produced is not used efficiently by the body, it causes a condition called diabetes. Monitoring blood glucose helps to understand the fluctuations in blood sugar in response to diet, medications, exercise, or other pathological processes.

What Is Oral Glucose Tolerance Test?

The oral glucose tolerance test helps to know how the blood sugar moves from the blood to the tissues, like the muscles and fat in the body. The test is generally used in the diagnosis of diabetes. Screening of impaired glucose tolerance and diabetes mellitus can be done by this test. It checks the insulin sensitivity or insulin resistance of the body. However, since easier methods are available, like fasting blood glucose or the HbA1c, oral glucose tolerance test is usually reserved for specific clinical scenarios like gestational diabetes.

What Are the Indications of an Oral Glucose Tolerance Test?

The oral glucose tolerance test can identify imbalances that are missed by other tests. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) advises glucose tolerance tests for:

• Screening and diagnosis of prediabetes or impaired glucose tolerance.

• Screening and diagnosis of type 2 diabetes.

• Screening and diagnosis of gestational diabetes.

• Reactive hypoglycemia.

• Acromegaly (a condition caused by the hyperactive pituitary gland).

• Beta cell dysfunction.

• Rare disorders that affect carbohydrate metabolism, like fructose intolerance.

How Is a Glucose Tolerance Test Done?

The patient is advised to follow a normal diet for three days prior to the test. It should contain approximately 5.36 ounces (oz) of carbohydrate per day. The patient should be advised to arrive for the test fasting for eight hours. The test should be rescheduled if the patient is not arriving on fast to check the blood sugar levels (unless they are advised for random tests).

The fasting blood sample can be collected, and the time of the test should be noted. The patient is then given a standard amount of glucose (about 2.68 ounces), which should be consumed within a maximum of five minutes. Additional samples can be collected at intervals of 30 minutes, 60 minutes, 90 minutes, and 120 minutes as advised by the healthcare provider. Some healthcare professionals advise collecting a simple sample at 120 minutes. The samples should be processed as required and sent to the laboratory for analysis.

What Does the Report of an Oral Glucose Tolerance Test Signify?

The normal expected levels for a 2.68 ounce (oz) oral glucose tolerance test should be:

• Fasting: 60-100 mg/dl.

• After 1 hour: less than 200 mg/dl.

• After 2 hours: Less than 140 mg/dl.

• Levels between 141 mg/dl and 200 mg/dl after two hours are considered an 'impaired glucose tolerance test'.

• Levels greater than 200 mg/dl after two hours are indicative of diabetes.

Diabetes, insulin resistance, impaired beta-cell function, reactive hypoglycemia, acromegaly, and other disorders of carbohydrate metabolism can be checked by this procedure.

What Are the Advantages of Oral Glucose Tolerance Test?

• It is more sensitive than a fasting plasma glucose test (or fasting blood glucose test). It is usually suggested when the healthcare provider suspects a person to be diabetic but has normal values for fasting blood glucose.

• It helps to detect issues related to blood glucose in their initial stages itself. Hence, a person in the prediabetic stage will be able to manage their blood glucose levels through changes in diet and lifestyle.

• It is the only test that definitively diagnoses an impaired glucose tolerance.

What Are the Limitations of the Oral Glucose Tolerance Test?

The limitations of oral glucose tests can include the following:

• It is time-consuming.

• Requires people to fast for eight hours prior to the test.

• The results can be altered by stress, medications, etc.

• Since blood is very unstable after collection, the results can be hampered if the sample is not handled or stored properly.

Is Oral Glucose Tolerance Test Accurate?

The test is very accurate. The sensitivity or probability that the test provides the correct results ranges from eighty percent to ninety-one percent. On the other hand, the sensitivity of fasting blood glucose ranges from forty-five to fifty-five percent.

What Is Fasting Glucose Test?

The blood sugar levels while fasting are checked by a fasting glucose test. It is a simple, easy, and the most common method to check prediabetes, diabetes, or gestational diabetes. These conditions might not show any symptoms during their early or initial stages. They might be unaware that they have such a condition in their body. To manage diabetes, monitoring and maintaining a healthy fasting blood sugar level is necessary.

What Are the Indications of a Fasting Glucose Test?

The test can generally be done under the following conditions:

• As a part of the normal routine blood check-up to test one’s blood sugar levels.

• Pregnant women to ensure that their blood sugar levels are not altered by pregnancy hormones.

• A person who has symptoms of diabetes like excessive thirst, frequent urination, fatigue, etc.

• A person with a family history of diabetes.

• A person with risk factors that can lead to diabetes (like obesity).

• A previous test showed an elevation in blood sugar levels.

How Is the Test Done?

The patient should be advised to fast for eight to twelve hours prior to the test. The test should be rescheduled if the patient is not arriving fast. The sample can be collected and stored appropriately to be sent to a laboratory, where it is analyzed.

Conclusion

Monitoring the blood glucose levels is a critical part of managing diabetes or impaired glucose tolerance. Managing diabetes is a multidisciplinary approach. However, testing the blood glucose levels should be considered after thinking of the complex impact diabetes has on the various body systems- the effects on the metabolism, microangiopathies, and the adverse effects of hypoglycemia and hyperglycemia.