All about Diabetes > Diabetes Simplified > Tests


Tests:

The following tests are used for diagnosis:

  • A Fasting Plasma Glucose (FPG) test measures blood glucose in a person who has not eaten anything for at least 8 hours. This test is used to detect diabetes and pre-diabetes.
  • An Oral Glucose Tolerance Test (OGTT 75 gm) measures blood glucose after a person fasts at least 8 hours and 2 hours after the person drinks a glucose-containing beverage. This test can be used to diagnose diabetes and pre-diabetes.
  • A Random Plasma Glucose Test, also called a casual plasma glucose test, measures blood glucose without regard to when the person being tested last ate. This test, along with an assessment of symptoms, is used to diagnose diabetes but not pre-diabetes.

Test results indicating that a person has diabetes should be confirmed with a second test on a different day.

FPG Test

The FPG test is the preferred test for diagnosing diabetes because of its convenience and low cost. However, it will miss some diabetes or pre-diabetes that can be found with the OGTT. The FPG test is most reliable when done in the morning. Results and their meaning are shown in Table 1. People with a fasting glucose level of 100 to 125 milligrams per decilitre (mg/dL) have a form of pre-diabetes called impaired fasting glucose (IFG). Having IFG means a person has an increased risk of developing Type 2 diabetes but does not have it yet. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.

Table 1. FPG test

Plasma Glucose Result (mg/dL) Diagnosis

99 or below

Normal

100 to 125

Pre-diabetes
(impaired fasting glucose)

126 or above

Diabetes*

*Confirmed by repeating the test on a different day.

OGTT

Research has shown that the OGTT is more sensitive than the FPG test for diagnosing pre-diabetes, but it is less convenient to administer. The OGTT requires fasting for at least 8 hours before the test. The plasma glucose level is measured immediately before and 2 hours after a person drinks a liquid containing 75 grams of glucose dissolved in water. Results and their meaning are shown in Table 2. If the blood glucose level is between 140 and 199 mg/dL 2 hours after drinking the liquid, the person has a form of pre-diabetes called impaired glucose tolerance (IGT). Having IGT, like having IFG, means a person has an increased risk of developing Type 2 diabetes but does not have it yet. A 2-hour glucose level of 200 mg/dL or above, confirmed by repeating the test on another day, means a person has diabetes.

Table 2. OGTT

2-Hour Plasma Glucose Result (mg/dL) Diagnosis

139 and below

Normal

140 to 199

Pre-diabetes
(impaired glucose tolerance)

200 and above

Diabetes*

*Confirmed by repeating the test on a different day.

Gestational diabetes:

Gestational diabetes is also diagnosed based on plasma glucose values measured during the OGTT, preferably by using 100 grams of glucose in liquid for the test. Blood glucose levels are checked four times during the test. If blood glucose levels are above normal at least twice during the test, the woman has gestational diabetes. Table 3 shows the above-normal results for the OGTT for gestational diabetes.

Table 3. Gestational diabetes: Above-normal results for the OGTT*

When Plasma Glucose Result (mg/dL)

Fasting

95 or higher

At 1 hour

180 or higher

At 2 hours

155 or higher

At 3 hours

140 or higher

Note: Some laboratories use other numbers for this test.
*These numbers are for a test using a drink with 100 grams of glucose.

Random Plasma Glucose Test

A random, or casual, blood glucose level of 200 mg/dL or higher, plus the presence of the following symptoms, can mean a person has diabetes:

  • increased urination
  • increased thirst
  • unexplained weight loss

Other symptoms can include fatigue, blurred vision, increased hunger and sores that do not heal. The doctor will check the person's blood glucose level on another day using the FPG test or the OGTT to confirm the diagnosis.

The HbA1c blood test

If you have diabetes, your HbA1c level may be done every 2-6 months by your doctor or nurse. This test measures your recent average blood glucose level. The test measures a part of the red blood cells. Glucose in the blood attaches to part of the red blood cells. This part can be measured and gives a good indication of your average blood glucose over the previous 2-3 months.

For people with diabetes, treatment aims to lower the HbA1c level to below a target level which is usually agreed between you and your doctor. The target level is usually somewhere between 6.5% and 7.5%. If your HbA1c level is above your target level, then you may be advised to 'step up' treatment to keep your blood glucose level down. (For example, by increasing the dose of medication, improving your diet, etc.)

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