Tests For Diabetes - Diagnosing Diabetes: Diabetes

Content provided by the Faculty of the Harvard Medical School

Tests for diabetes

If you display any of these symptoms or suspect that you might have diabetes, see a doctor promptly. Your doctor will take a full medical history and will probably perform one of three simple blood tests: a random plasma glucose test, a fasting plasma glucose test, or an oral glucose tolerance test. A fourth test, the glycosylated hemoglobin test, is generally used to monitor diabetes after a diagnosis has been made. All these tests require a small blood sample that a lab will analyze for glucose content.

Random plasma glucose test

The random plasma glucose test measures glucose levels in your blood. If the glucose level exceeds 200 mg/dL, you probably have diabetes, especially if you've noticed symptoms. You don't have to refrain from eating before having this test done, but the glucose content of your meals can affect the results, so it's considered less reliable than the fasting plasma glucose test.

Fasting plasma glucose test

This test is the current standard for diabetes diagnosis. Like the random plasma glucose test, it measures blood glucose levels, but in this case, blood is drawn after you've refrained from eating for at least eight hours so there isn't any chance of a meal interfering with the results. Normal fasting plasma glucose levels are less than 100 mg/dL, and levels at or above 126 mg/dL indicate diabetes. However, if your first test indicates diabetes, have a second one to confirm the results.

Oral glucose tolerance test

Currently, the American Diabetes Association doesn't recommend an oral glucose tolerance test for detecting diabetes. This doesn't reflect any doubts about the test's accuracy. In fact, the oral glucose tolerance test is probably more sensitive than the fasting test. Instead, it's an issue of practicality: The test is considerably more time-consuming and cumbersome than the fasting plasma glucose test.

For the oral glucose tolerance test, your glucose level is measured after you've fasted overnight. You drink a sugary solution, and then another blood sample is drawn two hours later. Glucose levels will rise and fall quickly in healthy people. But they rise above normal and decrease slowly in those with diabetes. A person whose glucose level is 200 mg/dL or higher when the second blood sample is drawn has diabetes. This test, like the fasting plasma glucose test, should be repeated on another day to confirm the diagnosis.

Glycosylated hemoglobin test

Another measurement widely used in diabetes management is the glycosylated hemoglobin test. This blood test reflects the average blood sugar level over the preceding two to three months and will help your doctor monitor your efforts to keep your blood sugar as close to normal as possible. Because having food or a drink before the test won't skew the results, a glycosylated hemoglobin test can be performed at any time of day, even after a meal.

Hemoglobin is the oxygen-carrying protein in red blood cells. When glucose in the blood attaches to hemoglobin, the bound product is called HbA1c. (It's also known as glycosylated hemoglobin, glycated hemoglobin, or glycohemoglobin.)

Healthy, nondiabetic people usually have an HbA1c level of about 5%, meaning that approximately 5% of their hemoglobin molecules have glucose attached to them. If your diabetes has been well controlled during the previous two to three months, the HbA1c level should be close to normal, that is, less than 7%. If your blood sugar has been high, the level will be elevated. Studies have shown that keeping HbA1c levels low reduces the chances of developing complications of diabetes. It is therefore wise to keep HbA1c levels as close to the nondiabetic range as is safely possible. The American Diabetes Association advises people with the disease to strive for an HbA1c level of 7% or less.

Not quite diabetes

Some of these tests can also uncover lesser degrees of abnormal glucose metabolism, which may eventually develop into full-blown diabetes. A fasting plasma glucose reading between 100 mg/dL and 126 mg/dL indicates impaired fasting glucose. If this is the case and an oral glucose tolerance test is performed, a two-hour glucose result between 140 mg/dL and 200 mg/dL is considered impaired glucose tolerance. Both conditions raise your risk for diabetes and cardiovascular disease (see "What causes type 2 diabetes?"). To monitor these conditions, it's best to have annual glucose tests.

Last updated:	January 23, 2007