A fasting plasma glucose level above 126 indicates diabetes, according to the National Diabetes Information Clearinghouse. Fasting plasma glucose levels of 99 or below are normal while levels from 100 to 125 indicate prediabetes.
Doctors use results from the fasting plasma glucose test, the A1C test or the oral glucose tolerance test to diagnose diabetes. During a checkup, doctors sometimes order a random plasma glucose test. Results above 200 indicate the potential for diabetes and require further testing, reports the National Diabetes Information Clearinghouse.
Both the fasting and oral tolerance test require patients fast for eight hours before testing. Patients taking the oral tolerance test drink a solution containing 75 grams of glucose in water and return for testing after two hours without eating anything else. While this test is more accurate, it is also less convenient for the patient, so the fasting test is more common, notes the National Diabetes Information Clearinghouse.
The A1C test provides results that indicate a patient's average blood sugar levels for the past three months. However, some A1C test results give false positives for diabetes according to the National Diabetes Information Clearinghouse. The test is also not useful for diagnosing type 1 diabetes. A1C results below 5.7 percent are normal, levels from 5.7 percent to 6.4 percent indicate prediabetes, and levels above 6.4 indicate diabetes. A prediabetic with a level of 6.0 percent or above is at a high risk for diabetes.