Diabetes mellitus is a disorder characterized by abnormally high blood glucose levels (hyperglycemia). Glucose, a type of sugar, is the body's primary source of energy. In type 1 diabetes, blood glucose is elevated because the pancreas produces little or no insulin—the hormone that permits cells to remove glucose from the blood; in type 2 diabetes, high blood glucose develops because the body’s cells are resistant to insulin. The following glucose tests are performed to diagnose or monitor diabetes.

A single blood sample taken anytime after eating that day may be sufficient for a diagnosis; a blood glucose level above 200 milligrams per deciliter (mg/dL) associated with the classic symptoms of hyperglycemia - thirst, frequent urination, and weight loss - indicates that diabetes is present.

Fasting blood glucose measures blood glucose levels after a 12- to 14-hour fast. While levels normally decrease during fasting, they remain persistently high in people with diabetes. A fasting glucose value above 125 mg/dL on at least 2 tests indicates diabetes.

Postprandial blood glucose measures blood glucose levels 2 hours after eating a meal. Postprandial blood glucose is usually done in people who have symptoms of hyperglycemia, or when the results of a fasting glucose test suggest possible diabetes, but are inconclusive. Values of 200 mg/dL or more indicate diabetes.

The oral glucose tolerance test is not necessary in most cases, but is the method of choice to detect diabetes when results from the fasting and postprandial tests are borderline or inconclusive. In oral glucose tolerance, the glucose levels in the blood and urine are measured periodically for several hours following the ingestion of a beverage containing a specified dose (usually 75 grams) of glucose.

Hemoglobin A1c (HbA1c), also known as the glycosylated hemoglobin or glycohemoglobin test, is used to monitor the effectiveness of therapy in people already diagnosed with diabetes. HbA1c measures the amount of glucose attached to hemoglobin (the oxygen-carrying protein in red blood cells), which increases as blood glucose levels rise. Since hemoglobin circulates in the blood until the red blood cells die (half the red blood cells are replaced every 12 to 16 weeks), the HbA1c test is a useful tool for measuring average blood glucose values over the previous 2 to 3 months.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at Healthcommunities.com

Published: 12 Jan 2012

Last Modified: 11 Sep 2015