A glycohemoglobin, or GHB, test measures the amount of glucose that has bound itself to hemoglobin in the blood. This combination of glucose and hemoglobin is also called glycated or glycosylated hemoglobin, and accumulates over a period of time. Measuring the levels of glycohemoglobin allows doctors to determine the average blood sugar level over a period of a few months. That makes this test an effective diagnostic and monitoring tool for people with diabetes and related disorders. 

A GHB test may be administered upon a patient's initial diabetes diagnosis, to determine how elevated his or her blood glucose level has been over the previous several months. Patients who have had a diagnosis of diabetes should also have this kind of test performed regularly, usually every two to six months. This kind of routine testing allows patients and their doctors to identify long term increases in blood sugar level and address them before they can cause organ and tissue damage. 

Unlike a fasting blood test, glycohemoglobin levels are unlikely to be affected by a patient's most recent eating habits, so it is unnecessary to avoid food before the test. This kind of test is not effective on people who have a pre-existing blood disorder, recent transfusion or serious bleeding, or people who are pregnant or recent sufferers of liver or kidney disease. In most healthy people, it will produce blood GHB levels between 4 and 6 percent, with levels above 5.7 percent indicating prediabetes and levels above 6.5 percent indicating full-blown diabetes.