Hemoglobin A1c is frequently used to help diagnosed diabetics determine how elevated their uncontrolled blood glucose levels have been. It may be ordered several times while control is being achieved, and then several times a year to verify that good control is being maintained. The A1c test may be used to screen for and diagnose diabetes. However, A1c should not be used for diagnosis in pregnant women, people who have had recent severe bleeding or blood transfusions, those with chronic kidney or liver disease, and people with blood disorders such as iron-deficiency anemia, vitamin B12 anemia, and hemoglobin variants.

Also, only A1c tests that have been referenced to an accepted laboratory method (standardized) should be used for diagnostic or screening purposes. In these cases, a fasting plasma glucose or oral glucose tolerance test may be used for screening or diagnosis. Currently, point-of-care tests, such as those that may be used at a doctor's office or a patient's bedside, are too variable for use in diagnosis but can be used to monitor treatment (lifestyle and drug therapies).

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