A1C and fasting glucose testing are less burdensome than the oral glucose challenge. A1C does not require fasting but is more costly than fasting blood glucose and may be unreliable in certain conditions. People in the early stages of diabetes or with prediabetes may be identified by one test but not the other. Therefore, confirmation with the same test that is above the diagnostic threshold for diabetes should be considered.
The threshold for diagnosis of diabetes, defined by the cut points referenced in Table 2, reflects levels at which risk of microvascular disease increases. There is some variation among guidelines on the cut point for diagnosis of prediabetes and the preferred test. All agree, however, that the risk of diabetes is continuous, extending below the lower limit of the range and becoming disproportionately greater at the higher end of the range.
If health care professionals choose A1C testing for diagnostic purposes, it should be performed in a laboratory using a method that is NGSP certified. Point-of-care A1C tests and finger-stick blood glucose testing should not be used for diagnosis. The A1C test may not be accurate in people with anemia, kidney failure, or liver disease and should not be used for diagnosis of GDM or cystic fibrosis–related diabetes. Some A1C tests give false results in people with hemoglobin variants (e.g., sickle cell trait), but most are free from such interference.