HbA1 c is a test which measure the blood sugar control over last few weeks or months. Its normal levels are below 6%. Higher levels indicate that your blood sugar levels were not controlled properly during last few weeks or months. Its levels 7 mean your diabetes control was not good as it should be. As the levels of HbA1 c continue to increase, chances of developing diabetes complication also become high. So, this test is not done to confirm the diagnosis of diabetes but is done in people who are already diabetic and to monitor the sugar control in last few weeks/months.
I think this test is highly overrated. For many years I had normal A1C levels, but my blood sugar was often climbing to the 200-300 range and causing damage.