The A1C blood test monitors the effectiveness of blood sugar control over a 6-to-8 week period by measuring the percentage of hemoglobin molecules in the blood that are glycated and reflecting the average level of glucose in the blood over the past 2-3 months. While the less common GSP test measures glycated serum proteins over a shorter 2 week period, the A1C test is preferred for assessing long-term glycemic control.