The fasting plasma glucose (FPG) test measures the concentration of glucose in the plasma after fasting for at least 8 hours (usually between 8 and 12 hours). This measurement reflects the body's ability to maintain normal blood sugar levels while fasting and is a key test for diagnosing type 2 diabetes and prediabetes (impaired glucose tolerance). It is also used to monitor people with known diabetes. It is one of the diagnostic criteria established by international diabetes associations.