This test measures the amount of glucose (sugar) in blood. This test is done after fasting. This test is used to diagnose diabetes mellitus, prediabetes, hypoglycemia (low blood sugar), and hyperglycemia (high blood sugar)[1].his test measures the amount of glucose (sugar) in blood. This test is done after fasting. This test is used to diagnose diabetes mellitus, prediabetes, hypoglycemia (low blood sugar), and hyperglycemia (high blood sugar)[1].