The complement C3 test measures the activity of a protein called complement component 3 (C3) in your blood. This protein is part of the immune system and helps protect your body from infections. Doctors use this test to diagnose and monitor autoimmune diseases and conditions that cause inflammation. For diabetes patients, this test is important because elevated C3 levels can indicate inflammation, which is common in diabetes. The test involves drawing blood from a vein, usually from the back of the hand or the inner side of the elbow. Normal C3 levels range from 80 to 160 milligrams per deciliter. Low C3 levels can suggest an inherited deficiency or autoimmune diseases like lupus. High C3 levels may indicate inflammation due to conditions like ulcerative colitis or certain cancers. However, high C3 levels are also seen in diabetes, so additional tests are needed to confirm the diagnosis. Blood tests are generally safe but can have minor risks like bruising or dizziness.