The Complement 4 (C4) test is a blood test that measures the level of C4 protein in the blood. C4 is a protein that is part of the complement system, which is a group of proteins that work together to help the immune system fight infections and other foreign invaders. The test is primarily used to help diagnose and monitor certain autoimmune diseases, such as lupus, where the complement system can be overactive and lead to inflammation and tissue damage. The test is also used to monitor the progression of kidney disease, as C4 levels can be affected by damage to the kidneys. The test is performed on a blood sample, and the results are typically reported as a numerical value that is compared to the laboratory’s reference range to determine if the level of C4 protein in the blood is within a normal range or if further investigation is necessary. A healthcare provider may recommend additional tests or imaging studies to confirm a diagnosis or monitor the progression of the disease based on the results of the test and other factors such as symptoms and medical history.