Also known as: Complement component 4, C4
This test measures the amount of C4 proteins in your blood. These proteins are part of your complement system, an important part of your immune system that helps kill disease-causing bacteria and viruses.
By measuring complement C4 levels, especially in how they compare with other parts of the complement system, your healthcare provider can diagnose and keep track of the treatment of certain diseases. One of the diseases that commonly involves abnormal C4 levels is systemic lupus erythematosus, or lupus, an autoimmune disorder.