The complement system important part of the immune system. There are certain plasma proteins involved in the complement system, which destroy the microbes. C3 protein is the critical and most abundant protein in the complement system that helps in killing bacteria and viruses that cause disease. Complement 3 test measures the amount of C3 proteins in the blood. Reduced levels of C3 can help in the diagnosis and monitoring of the treatment of certain diseases. Systemic Lupus Erythematosus (SLE) is one of the autoimmune disorders that often cause abnormal C3 levels. Also, many recurrent bacterial infections are known to occur when there is a deficiency of C3.