Complement Levels - C3, C4 are blood tests that measure the levels of two proteins, C3 and C4, that are part of the complement system. This system plays a role in the bodys immune response by helping to destroy foreign invaders such as bacteria and viruses. The test is performed by taking a blood sample from the patient and analyzing it in a laboratory. Abnormal levels of C3 and C4 can indicate certain autoimmune diseases or infections.