Elevated complement C1q is associated with many autoimmune related diseases, such as systemic lupus erythematosus, rheumatoid arthritis, as well as scleroderma, vasculitis, or allergic purpura, as well as gout and osteomyelitis. Complement C1q has a very good significance in the diagnosis of these diseases, as well as in the monitoring of the efficacy. This is because complement ensures the transport and clearance of circulating immune complexes in the body. Complement C1q may decrease transiently during the acute and active phase of autoimmune diseases such as SLE, and then increase when these diseases enter remission and recovery. In addition, complement C1q can help diagnose some congenital complement deficiency disorders.