Human serum complement levels are relatively stable, and the total amount of serum complement or its components may change only in the presence of certain diseases. Currently, the total complement level can be determined based on the hemolytic activity of complement, and the level of certain complement components can also be determined by immunodiffusion. In a few patients with malignant tumors and other diseases, the total serum complement level may be 2 to 3 times higher than normal, but the significance of this is not clear. Compensatory increases may also be seen in some infectious diseases. A lower-than-normal total serum complement is called hypocomplementemia. Hypocomplementemia can be seen in the following conditions: ① Large depletion of complement components: can occur in serum sickness, glomerulonephritis after streptococcal infection, systemic lupus erythematosus, autoimmune hemolytic anemia, rheumatoid arthritis, and allograft rejection. In these diseases, in addition to a decrease in total complement, there may also be a decrease in each component of Clq, C4, C2, C3 and C5. ② Large loss of complement is seen in patients with trauma, surgery and major blood loss. Loss of complement components with expanded loss of serum proteins and hypocomplementemia occurs. (iii) Insufficient complement synthesis: mainly seen in liver patients, such as cirrhosis, chronic active hepatitis and severe cases of acute hepatitis.